antihypertensive therapy and arterial function in experimental hypertension

Gen. Pharmac. Vol. 27, No. 2, pp. 221-238, 1996 Elsevier Science Inc. Printed in the USA.

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ISSN 0306-3623/96 0306-3623(95)02015-6

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Antihypertensive Therapy and Arterial Function in Experimental Hypertension

Mika Kah6nen, l* Pertti Arvola, 2 Heikki Makynen I and Ilkka P6rsti 2

1MEDICAL SCHOOL, UNIVERSITY OF TAMPERE AND 2DEPARTMENT OF INTERNAL MEDICINE, P. O. Box 607, TAMPERE UNIVERSITY HOSPITAL, HN-33101 TAMPERE, FINLAND

ABSTRACT. 1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure- lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds.

2. A number of reports have suggested that ACE inhibitors can improve arterial function in hyperten. sion, whereas the knowledge about the vascular effects of other antihypertensive drugs, like {~.blockers, calcium channel blockers, and diuretics remains rather limited.

3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed. OEN PHARMAC 27;2:221-238, 1996.

KEY WORDS. Antihypertensive therapy, arterial smooth muscle, blood pressure, endothelium, hypertension

I N T R O D U C T I O N

Arterial tone which maintains the peripheral resistance in the circulation is a crucial controller of blood pressure (Nelson et al. 1990). An increase in arterial resistance is a characteristic finding in established essential hypertension in humans and in genetic models of hypertension, whereas the other major determinant of blood pressure, cardiac output, usually remains normal. Since peripheral resistance could increase as a result of either enhanced contractility or impaired relaxation of resistance arteries, functional changes in arterial smootl~.muscle and endothelium may be important in the establishment of hypertension.

Although hypertension has been treated with blood pressure-lowering compounds for several decades, the precise mechanisms of action for many agents in this class still remain somewhat obscure. A number of studies have documented that several antihypertensive drugs have favorable influences on the function of arterial smooth muscle and endothelium, which could contribute to the chronic antihypertensive actions of these compounds. Detailed knowledge about the influences of long-term pharmacological therapies on arterial function and structure would be helpful to permit a more appropriate basis for the development of optimal compounds and for cause-oriented treatment of hypertension.

In this article, the effects of antihypertensive therapies on the function arterial smooth muscle and endothelium in human and experimental hypertension are reviewed. In the first section, the findings concerning altered arterial function in hypertension are summarized, and in the second section the effects of antihypertensive therapies on these variables are discussed.

ARTERIAL F U N C T I O N IN HYPERTENSION E n d o t h e l i a l f u n c t i o n

ENDOTHELIUM-DERIVED RELAXING FACTOR/NITRIC OXIDE. In 1980, Furchgott and Zawadzki demonstrated that vascular relaxation induced by acetylcholine (ACh) was dependent on the presence of the endothelium and provided evidence that this effect was mediated by

*To whom correspondence should be addressed. Received 18 May 1995.

a labile humoral factor, later known as the endothelium-derived relaxing factor (EDRF). Nitric oxide (NO) release from the endothelium has been reported to account for the biological activity of EDRF (Feelisch et al., 1994; Palmer et al., 1987), suggesting that EDRF and NO are identical. NO is formed from the conversion of L-arginine to L-citrulline by the enzyme NO synthase (Palmer eta/ . , 1988), which can be inhibited by L-arginine analogs, like NG-monomethyl-L-arginine (L-NMMA) and NC-nitro-L-arginine-methylester (L-NAME) (Moncada et al., 1991). Subsequently, NO relaxes vascular smooth muscle via the stimulation of soluble guanylate cyclase and elevation ofintracellular cyclic guanosine monophosphate (GMP) (Moncada et al., 1991). There seems to be a continuous and spontaneous basal release of NO from the endothelium, the amount of which regulates the level of vascular tone (for review, Schulz and Triggle, 1994).

Since NO appears to play a fundamental role in the regulation of blood flow and pressure, endothelial dysfunction may contribute to the increase in arterial resistance in hypertension (see Moncada et al., 1991). Indeed, endothelium-dependent relaxation has been reported to be impaired in blood vessels from hypertensive subjects, suggesting that endothelial dysfunction could be an underlying factor in hypertension. Abnormal relaxation of arteries to ACh has been found in patients with essential hypertension (Egashira et al., 1995; Falloon and Heagerty, 1994; Panz a et al., 1990; Treasure et al., 1992 ), in patients with renov ascu- lar and primary aldosteronism (Taddei et al., 1993), as well as in various models of experimental hypertension including the spontaneously hypertensive rat (SHR) (Arvola et al., 1993a; K~ing and L(ischer, 1995; Shirasaki et al., 1988), aortic coarctation-induced hypertension (Lock- ette et al., 1986), and deoxycorticosterone acetate-NaCl hypertension (M/ikynen et al., 1994). Since the responses to endothelium-independent v asodilators like sodium nitroprusside and SIN-1 have usually remained unchanged in essential and experimental hypertension (Clozel et al., 1990; Egashira et al., 1995; K(ing and L~ischer 1995; M/ikynen et al., 1995; Panza et al., 1993a, 1995), the ACh test has been interpreted to • indicate impaired ability of the endothelium to produce and release relaxing factors in hypertension. Furthermore, patients with essential hypertension as well as SHR have impaired endothelium-dependent responses not only to ACh, but to several other agonists as well (which

222 M. K/ih6nen et al.

exert action via different initial signal transduction pathways), suggesting generalized abnormality of endothelial vasodilator function (Panza et al., 1995; Rubanyi et al., 1993).

Increasing numbers of results have suggested that the function of the L-arginine-NO pathway is abnormal in hypertension. The NO synthesis inhibitor, L-NMMA, was less effective in reducing forearm blood flow in hypertensive than normotensive subjects, suggesting abnormality of basal NO-mediated dilation (Calver et al., 1992). Also, in hypertensive rats, NO synthesis inhibition less effectively increased arteriolar tone than in normotensive controls, and c-arginine consis- tently caused vasodilation only in normotensive but not in hypertensive rats (Boegehold, 1992). Moreover, ACh elevated cyclic GMP levels in aortic rings of SHR less effectively than in the rings of normotensive Wistar-Kyoto rats (WKY) (Shirasaki et al., 1988). Either L-NAME or endothelium removal altered flow-diameter relations in mesenteric arteries of WKY but not in those of SHR, suggesting impaired endothelium-dependent regulation of diameter by flow in hypertension (Qiu et al., 1994a). We have found that vascular endothelium effectively attenuated arterial contractions induced by cumulative addition of Ca 2+ in SHR and WKY, while the effect of NO synthesis inhibition by L-NAME on the responses was comparable in both strains. However, the reversing effect of L-arginine on the influence oft-NAME was more pronounced in WKY than SHR (K~ih6nen et al., 1994a). This result paralleled recent findings in humans whereby increased availability of L-arginine did not modify endothelium-mediated vasodilation in hypertensive subjects but significantly increased the production of NO in normotensive humans, suggesting altered function of the t-arginine- NO pathway in hypertension (Panza et al., 1993b).

However, many reports have also suggested that the NO pathway may not be deficient in hypertension: L-NAME potentiated pressor responses elicited by sympathetic nerve stimulation more effectively in the pithed SHR than WKY (Tabrizchi and Triggle, 1991), basal NO formation was higher and bradykinin-induced release of NO was more pronounced in the coronary circulation of SHR than WKY (Kelm et al., 1992, 1995). These investigators suggested that such an increase in the responses of SHR may reflect a compensatory response to the elevated blood pressure and vascular resistance. Moreover, intravenous administration of ACh comparably reduced mean arterial pressure in SHR and WKY, accompanied by a significant and corresponding rise in serum NO metabolites (nitrate and nitrite) (Sawada et al., 1994). In addition, L-NAME comparably potentiated a-adrenoceptor- and KCl-mediated pressor responses in the perfused mesenteric arterial bed of SHR and WKY (Adeagbo et al., 1994). Finally, cyclic GMP accumulation elicited by exogenous nitrovasodilators, or by EDRF released either basally or in response to bradykinin and calcium ionophore, was greater in smooth muscle from SHR than WKY. This could have resulted from upregulation of guanylate cyclase to compensate for the increased total peripheral resistance and/or endothelial dysfunction and chronic EDRF deficiency associated with the development of hypertension (Papapetro- poulos et al., 1994).

PROSTACYCLIN. Prostacyclin (PGIz) is the major prostanoid produced by the vascular endothelial cells (see Busse and Fleming, 1993). PGIz exerts its actions (vasodilation and inhibition of platelet aggregation) by binding to its membrane receptor which activates adenylyl cyclase and subsequently increases the intracellular concentration of cAMP. In contrast to the inhibition of NO biosynthesis, blockade of endothelial PGI2 formation has practically no effect on systemic blood pressure (see Busse and Fleming, 1993). Nevertheless, impairment of endothelial PGIz synthesis or release has been suggested to underlie the attenuated endothelium-dependent relaxations to bradykinin in striated muscle arterioles from renovascular hypertensive rats (Naka-

mura and Prewitt, 1991). Moreover, if there is a dysfunction of the endothelium in hypertension, impaired generation of both NO and PGIz could be expected, because the release of these autacoids show fairly similar patterns (Mitchell et al., 1992).

ENDOTHELIUM.DERIVED HYPERPOLARIZATION. Endothelial cells have also been found to mediate hyperpolarization of arterial smooth muscle cell when stimulated by agonists (Bray and Quast 1991; Chen et al., 1988; Feletou and Vanboutte, 1988; Garland and McPherson, 1992). This hyperpolarization cannot be blocked by inhibition of the NO synthase or cyclooxygenase pathways, indicating that a factor distinct from NO or PGI2 is released from the endothelium (Bray and Quast, 1991; Chen etal. , 1991 ; Garland and McPherson, 1992; Vanheel et al., 1994). This substance has been termed endothelium-derived hyperpolarizing factor (EDHF), which probably causes opening of K + channels in the smooth muscle membrane (Bray and Quast, 1991; Garland and McPherson, 1992; also, see Garland et al., 1995), although the chemical identity of EDHF is still unknown (for review see Garland et al., 1995). The K + channel-opening action is supported by the finding that apamin and charybdotoxin, blockers of Ca2+-activated K + channels, abolished the L-NAME insensitive relaxations to ACh in the rat mesenteric artery (Waldron and Garland, 1994). Recent findings suggest that EDHF released by bradykinin in porcine coronary artery is a cytochrome P450-derived arachidonic acid metabolite, which indeed opens Kca channels in smooth muscle (Bauersachs et al., 1994; Hecker et al., 1994b). An alternative possibility for the release of putative EDHF is that the hyperpolarization of the endothelial cells may electrotonically spread into adjacent vascular smooth muscle cells (Busse et al. 1988; Garland and McPherson 1992). This concept is supported by data of K~ihnberger et al. (1994), whereby the non-NO-, non-prostanoid- mediated relaxation in the porcine coronary artery required function- ally intact myoendothelial junctions.

The contribution of endothelium-derived hyperpolarization to the control of blood flow remains largely unresolved, even though Adeagbo and Triggle (1993) have suggested that in the rat mesenteric arterial bed EDHF plays a significant role in the regulation ofperfusion pressure. Nevertheless, in vitro experiments have indicated that the EDHF pathway contributes notably to relaxation responses in mesenteric, renal, and femoral arteries but not in larger conduit arteries (aorta, iliac artery), which may explain why these smaller vessels are more sensitive to endothelium-dependent relaxants than large arteries (Nagao eta/. 1992; for review see Garland et al., 1995).

Interestingly, endothelium-dependent hyperpolarization to ACh has been reported to be markedly impaired in SHR when compared with WKY, and this mechanism could contribute to the attenuated relaxation to ACh in SHR mesenteric arteries (Fujii et al., 1992, Fujii et al., 1993; K~ih6nen eta/., 1994a, 1995b; Li et al., 1994). The endothelium- dependent hyperpolarization appears to be impaired in both WKY and SHR with aging (Fujii et al., 1993). Recent experiments performed in vivo with rats support this, because the indomethacin- and L-NAME- resistant vasodilatory action of ACh was found to be impaired by both hypertension and aging (Tominaga et al., 1994). Furthermore, the results of Yokota et al., (1994) indicate that the attenuating effect of endothelium on serotonin-induced vasoconstriction is decreased in SHR when compared with WKY, which may be explained by the inability of the endothelium to release an EDHF-Iike substance. Recent findings of ours also support the view that impaired endothelium-dependent agonist-induced relaxations can be attributed to attenuated endothelium- d e r i v e d h y p e r p o l a r i z a t i o n i n S H R ( K ~ h 6 n e n e t a l . , 1994a, 1995b). Taken together, these findings indicate that decreased endothelium-derived hyperpolarization may significantly contribute to the endothelial dysfunction observed in hypertension.

Antihypertensive Therapy and Arterial Function 223

NORMOTENSION HYPERTENSION

~ ENDOTHELIUM

SMOOTH MUSCLE

FIGURE 1. Summary diagrams of the regulation of arterial smooth muscle function by vascular endothelium and its alterations in hypertension (see text for detailed explanations). Abbreviations: NO=nitric oxide; PGI2=prostacyclin; EDHF=endothelium- derived hyperpolarizing factor; EDCF = endothelium.derived con. tracting factor(s). Symbols: arrows = hypertensive aberrations from normal endothelial function are indicated by wide and thin arrows (increased and decreased action, respectively).

ENDOTHELIUM-DERIVED CONTRACTILE FACTORS. ACh can con- strict arteries of both prehypertensive and hypertensive SHR via the release of endothelium-derived contractile factor(s) (EDCF), such responses being absent in young and adult WKY (Auch-Schwelk et al., 1992a; Jameson et al., 1993; Lfischer and Vanhoutte, 1986), although production of EDCF has been described in the aorta from old WKY (Koga et al., 1989). Several groups have suggested that endothelium- mediated relaxations are impaired in adult SHR because of increased production of cyclooxygenase-derived EDCF which antagonize the relaxing properties of EDRF, because they found no differences in relaxations to ACh in SHR and WKY during cyclooxygenase inhibition (Ito and Carretero, 1992; Jameson et al., 1993; Koga et al., 1989; Li and Bukoski, 1993; Ltischer and Vanhoutte, 1986; Takase et al., 1994). Prostaglandin H2 (PGH2) has been proposed to be a mediator ofendothe- lium-dependent contractions in the aorta (Iwama et al., 1992; Kting and Ltischer, 1995) and renal arteries of SHR, while in the mesenteric artery this contractile factor was suggested to be the superoxide anion (Fu-Xiang et al., 1992; Jameson et al., 1993). Taken together, EDCF may at least partially account for the impaired endothelium-mediated relaxation in SHR. The physiological and pathophysiological signifi- cance ofcyclooxygenase-derived contracting factors in the whole organ- ism remains unknown. However, it is interesting that the production of EDCF appears to parallel closely the increase in blood pressure in SHR (Iwama et al., 1992).

CONCLUSIONS. Most of the evidence supporting endothelial dysfunction in different forms of hypertension comes from the impaired relaxation revealed by the "ACh test" in a variety of isolated preparations. Angus and Lew (1992) have stated that the available evidence from the ACh test is conflicting and therefore the question of endothelial dysfunction remains far from proven.

Another important question has been whether the endothelial dysfunction in hypertension is a primary defect or merely a result of high blood pressure and a subsequent damage to the endothelium. In young 3-week-old SHR, the relaxation responses to ACh were even more pronounced than in WKY (Watt and Thurston, 1989), while no differences were found in responses to ACh or in cGMP concentrations of smooth muscle between 5-6-week-old SHR and WKY (Li and Joshua, 1993; Papapetropoulos et al., 1994; Shirasaki et al., 1988). Moreover, reversal of hypertension in deoxycorticosterone-NaCl-treated rats and, in one-kidney/one-clip renovascular rats resulted in normalization of the endothelium-dependent relaxation to ACh (Lockette et al., 1986).

NORMOTENSION HYPERTENSION

Ca2+ Ca2+

FIGURE 2. Summary diagrams of the regulatory systems for arterial smooth muscle function and the alterations in hypertension (see text for detailed explanations). Abbreviations: VOC = voltage- operated Ca 2 + channel; ROC= receptor-operated Ca 2+ channel; SR=sarcoplasmic reticulum; IP3=inositol 1,4,5.triphosphate; ATP pump utilizing adenosine 5'-triphosphate-derived energy. Symbols: arrows=hypertensive aberrations from normal arterial smooth muscle function are indicated by wide and thin arrows (increased and decreased action, respectively).

Therefore, endothelial dysfunction may only be a secondary rather than a primary alteration in hypertension. Nevertheless, forearm vasodilation to ACh in vivo has been reported to be decreased in normotensive subjects with a familial history of essential hypertension, suggesting that this abnormality can precede the appearance of hypertension (Tad- dei et al., 1992). In addition, the endothelium-dependent contractions in response to high concentrations of ACh in SHR, but not in WKY, may appear as early as 4 weeks of age, before any significant rise in blood pressure (Jameson et al., 1993). Regardless of whether the endothelial dysfunction in hypertension is primary or secondary in nature, impaired endothelium-mediated vasodilation may be of fundamental importance in the maintenance of sustained hypertension.

The aforementioned controversial findings may be attributed to differences in various forms and stages of hypertension, heterogeneous endothelial influences in various vascular regions, and varying experimental conditions used in the evaluation of endothelial function. How- ever, to date the detailed contributions of each of these factors have not been fully characterized. Anyway, endothelial function in general appears to be normal in the prehypertensive phase and very early stages of experimental hypertension (see above). The subsequent increase in blood pressure may initially activate compensatory mechanisms and augment the release of endothelium-derived relaxing factors, as suggested by enhanced endothelial dilator actions observed during the early development of elevated blood pressure (Kelm et al., 1992; Mourlon-Le Grand et al., 1992; Tabrizchi and Triggle, 1991). This kind of compensa- tion, however, appears to be insufficient in the long run and high blood pressure finally results in endothelial damage and loss of vasodilation in the more established stages of hypertension (Ciriaco et al., 1993).

Arterial smooth muscle Function

CONTRACTILE MACHINERY. Vascular smooth muscle contraction is initiated by an increase in the cytosolic Ca 2+ level ([CaZ+D (Karaki and Weiss, 1988; Stull et al., 1991). Because of this crucial rote, [Ca2+]~ within the smooth muscle cell is tightly regulated, the major mechanisms being Ca z÷ entry and extrusion across the plasmalemma and Ca z+ release and uptake by the sarcoplasmic reticulum (SR) (Stull et al., 1991). Subsequently, cytosolic free Ca 2+ binds to calmodulin and this complex then activates myosin light-chain kinase (MLCK). The activated MLCK phosphorylates the regulatory light chain subunit of myosin, which leads to an increase in myosin's ATPase activity. The


phosphorylated myosin binds to actin filaments in cycles producing contractile force (for review see Morano, 1992; Rembold, 1992; Stull et a/., 1991). The actomyosin filaments and their function per se appear to be unaltered in hypertension, although this subject has not been investigated in very great detail (see also Bohr and Webb, 1984; Domi- niczak and Bohr, 1990; Hermsmeyer, 1987).

RECEPTOR.MEDIATED CONTRACTION. Stimulation of specific receptors by several agonists like noradrenaline (NA) and serotonin leads to a G protein-regnlated activation of phospholipase C (PLC), which in turn results in the hydrolysis of phosphatidylinositol 4, 5-biphosphate 139 k . .

and rapid generation ofD-myoinositol 1,4,5-triphosphate (IP3) and diac- 2] ylglycerol. IP3 in turn promotes Ca z+ release from SR of smooth muscle ¢.O cells (Pijuan et al., 1993, Pozzan et al., 1994). Accumulation of inositol (1) phosphates in response to a-adrenoceptor stimulation has been reported ~._ to be enhanced in arteries from SHR (Brodde and Michel, 1992; Guild C)_ et al., 1992; Turla and Webb, 1990; Vila et al., 1993), and Kanagy and - O Webb (1994) have found enhanced arterial reactivity to the G-protein O activator mastoparan in SHRSP when compared with WKY. The above O reports support the hypothesis that increased responsiveness of G pro- teins leads to elevated PLC activity, which may contribute to the enhanced Ca z+ mobilization and elevated vascular responsiveness in experimental hypertension (Kanagy and Webb, 1994).

CELLULAR CALCIUM HANDLING. The [CaZ+]i in aduh hypertensive animals has been found to be abnormally high in blood cells (Arvola "et al., 1993a; Ishida-Kainouchi et al., 1993; Oshima et al., 1991; Wuorela et al., 1992), cultured aortic and mesenteric arterial smooth muscle cells (Bendhack et al., 1992; Osanian and Dunn, 1992; Sugiyama et al., 1990), freshly isolated aortic smooth muscle cells (Papageorgiou and Morgan, 1991), and intact aortas and renal arteries (Jelicks and Gupta, 1990; Sada et al., 1990; Spieker et al., 1986). Although basal whole cell [Ca2+]i was unaltered in neonatal SHR, abnormally high [Ca2+]i in response to a Ca 2+ channel activator was observed in cultured azygous venous cells from 3-day-old animals, in which blood pressure was still normal (Erne and Hermsmeyer, 1989). These findings suggest increased [Ca2÷]i in vascular smooth muscle cell in hypertension, which could increase arterial tone and total peripheral resistance. In contrast, some recent reports have suggested that in several vessels and in primary and first-passage-cultured myocytes from SHR and stroke-prone SHR (SHRSP) basal [CaZ+]i is similar to that of WKY (Bian and Bukoski, 1995; Bukoski, 1990; Bukoski et al., 1994; Liu et al., 1994; Neusser et al., 1994). Moreover, no differences were found in [Ca z+]i mobilized in response to high levels of NA or K ÷ in isolated mesenteric resistance artery smooth muscle cells between SHR and WKY (Bian and Bukoski, 1995; Bukoski et al., 1994). The explanation for these inconsistent results is not clear but could be related to the differences in experimental conditions, animal models and age, and vessel types among the investigations (Liu et al., 1994). As suggested by Bukoski et al., (1994), a potential reason to the elevated [Ca2+]~ in aortic smooth muscle cells could be the relatively high transmural pressure to which the aorta is subjected in vivo when compared with more distal resistance vessels like small branches of the mesenteric artery.

Voltage-dependent Ca z+ channels of smooth muscle have been di- vided by electrophysiological and pharmacological means into two different subgroups: one type is activated by small depolarizations and is inactivated quickly (T-type); whereas the other requires stronger depolarizations and inactivates more slowly (L-type) (Bean et a/., 1986; Spedding and Paoletti, 1992). Dihydropyridine CaZ+-channel antagonists selectively block the long-lasting Ca 2+ current (Nelson et a/., 1990). The relative proportion of these two currents was found to be different in smooth muscle of SHR and WKY: in 1-3-day-old WKY the contribution

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240

120

0 >- 01 01 01

I I I

*

01 I C/3 ©

FIGURE 3. Systolic blood pressures in untreated spontaneously hypertensive rats (SHR), atenolol-treated SHR (ASHR), quinapril- treated SHR (QSHR), trichlormethiazide-treated SHR (TSHR), and Wistar-Kyoto (WKY) rats. Symbols indicate means with SE means, n = 10-12 in each group; *P<0.05, A N O V A for repeated measurements .

of the transient current was greater, whereas in SHR the long-lasting current predominated (Rusch and Hermsmeyer, 1988). Smooth muscle cells isolated from cerebral arteries of SHRSP exhibited a higher Ca 2+ current density and increased maximal inward current, the T-type current having the same magnitude and current-voltage relation between these strains, giving an L-type/T-type ratio of 3.85 for WKY and 6.25 for SHRSP cells (Wilde et a/., 1994). Moreover, previous reports have shown that arterial smooth muscle contractions of SHR were more sensitive to the CaZ+-channel antagonist nifedipine than those of WKY (Arvola eta/., 1992, K~ih6nen eta/., 1994a; Lederballe-Pedersen eta/., 1978; P6rsti, 1992). In addition, agonists of Ca 2 ÷ channels induced a greater rise in contractile force and [Ca2+]i in femoral arteries and aortas from SHR than WKY (Aoki and Asano, 1986; Sada eta/ . , 1990). 4SCa2+ influx in carotid artery strips ofSHR (Asano et a/., 1993) and aortic segments of SHRSP (Kanagy et al., 1994) was also significantly higher when compared with WKY, and this increase in SHR was abolished by nifedipine. These results suggest enhanced Ca 2÷ entry via voltage-dependent channels in vascular smooth muscle membrane of SHR, which could partially account for altered calcium homeostasis and increased vascular reactivity, and thus contribute to the genesis of hypertension and vasospasm. However, Storm et al. (1992) reported that activation of voltage-dependent Ca z÷ channels and the subsequent rise in [Ca2+]i did not differ in small arterioles of SHR and WKY, suggesting that enhanced voltage-dependent Ca 2+ entry may not be present in very small vessels.

The passive Ca 2+ leak may be defined as Ca z+ influx under resting


conditions in the absence of stimulation by stretch, agonists, or depolarization. This Ca 2+ influx is insensitive to Ca 2+ antagonists that block excitable Ca 2+ channels (see van Breemen et a/., 1986). The intracellular environment has been suggested to be protected from Ca 2+ overload by a mechanism whereby an increase in extracellular Ca 2+ concentration makes the membrane less permeable not only to c a 2÷ but also to other ions (Dominiczak and Bohr, 1990). In genetic hypertension less calcium than normally seems to be bound to the plasma membrane, which could lead to a more rapid influx of Ca 2+ (Lamb et al., 1988), and thereby contribute to the enhanced activation of arterial contractile machinery and thus elevate peripheral resistance.

The most important intracellular Ca 2+ store participating in contractile responses is considered to be the SR (Ashida et al., 1988; Casteels eta/., 1992; Karaki and Weiss, 1988), which can actively sequester Ca 2+ (DeLong and Blasie, 1993; Martonosi et al., 1990) and release it following plasmalemmal receptor activation (Minneman, 1988). Three major factors have been suggested to remove Ca 2+ ions from the cytoplasm in rat mesenteric arterial smooth muscle: a caffeine- and NA-sensitive store of SR (43%); a caffeine-sensitive but NA-insensitive store of SR (36%); and the sarcolemmal CaZ+-ATPase (16%) (Barb and Eisner, 1995). Na+-Ca 2+ exchange has been proposed as an additional Ca z+ extrusion mechanism (Dominiczak and Bohr, 1990), but its function does not appear to be altered in SHR (Matlib et al., 1985; Zhu et al., 1994). Dohi et al. (1990) and Wuorela et al. (1994) have studied the ability of arterial smooth muscle to sequester Ca 2 ÷ into the intraceUular Ca 2+ stores by means of caffeine- and NA-induced contractions after loading periods in different Ca 2+ concentrations. The subsequent responses to both caffeine and NA were lower in SHR than WKY, suggesting decreased ability of the SR to take up and store Ca 2 ÷. Moreover, Kojima et al. (1991) demonstrated that arterial SR sequestered Ca z+ which entered from the extracellular space during 20-mmol/Ipotassium- induced contraction, and that SR continued to sequester Ca 2+ also during the relaxation phase. Both of these sequestration phases were diminished in SHR compared with WKY. This attenuation in the ability of SR to sequester Ca 2+ could lead to increased contractility and impaired relaxation of SHR arteries.

Active ATP-dependent Ca 2+ pumps (Ca2+-ATPase) in the SR and plasmalemma in vascular smooth muscle are responsible for the maintenance of Ca 2+ homeostasis and for the return of [Ca2+]~ to the resting level after contraction. Studies with Ca 2+ pump inhibitors have supported the hypothesis that dysfunction of Ca z÷-ATPase in the SR could contribute to the development of experimental genetic hypertension (see Kwan et al., 1994; Neusser e ta / . , 1994). During aging there is a change in the isoform of SR CaZ+-ATPase in SHR, which could impair the control of intracellular Ca 2+ (Le Jemtel et al., 1993). However, Kanagy eta/. (1994) have suggested that alterations in SR stores result from enhanced Ca 2+ influx across the sarcolemma rather than from impaired recycling of the cation by Ca2+-ATPase in SHRSP arteries. Taken together, the diminished ability of SR in arterial smooth muscle to sequester Ca 2+ in hypertension could participate in the elevation of peripheral vascular resistance.

OTHER ION TRANSPORT SYSTEMS. It has been proposed that hypertension is associated with, and possibly even caused by, increased Na ÷ content, Na + permeability, and altered Na + transport in vascular smooth muscle cells (Blaustein, 1977; Friedman, 1990; Friedman and Tanaka, 1987). Na+,K÷-ATPase plays an important role in the maintenance of transmembrane gradients for Na ÷ and K + in vascular smooth muscle cell by mediating the active extrusion of Na ÷ from the cell, coupled with pumping of K ÷ into the cell. The vascular Na ÷,K ÷ pump can be inhibited by cardiac glycosides, the prototype for this class of pharmacological compounds being ouabain (see O'Donnell and Owen, 1994).

Altered Na+,K÷-ATPase function has been proposed to play a role in the pathogenesis of both essential and experimental hypertension (see Hermsmeyer, 1987), but very controversial results on Na÷,K ÷- ATPase have been reported: The activity of Na ÷,K+-ATPase may be enhanced in hypertension because of an increase in passive membrane permeability to Na ÷. However, the pump does not appear to fully compensate for this, and subsequently the vascular tone is elevated. Alternatively, the activity of N a +,K ÷ -ATPase may be depressed in hypertension, either because of an inherent defect or a circulating ouabainlike inhibitor of the pump, leading to increased levels of intracellular Na ÷ and elevated vascular tone (see Blaustein, 1993). The reduced function of Na ÷,K ÷- ATPase in hypertensive animals (Chen and Lin-Shiau, 1986; Manjeet and Sim, 1987) would favor depolarization and subsequently enhance voltage-dependent Ca z+ entry in arteries, and possibly also decrease Ca z+ efflux via a reduced Na+-Ca 2+ exchange. The recent findings of Zhu et al. (1994) suggest that the increase in [Ca2+]i after Na+,K +- ATPase inhibition is not a result of modulated Na ÷-Ca z+ exchange, but merely secondary to enhanced Ca 2÷ influx through voltage-dependent channels in vascular smooth muscle cells of SHR.

However, Na ÷,K ÷ pump activity of cultured vascular smooth muscle cells has been found to be normal or even enhanced in SHR when compared with WKY (Orlov eta/., 1992, Rinaldi and Bohr, 1989; Tamura eta/., 1986). In contrast, potassium-induced vasodilation of human forearm vasculature was reported to be attenuated by salt loads (Fujita and Ito, 1993), and potassium relaxation rate of mesenteric arterial rings of SILK was clearly reduced, probably reflecting decreased activity of Na ÷,K +- ATPase (Arvola eta/., 1992; Ptrsti et al., 1992). Moreover, ouabain administration has been shown to induce the development of hypertension in the rat (Huang eta/ . , 1994; Yuan et al., 1993), possibly partially via an increase in sympathetic activity (Huang et a/., 1994). The above contradictory results on vascular Na ÷,K+-ATPase in hypertension possibly reflect the fact that the pump's function al, enzymatic and biochemical properties may be dissimilarly changed in hypertension (Young et al., 1988), and that the type and duration of hypertension profoundly affect these results. Whether the altered activity of the Na ÷,K + pump in vascular smooth muscle is a key factor in hypertension remains to be clarified (for review see O'Donnell and Owen, 1994).

Ca2+-activated K ÷ channels (Kca) in arterial smooth muscle serve to hyperpolarize the cell membrane, the action of which is triggered by increases in [Ca2+]i during contractions (Brayden and Nelson, 1992; Edwards and Weston, 1990). These channels probably serve as a negative feedback pathway to control the degree of membrane depolarization and the level of vasoconstriction (Brayden and Nelson, 1992). Carotid and aortic vascular segments of SHR showed dose-dependent contractions during blockade of l~a by tetraethylammonium, whereas WKY vascular segments did not (Rusch eta/., 1992). In addition, K~a was found to have fivefold higher open-state probability in aortic smooth muscle cells of SHR as compared with WKY (England eta/ . , 1993). K~a activity was found to be increased also in the aortic smooth muscle cells of SHRSP, probably as a result of an increased Ca z+ influx to the cells (Liu et al., 1994). Also, in the carotid artery of SHR, enhanced activity of l~a appeared to be secondary to increased Ca z+ influx via L-type voltage-dependent Ca 2+ channels (Asano et al., 1993). As suggested by England et al. (1993) and Rusch et al. (1992), the enhanced Ca 2+ sensitivity ofK ÷ channels in arterial smooth muscle probably represents a cellular compensatory mechanism to limit the arterial constriction in hypertension and to buffer the further increases in blood pressure. This view is supported by the findings ofLiu et al. (1994), which showed increased Ba 2+ influx (probably reflecting increased calcium influx) together with enhanced activity of K~a in aortic smooth muscle cells of SHRSP despite no detectable elevation in resting [Ca 2+]i. Thus, enhanced Ca z+ cycling across the sarcolemma possibly explains why the aforemen-

226 M. Kah6nen et al.

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FIGURE 4. Relaxations to acetylcholine (upper panel) and K + (lower panel) in isolated endothelium-intact mesenteric arterial rings from untreated spontaneously hypertensive rats (SHR), atenolol-treated SHR (ASHR), quinapril.treated SHR (QSHR), trichlormethiazide-treated SHR (TSHR), and Wistar-Kyoto (WKY) rats. The relaxations to acetylcholine were induced after precon. traction with 1 pM noradrenaline, and to K + after contractile response induced by K+-free buffer solution. Symbols indicate means with SE means, n = 10-12 in each group; *P<0.05, ANOVA for repeated measurements.

tioned increases in calcium currents of smooth muscle cell membranes do not necessarily always lead to elevated [CaZ+]i in hypertension.

CONCLUSIONS. Numerous studies have shown that vascular responsiveness is altered in hypertension. However, the observations in different studies are rather variable and contradictory results for most of the aspects of arterial smooth muscle function in hypertension have been reported. Several factors may contribute to this marked variability observed in vascular function: type of hypertension; development and duration of elevated blood pressure; species, age, and gender of the subject; region of the vasculature studied; experimental setting used to evaluate vascular changes; and the vasoactive stimulus employed. Nevertheless, enhanced vascular contractions and attenuated relaxations are likely to be related to abnormal mechanisms of Ca 2+ handling

and to deficient membrane functions in hypertension (see also Bohr and Webb, 1984; Dominiczak and Bohr, 1990; Hermsmeyer, 1987). Interestingly, Dominiczak et al: (1991) have found that membrane mi- croviscosity, measured with fluorescent polarization, was greater in the membranes isolated from smooth muscle cells of SHRSP as compared with those of WKY. This defect could be responsible for the multiple abnormalities of ion transport systems in hypertension, resulting in increased membrane permeability to K +, Na +, and Ca z+.

ANTIHYPERTENSIVE T R E A T M E N T AND ARTERIAL F U N C T I O N A n g i o t e n s i n . C o n v e r t i n g E n z y m e Inhibitors

The antihypertensive action of ACE inhibitors is primarily based on the inhibition of systemic and local angiotensin II (Ang II) formation. ACE is largely located on the luminal surface of the arterial endothelium (Caldwell et al., 1976). Moreover, the vascular wall appears to have a renin-angiotensin system of its own which ACE inhibitors direcdy inhibit (Levy et d. , 1990), even more effectively than ACE circulating in the bloodstream (Baudin and Dr0uet, 1989). However, part of the effects of ACE inhibitors on vasculature and blood pressure may be mediated via additional pathways which are not related to the inhibition of Ang II formation (Sunman and Sever, 1993), and an interference with the degradation of bradykinin has been suggested to play a role in the actions of ACE inhibitors (Bao et al., 1992). Nevertheless, since the blood pressure-lowering effect of treatment with the ACE inhibitor benazeprilat for 7 days, as well as its acute antihypertensive effect in adult SHR, were practically identical to those obtained with the Ang II receptor (AT1) antagonist losartan, the major mechanism whereby ACE inhibitors lower blood pressure in SHR is probably reduced formation of Ang II (Bunkenburg et al., 1991). While the beneficial effects of ACE inhibitors on the functional vascular properties may predomi- nate in their initial antihyper tensive actions, the delayed and prolonged changes in structural and mechanical properties of arteries such as reversal of hypertrophy, prevention of microvessel rarefaction, and increase in arterial wall compliance (Harrap et al., 1990; Lee et d. , 1991; Safar et al., 1992; Unger et al., 1992) have been suggested to be important determinants of their blood pressure-lowering effects during long-term treatment (Richer et al., 1991).

Long-term ACE inhibitor treatment has been shown to prevent the development of hypertension and cardiac and arterial wall hypertrophy in SHR (Lee et al., 1991; Richer et al., 1991), an effect which also persisted for several weeks after the treatment was withdrawn (Richer et al., 1991). In hypertensive rats, 1-year treatment with a non- antihypertensive dose oframipril was found to prevent cardiac hypertrophy and myocardial fibrosis, and this effect also was still present after 6 months following the withdrawal of the treatment (Linz et d. , 1992). Moreover, even 4-6-week-long therapy during the development of the hypertension induced long-term blood pressure reduction in SHR (Ad- ams et al., 1990; Harrap et al., 1990; Kost et al., 1995). In contrast, when the treatment was introduced in the established phase of hypertension, the vascular changes attained were transient and cardiac hypertrophy redeveloped if the therapy was withdrawn (Adams et al., 1990; Harrap et al., 1990). These treatment withdrawal studies suggest that long-term ACE inhibition indeed has profound effects on the morphology and function of the cardiovascular system in experimental hypertension.

EFFECTS ON RELAXATIONS AND ENDOTHELIAL FUNC~FION. Several reports have shown that ACE inhibition has beneficial effects on arterial dilator responses. Nine-month-long treatment of normotensive rats with a low dose of lisinopril, which had no effect on blood pressure, was


0

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I I I I I I I I I I I I I I I

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Acetylcholine (-IogM) Acetylcholine (-IogM) Acetylcholine (-IogM) with 10 nM bradykinin with 10 nM bradykinin with 10 nM bradykinin

FIGURE 5. Relaxations to acetylcholine in the absence (upper panels) and presence of 10 nM bradykinin (lower panels) in endothelium- intact mesenteric arterial rings from untreated spontaneously hypertensive rats (SHR, D), quinapril.treated SHR (i) , untreated Wistar- Kyoto rats (WKY, O), and quinapril-treated WKY rats (@). The relaxations were induced after precontraction with 1 pM noradrenaline in the absence (left panels) and presence (middle panels) of 0.1 mM N°-nitro-L.arginine methylester (L-NAME), and after precontraction with 60 mM KCI without L-NAME (fight panels). Symbols indicate means with SE means, n = 10-12 in each group; *P<0.05, ANOVA for repeated measurements.

found to improve aortic distensibility (Makki et al., 1994). Furthermore, long-term ACE-inhibitor therapy augmented endothelium-dependent relaxation to ACh in SHR (Arvola eta/., 1993a; Clozel et al., 1990; Gohlke et al., 1993a, 1993b; Rubanyi et al. 1993; Tschudi et al., 1994) (see also Fig. 5) and in normotensive animals (Bossaller 1992), and enhanced ACh-induced vasodilation in vivo in normotensive humans (Nakamura et al., 1992). In addition, cyclosporin A-induced endothelial dysfunction (reflected as impaired relaxation to ACh and calcium ionophore) was prevented by cotreatment with lisinopril or Ang II receptor antagonist (Auch-Schwelk et al., 1994). Interestingly, long-term ramipril administration in SHR, in doses that did not affect blood pressure, still enhanced relaxation to ACh and cGMP accumulation in the aorta. This increase in cGMP was due to bradykinin potentiating the action of ACE inhibition, since it was abolished by coadministration of the B~-kinin receptor antagonist Hoe 140 (Gohlke et al., 1993a, 1993b). In contrast, 2-month-long captopril and enalapril treatments failed to improve the abnormal endothelium-dependent forearm vasodilation to metacholine in hypertensive patients (Creager and Roddy, 1994).

Unlike the endothelium-dependent responses, the endothelium-independent relaxations to NO donors like SIN-1 or nitroprusside have usually been found to remain unaffected by long-term ACE inhibition (Clozel et al., 1990; K/ih6nen et al., 1995b; Novosel et al., 1994; Tschudi et al., 1994), although some reports have suggested enhanced endothelium-independent relaxations (Arvola et al., 1993a; Mervaala et al., 1994). Taken together, the above findings support the concept that long-term ACE inhibition favorably alters the function of the endothelium in hypertension.

We have studied the cardiovascular effects of quinapril, a nonsulfhy- dryl, nonpeptide ACE inhibitor in SHR. Quinapril is hydrolyzed in vivo to the active metabolite, quinaprilat, which induces prolonged ACE inhibition especially in vascular, heart, and kidney tissues (Fabris et al., 1990). In human subjects quinapril has been found to lower blood pressure without an increase in plasma catecholamines, reflex tachycardia, or cardiac output, and to decrease peripheral arterial resistance (Gupta et al., 1990). In SHR quinapril therapy markedly enhanced the endothelium-dependent relaxations to ACh and ADP, and the

228 M. K~ih6nen et al.

responses did not differ from those of normotensive WKY. Moreover, NO synthesis inhibition with L-NAME less effectively attenuated the endothelium-dependent relaxations in quinapril-treated SHR than 'in untreated SHR and WKY. Interestingly, when endothelium-dependent hyperpolarization was prevented by precontracting the preparations with KC1 (Adeagbo and Triggle, 1993; Feletou and Vanhoutte, 1988), no differences were found in relaxations to ACh and ADP between the above groups. These findings were also confirmed by the use of the K + channel blockers apamin and glibendamide. Since the relaxations to ACh and ADP in quinapril-treated SHR were augmented in the absence and presence of NO synthesis inhibition but not under conditions of prevented hyperpolarization, enhanced endothelium-dependent relaxation following the quinapril therapy could be attributed to promoted endothelium-dependent hyperpolarization after long-term ACE inhibition (K~h6nen et al., 1995b) (see Fig. 5).

The potent blood pressure-lowering action of ACE inhibitors per se appears unlikely to be the sole explanation for the recovery of impaired endothelial function in hypertension. Long-term enalapril treatment augmented aortic relaxations to ACh and ADP also in normotensive rats (Bossaler et al., 1992), in which blood pressure was affected very little (Frohlich and Horinaka, 1991; Yang et al., 1993). Furthermore, short-term ACE inhibitor administration enhanced forearm vasodilation elicited by ACh in vivo in both normotensive humans (Nakamura et al., 1992) and patients with essential hypertension (Hirooka et al., 1992). In addition, cilazaprilat treatment in vitro potentiated endothelium-dependent relaxations induced by bradykinin, ADP, and aggregat- ing platelets in isolated arteries from normotensive animals (Mombouli etal . , 1991). Therefore, ACE inhibitor therapy seems to affect endothelial function at least partially by specific mechanism(s) not entirely dependent on their effects on blood pressure.

The role of vasoactive kinins in the antihypertensive effects of ACE inhibitors has attracted great interest, since bradykinin is a potent endogenous endothelium-dependent vasodilator via the stimulation of the Bz-kinin receptor on the endothelium. As ACE is identical to the enzyme responsible for the inactivation of bradykinin (kininase U), a significant portion of the hypotensive effect of ACE inhibitors may be mediated via reduced degradation of bradykinin. Indeed, ACE inhibition in vitro has been found to diminish the breakdown of bradykinin liberated from the endothelial cells, which in turn stimulated the synthesis of PGI2 and NO in endothelial cells (Gr/ife et al., 1993; Linz et al., 1992; Wiemer et al., 1991) and decreased the secretion of endothelin-1 (Momose et al., 1993). The endothelium-dependent hyperpolarization of arteries induced by bradykinin was also potentiated by ACE inhibition in vitro (Illiano et al., 1994; Nakashima et al., 1993). In addition, ACE inhibitors enhanced the relaxations to bradykinin in human, bovine, and canine coronary arteries, an effect which was independent of changes in plasma bradykinin concentrations (Auch-Schwelk et al., 1992b) and involved augmented release of both NO and EDHF (Mom- bouli et al., 1992).

Interestingly, exogenous bradykinin has also been found to enhance the relaxations to ACh in mesenteric arterial rings of quinapril-treated SHR (K~ih6nen eta/., 1995b). In addition, the bradykinin-potentiated relaxations to ACh were effectively inhibited by L-NAME, and the responses to ACh in the presence ofbradykinin and L-NAME appeared very similar to the responses elicited without exogenous bradykinin under the same conditions (Fig. 5). Thus, this potentiation resulted from enhanced endothelium-derived NO release following the long- term ACE inhibition (K~h6nen et al., 1995b). These findings suggest that bradykinin might also promote vasodilation in vivo during ACE inhibitor therapy by potentiating the effects of other endothelium- mediated vasorelaxants. Furthermore, ACE inhibitors have recently

been suggested to potentiate the actions of bradykinin at the level of the B2-kinin receptor independently of the inhibition of ACE (Auch-

• Schwelk et al., 1993; Hecker et al., 1994b). According to the above results, such an interaction could also promote endothelial dilator responses to other agonists as well (K/ih6nen et al., 1995b).

In spite of the large number of in vitro studies, the contribution of bradykinin to the antihypertensive effects of ACE inhibitors still remains somewhat obscure. The hemodynamic responses to ACE inhibition and kinins express marked variability in different vascular regions, which makes interpretation of the role of bradykinin on the observed blood pressure changes and of the influence of ACE inhibitors there- upon difficult (Gardiner et al., 1993). However, kinin antagonist administration can attenuate the short-term blood pressure-lowering effect ofcaptopril and ramiprilat in SHR, suggesting that kinins do participate in the acute antihypertensive effects of ACE inhibitors (Cachofeiro et al., 1992). Inhibition of B2-kinin receptors was also found to enhance the pressor effect of infused Ang II, suggesting that endogenous kinins could buffer the chronic pressure effect of an excess of Ang II (Madeddu et al., 1994). Chronic blockade of Bz-kinin receptors by subcutaneous infusion of the antagonist Hoe 140 attenuated the antihypertensive effect of ramipril in the two-kidney/one-clip hypertensive Wistar rat but not in SHR (Bao et al., 1992). Interestingly, long-term reduction of blood pressure in SHR by ACE inhibition may partially depend on the actions of bradykinin, since withdrawal of the therapy in young SHR treated with ramipril plus B2-kinirt receptor antagonist resulted in a more pronounced increase in blood pressure than in those treated with ramipril alone (O'Sullivan and Harrap, 1995). However, in rats with coarctation-induced hypertension and in SHRSP, kinins did not participate in the chronic antihypertensive and antihypertrophic effects of ramipril (Gohlke et al., 1994; Rhaleb et al., 1994). Thus, the question whether or not the endogenous kinins significantly contribute to the antihypertensive and cytoprotective actions of ACE inhibitors in hypertension remains to be studied.

Another possible mechanism whereby ACE inhibition could enhance endothelium-dependent dilations is increased accumulation of the angiotensin I metabolite, angiotensin-(1-7) (Yamamoto et al., 1992), which has been reported to dilate isolated porcine coronary arteries via the release of NO from the endothelium (P6rsti et al., 1994). Moreover, sulfhydryl-group-containing ACE inhibitors may possess an additional endothelium-mediated component of vasodilation by virtue of their ability to scavenge oxygen radicals, thereby protecting endogenous NO as well as enhancing the effects of exogenous nitrovasodilators (van Gilst et al., 1991; Goldschmidt and Tallarida, 1991).

The antihypertensive effect of quinapril therapy in SHR has been found to be associated with augmentation of potassium relaxation rate (Arvola et al., 1993b), probably reflecting improved function ofNa ÷,K ÷ - ATPase in smooth muscle (see Fig. 4). In addition, nonpharmacological lowering of blood pressure has been reported to reduce plasma- digitalislike immunoreactivity (DIR) in rats with salt-induced hypertension (Doris, 1988), and to lower plasma ouabainlike activity (OLA) in SHR (Doris, 1994), suggesting a reduction in circulating Na ÷,K ÷ pump inhibitor concentration in these animals. Recently, we found that long- term quinapril treatment in parallel reduced DIR, enhanced arterial potassium relaxation (probably reflecting enhanced function of Na ÷,K ÷-ATPase), and normalized plasma Na ÷ :K + ratio in SHR (K/ih6- nen et al., 1995c). Thus, the reduction in plasma sodium pump inhibitor provides a possible link to explain the augmented potassium relaxation in quinapril-treated SHR. However, the mechanism by which plasma DIR is reduced after ACE inhibition in SHR is not clear. Nevertheless, the potent vasoconstrictor Ang II promotes the release of the mineralo- corticoid aldosterone, which in turn causes sodium and volume retention (Frohlich, 1989), whereas ACE inhibitors enhance sodium excre-


T A B L E 1. Effect of long . te rm angio tens in-conver t ing enzyme inhibitor treatment on arterial function in experimental animals

Exper imenta l Treatment Decrease in Effect on Reference Animal Length BP (mmHg) Arterial Function

Arvola eta/. (1993) SHR 15 weeks 90

Bossaller et al. (1992) Wistar rat 6 weeks Not measured

Clozel et al. (1990) SHR 20 weeks 86 Clozel et a/. (1991) SHR 4 months 70 Gohike eta/. (1993) SHR 20 weeks 65

K/~h6nen et al. (1995b, 1995d) SHR 10 weeks 105

Lee et al. (1991) SHR 28 weeks 75 Major eta/. (1993) SHR 1 week 31

Makki et al. (1994) Wistar rat 9 months 29 Novosel et al. (1994) SHRSP 8 weeks 47

Rubanyi eta/. (1993) SHR 2 weeks 35

Sada et al. (1989) SHR 20 weeks 66 Sada et al. (1990) SHR 8 weeks 81 Traub and Webb (1993) SHRSP 4 weeks 49

Tschudi et al. (1994) SHR 8 weeks 23

Weiss et al. (1993) SHR 5 weeks 70 Yang et al. (1993) SHRSP 3-6 months 95

Relaxations to ACh, sodium nitrite, and isoprenaline t; contractions to NA and KCI # (Mes) Relaxations to ACh and ADP t, contractions to PE and

Ang II "~ (Aor) Relaxation to ACh t, contraction to 5-HT ~ (Aor) Maximal arterial flow t (Cot) Relaxation to ACh, cGMP content t, contraciton to NA

(Aor) Relaxations to ACh, ADP, and Bk t; contractions to NA

and 5-HT ~ (Mes) Contraction to NA ~ (Mes) Contractions to PE and 5-HT 4, contractions to KCI and

phorbol ester ~" (Mes) Distensibility t (Aor) Relaxation to ACh t, and to SNP "-~ Contraction to NA and ET-I with and without L-NAME

"-" (Aor) Relaxations to ACh and ADP t, contractions to 5-HT and

PGF2~, and relaxation to SNP ~" (Aor)

Contractions to KC1 and Ca2+-channel activator # (Aor) Contractions to Ca 2+ and Ca2+-channel activator # (Aor) Contractile force development to Ca2+-channel activator

(Car) and caffeine (Aor) Relaxation to ACh t, SIN-1 "% contraction to 5-HT ~"

(Cot) Contraction to NA 4, and to Ang I "~ (Car) Dilation to Bk, A23187, NTG and adenosine (Cer) t

SHR, spontaneously hypertensive rats; SHRSP, stroke-prone SHR; Aor, aorta; Car, carotid artery; Cer, cerebral artery; Cor, coronary artery; Mes, mesenteric artery; ACh, acetylcholine; NA, noradrenaline; PE, phenylephrine; Ang I and II, angiotensin I and II, respectively; 5-HT, serotonin; SNP, sodium nitroprusside; PGF2,, prostaglandin Fz,; NTG, nitroglycerine; Bk, bradykinin; ET-1, endothelin-1; L-NAME, N°-monomethyl-L-arginine; t, increase; 4, decrease; ~', no change.

tion via inhibition of the renin-aldosterone loop. Since sodium retention and volume expansion increase plasma ouabainlike activity (Doris, 1988; Fujita and Ito, 1993; Keane et al. , 1991), the alteration in sodium balance by quinapril may partially explain the marked reduction in DIR in the treated SHR. Moreover, since plasma DIR was practically abolished from the plasma of SHR after quinapril therapy, ACE inhibit ion may have some specific effects on the regulation of plasma DIR concentration in SHR.

E ~ ON ARTERIAL CONTRACTIONS AND SMOOTH MUSCLE FUNCTION. Since ACE inhibitors are effective antihypertensive agents, their influences on the vascular function could be secondary to the reduced blood pressure. However, while the normalization of blood pressure in SHR by ACE inhibitor therapy clearly reduced aortic contractions to high concentrations of K ÷ (Sada eta/., 1989), chronic regional normotension in SHR femoral arteries obtained by a partial ligature in iliac artery had no effect on the sensitivity to K + when compared with the unprotected side (Field and Soltis, 1985). On the other hand, attenuated contractile responses to NA were observed in the mesenteric vasculature after treatment with captopril as well as in femoral arteries sheltered from high pressure stress in SHR (Field and Soltis, 1985; L e e et al. , 1991). However, even increased sensitivity to NA has been reported in femoral arteries made normotensive by ligature of otherwise hypertensive SHR (Bund et al., 1991). Furthermore, an effective antihypertensive dose ofquinapril for 7 days reduced vasoconstrictor responses to phenylephrine in SHR, while the same dose does not influence the responses in normotensive rats, and the administra-

tion of a subantihypertensive dose was without effect in SHR (Major et al., 1993). Thus, the ACE inhibit ion-induced blood pressure reduction per se may significantly contribute to the attenuated vasoconstrictor responses.

Several studies have reported that ACE inhibition attenuates vascular contractions to NA in experimental hypertension (Arvola et al., 1993a; Clozel et al. , 1990; Hoshino et al. , 1994; Lee et al., 1991; Sunano et al. , 1992), and reduces pressor responses to infused NA in man (Malini eta/., 1990; Uehlinger et al., 1989). The mechanism for the attenuated constrictions has been suggested to be reduced vascular wall hypertrophy with lower smooth muscle mass following ACE inhibition (Freslon and Giudicelli, 1983; Lee et al. , 1991). However, recent results of Arvola et a/. (1993) do not support this view, since medial thickness was only slightly reduced while arterial contractions to NA were markedly attenuated after quinapril therapy. Moreover, reduced smooth muscle mass is not equivalent to smaller contractions, since normotensive WKY with clearly lower medial thickness showed more pronounced responses than SHR (Arvola et al. , 1993a). Furthermore, even long-term low-dose treatment of SHR with ramipril, which did not affect blood pressure or vascular hypertrophy, was capable of decreasing vasoconstrictor responses to N A (Gohlke et al., 1993a, 1993b).

Both decreased release of endogenous NA from adrenergic nerves and reduced effect of catecholamines on arterial smooth muscle might contribute to the attenuated vasoconstrictor responses after ACE inhibition. Enalapril treatment was found to reduce tissue N A fractional rate constant (calculated from N A concentrations) in forelimb skeletal muscle of SHR, suggesting reduced vascular sympathetic nerve activity

230 M. K~h6nen et al.

TABLE 2. Effects of long-term antihypertensive therapies on arterial function in experimental animals

Experimental Therapeut ic Trea tment Effect on Reference Animal Agent Length Arterial Funct ion

Benetos et al. (1995) Dahl Chillon et al. (1992) SHR Clozel et al. (1990) SHR Clozel et al. (1991) SHR

SHR Godfraint et al. (1991) K/ih6nen et al. (1994b) SHR

K~ih6nen et al. (1995b) SHR

Levy et al. (1994) SHR Major et al. (1993) SHR Novosel et al. (1994) SHRSP

Rizzoni et al. (1994) SHR Traub and Webb (1993) SHRSP

Tschudi et al. (1994) SHR

rat Indapamide 5 weeks Thiazide Hydralazine 20 weeks Hydralazine 4 months Nisoldipine 28 weeks

Atenolol 10 weeks

Thiazide 10 weeks

Isradipine 12 weeks Hydralazine 1 week Verapamil 8 weeks

Nitrendipine 4-8 weeks Hydralazine, 4 weeks

thiazide Valsartan, 8 weeks nifedipine

Carotid arterial compliance t Aortic distensibility t Relaxation toACh "~, contraction to 5-HT ~" Maximal coronary arterial flow "~ Postcontraction tone 4, relaxation after KCI

depolarization f Ca z+ sequestration ability f, relaxation to ACh t,

contractions to NA and KC1 ~" Relaxations to ACh f, contractions to NA

and KCI ~ ' , Ca z+ sequestration ability t Total peripheral resistance 4, arterial compliance t Responses to PE, KCI, and phorbol ester ~" Relaxation to ACh t; contractions to ACh, NA, and

ET-1 "% relaxation to SNP "~" Relaxation to ACh t, contraction to NA Contractile force development to CaZ+.channel

activator thiazide and caffeine ~" Relaxation to ACh t, relaxation to SIN-l, and

contraction to 5-HT "

SHR, spontaneously hypertensive rats; SHRSP, stroke-prone SHR; 5-HT, serotonin; ACh, acetylcholine; ET-1, endothelin-1; NA, noradrenaline; PE, phenylephrine; SNP, nitroprusside; t, increase; 4, decrease; "-~, no change.

in vivo (Adams et al., 1990). The centrally acting sympatholytic drug, clonidine, has been found to partially reverse the Ang H-induced hypertension in rats, suggesting that circulating Ang II increases arterial pressure also via the sympathetic nervous system (Gorbea-Oppliger and Fink, 1994). Both the ACE inhibitor, perindoprilat, and the Ang II receptor antagonist, losartan, have been found to attenuate phenylephrine-induced vasoconstriction in vivo in the mesenteric artery of SHR and WKY, probably reflecting the role of endogenous Ang II in the potentiation of adrenergic responses (Qiu et al., 1994b). Moreover, the facilitation of sympathetic transmission by Ang II appeared to be greater in isolated mesenteric arteries from renovascular hypertensive rats than in normotensive controls (Faria and Salgado, 1992). Thus, the decrease in sympathetic neural activity after ACE inhibition probably results from the blockade of Ang II formation, since Ang II is well known to facilitate noradrenergic neurotransmission (Hilgers et al., 1993).

The vasoconstrictor effect of NA on arterial smooth muscle might be diminished because of suppressed Ang II generation during ACE inhibition, since Ang II has been suggested to amplify NA-induced contractions via activation of PKC, which increases the sensitivity of the contractile apparatus to Ca 2 + (Henrion et al., 1992). Vasoconstriction to serotonin has also been shown to be markedly diminished in SHR by ACE inhibition (Clozel et al., 1990; K~ih6nen et al., 1995b; Major et al., 1993). Interestingly, arterial contractile responses to phenylephrine and serotonin were reduced by treatment with lisinopril, but not by the Ang II receptor antagonist D8731 in normotensive rats (Auch- Schwelk et al., 1994). Thus, ACE inhibition appeared to reduce receptor- mediated vasoconstriction.

Ang II has been reported to stimulate the endothelial production of endothelin in situ and thereby potentiate contractions to NA in mesenteric arteries of SHR but not in WKY (Dohi et al., 1992a). Thus, attenuation of contractions to NA by ACE inhibitors in SHR could also result from reduction of Ang Ii and, subsequently, endothelin formation. Whether this mechanism is involved in the reduction of arterial constrictor responses remains to be studied.

Serotonin activates serotonergic receptors and NA also activates oh-receptors in the endothelium, and activation of ct2-receptors and

serotonergic receptors in endothelium release EDRF which decrease vasoconstriction induced by these agonists (Cocks and Angus, 1983; Lfischer and Vanhoutte, 1986). In SHR serotonin and NA also induce the release of EDCF from the endothelium (Auch-Schwelk and Van- houtte, 1991; Li and Bukoski, 1993). Long-term cilazapril treatment, but not short-term administration, was found to reduce the abnormal endothelium-dependent component of serotonin-induced contractions in SHR aorta, which was due to the release of PGH2 from the endothelium (Clozel et al., 1990; Clozel, 1991). However, the serotonin-induced endothelial PGH2 generation persisted in cilazapril-treated SHR (Clozel, 1991; Rubanyi et al., 1993). Since the positive effect of long-term cilazapril therapy on these contractions was completely reversed in vitro by inhibiting the action of NO with methylene blue, ACE inhibition was suggested to improve endothelial vasodilator function in SHR by increasing the release of NO from the endothelium (Clozel et al., 1990; Clozel, 1991). The increased release of endothelial NO after long-term ACE inhibition may in turn lead to more effective inactivation of endothelium-derived contracting factors (Auch-Schwelk et al., 1992a). We have found that NO release from the endothelium more effectively counteracted vasoconstriction induced by serotonin in quinapril- treated than in untreated SHR, as evaluated by the modulatory influence of NO synthase inhibition on the response (K~ih6nen et al., 1995d). However, in these animals, indomethacin had only minor effects on arterial contractions, suggesting that products of the cyclooxygenase pathway did not significantly modulate the responses. Taken together, the above findings indicate that ACE inhibitor therapy can favorably alter the effect of endothelium on vasoconstriction in SHR, with the modulatory role of NO on the responses being enhanced.

Seven-day administration of quinapril has not been found to affect depolarization-mediated contractile responses to KC1 in SHR (Major eta/., 1993). In contrast, 20-week treatment with the ACE inhibitor, CS-622 (Sada et al., 1989a), and 15-week therapy with quinapril (Arvola et al., 1993a) reduced contractile force generation to KCI in SHR. A very likely explanation for these diverse findings is that a 7-day-long treatment may not be long enough to alter vascular reactivity to KCI. However, the constrictor response to Ang II in the aorta of SHR has been reported to remain unaffected by long-term treatment with several


ACE inhibitors (Gohlke et al., 1993a, 1993b). Moreover, we recently studied contractions to KCI after elimination of the profound modulatory influences of the endothelium and neurotransmitter release from vascular adrenergic nerve endings, and found no differences between the responses of untreated and quinapril-treated SHR (K~ih6nen eta/., 1995c). Thus, the capacity for contractile force production by arterial smooth muscle appears to remain intact after ACE inhibition.

Long-term inhibition of ACE has been reported to suppress Ca 2+- channel agonist-induced contraction (Sada et al., 1989a; Traub and Webb, 1993) and decrease CaZ+-dependent tone of aorta in SHR (Sada et al., 1989b). Subsequently, ACE inhibition possibly decreased arterial tone in hypertension by reducing [Ca2÷]~ in smooth muscle (Sada et al., 1989a; Sada et al., 1990). This effect is probably associated with normalization of the function of voltage-dependent Ca 2+ channels, since Ca z+ channel antagonists have been found to more effectively inhibit contractions in untreated SHR than in SHR treated with ACE inhibitor and normotensive controls (Arvola et al., 1993a; Sada et al., 1990). The mechanism by which ACE inhibition affects Ca 2÷ channel function is not clear, but could be attributed to the normalization of blood pressure and cell membrane function (Dominiczak and Bohr, 1990) and to the withdrawal of the activation of voltage-sensitive channels by Ang II (Hausdorff and Catt 1988). Moreover, ACE inhibitors have been shown to directly reduce the cell membrane Ca z+ currents in vitro, such possible nonangiotensin effects by captopril being attributed to a direct blockade of voltage-dependent L-type Ca 2+ channels (see Sunman and Sever, 1993). Taken together, ACE inhibition seems to profoundly affect arterial cell membrane function in SHR, leading to normalization of cellular Ca 2+ handling. These favorable effects on [Ca2+]i in smooth muscle may take part in the long-term antihypertensive action of ACE inhibitors.

Rat arterial muscle cells show elevated Ca2÷-dependent K + efflux during the established phase of hypertension (see above). Interestingly, Rusch and Runnels (1994) found that K ÷ current density in membranes of aortic smooth muscle was reduced in SHR after ramipril therapy. Moreover, aortic segments from untreated SHR, but not those from WKY or ramipril-treated SHR, constricted strongly after blockade of Ca2÷-dependent K + channels by tetraethylammonium. The authors concluded that Ca2+-dependent K ÷ current density in arterial smooth muscle membranes showed a positive correlation with long-term arterial blood pressure level.

p . a d r e n o c e p t o r b l o c k e r s

While the beneficial influences of ACE inhibition on arterial function in hypertension have attracted great interest, the investigations concerning the vascular effects of other antihypertensive drugs, like I~-adrenoceptor blockers and diuretics, remain surprisingly few. Although [3-adrenoceptor blockers have been widely used in the treatment of hypertension for decades, the mechanisms of the long-term antihypertensive action of these drugs are not clear, but reductions in adrenergic activity, cardiac output, and renin release have been proposed to underlie the lowering of blood pressure (see Man In't Veld et al., 1986; Meiracker et al., 1989). Since different ~-adrenoceptor blockers with quite dissimilar effects on these variables have comparable effects on blood pressure, Man In't Veld et al. (1986) have suggested that the reductions observed in these factors are not essential for the antihypertensive action. The acute hemodynamic effect oflS-adrenoceptor antagonism is a fall in cardiac output (Lysbo Svendsen et al., 1979), and because of baroreflex-mediated compensatory mechanisms, total peripheral resistance initially rises (Lund-Johansen, 1979). The long-term effect, however, is a reduction of arterial resistance associated with return of cardiac output toward baseline values. Therefore, it appears

that 15-blockers lower blood pressure by influencing the regulation of vascular tone, which leads to vasodilation in the resistance vasculature (Man In't Veld, 1991).

The mechanism of the vasodilation during I~-adrenoceptor antagonism is not fully understood. Interference with vasoconstrictor nerve activity through blockade of prejunctional lS-adrenoceptors with subsequent inhibition of NA release could lead diminished arterial contractility, and thus explain the antihypertensive effect (Meiracker et at., 1989). In addition, endothelial influences may play a role, since arterial preparations ofnormotensive animals have been shown to relax in the presence of I~-blockers if endothelium is left intact (Gao et al., 1991; Janczewski et al., 1987; Mostaghim et al., 1986). Moreover, treatment with atenolol has been reported to stimulate vasodepressor prostaglandin generation in the kidney and aorta of hypertensive rats (Hirawa et al., 1991). Some negative findings concerning the effects of 15-blockers on arterial function have also been reported: lisinopril but not metoprolol therapy improved aortic distensibility in hypertensive humans (Barenbrock et al., 1994). Moreover, Schiffrin eta/. (1994) showed that 1-year-long treatment with cilazapril, but not with atenolol, regressed the media- lumen ratio and shifted the responses to several vasoconstrictor agents toward normal in subcutaneous resistance arteries of hypertensive patients.

We have recently found that the moderate antihypertensive effect ofatenololtherapy was accompanied by normalization of the relaxations to ACh in SHR (K/ih6nen et al., 1994b). Moreover, the recontractions after maximal dilations to ACh, which are considered to reflect EDCF release (LCischer and Vanhoutte, 1986; Jameson et al., 1993), were absent in atenolol-treated SHR. Thus, atenolol had favorable influences on endothelial function in SHR, the effect of which could be attributed to enhanced production of dilatory autacoids or reduced contractile factor release from the endothelium. Moreover, arterial contractile responses and Ca 2÷ sensitivity were not affected, whereas the responses reflecting the ability of the SR in smooth muscle to take up and store Ca 2+ were improved after the atenolol therapy. Interestingly, potassium relaxation rate was also enhanced in SHR by atenolol, indicating promoted recovery rate of ionic gradients across the cell membrane, probably via improved function of vascular Na+,K+-ATPase. These findings indicate that atenolol therapy had clear beneficial effects on arterial function in SHR, even though the antihypertensive effect was only moderate (see Fig. 4).

Recently, Mehta et al. (1994) reported that NO synthase activity of peripheral blood neutrophils was increased and superoxide anion generation was decreased by 8-week-long treatment of hypertensive patients with celiprolol. Future studies will clarify whether the drug- induced alterations in NO synthesis in neutrophils reflect the changes in endothelial constitutive NO synthase, and whether this mechanism could explain some of the cardioprotective effects of [~-blockers (Mehta et al., 1994).

T h i a z i d e d i u r e t i c s

The mechanisms beyond the long-term antihypertensive action ofthia- zide diuretics have not been fully clarified. After short-term administration the reduction of arterial pressure by thiazides is due to decreased cardiac output resulting from the natriuretic and diuretic effect (Vil- larreal et al., 1962). Extracellular fluid and plasma volumes, however, are later restored to their initial levels and cardiac output is thus normalized (Gifford et al., 1961). Nevertheless, long-term thiazide treatment is accompanied by reduction of total peripheral resistance below pretreat- ment levels (Conway and Palmero, 1963; Shah et al., 1978; Villarreal etal. , 1962), suggesting that vasodilation in the vasculature could explain the lowering of blood pressure (van Brummelen et al. 1980).


It has been proposed that thiazides reduce peripheral arterial resistance by increasing the biosynthesis of PGI2 (Webster eta/ . , 1980). However, the antihypertensive efficacy of hydrochlorothiazide is not PGI2 dependent in humans (Gerber eta/., 1990), and trichlormethiazide has even been found to diminish PGI2 synthesis in vascular smooth muscle cells of SHR (Numabe et al., 1989). Indapamide, a diuretic with vascular actions, attenuated constrictor responses in isolated vascular preparations of the rat (Finch et al., 1977), facilitated endothelium-dependent relaxation and EDRF release in canine femoral arteries in vitro (Schini et al., 1990), and after long-term therapy prevented the disturbed mechanical properties (acting on both arterial structure and function) of the carotid artery of salt-sensitive Dahl rats (Benetos et al., 1995). In addition, contractile responses of guinea pig aorta in vitro to several agonists were attenuated in the presence of thiazides (Mironneau et al. 1981).

In hypertensive patients, 9-day administration of hydrochlorothiazide has been found to reduce pressor responses to infused NA (Malini et al., 1990). However, long-term treatment with hydrochlorothiazide failed to affect contractile responses to Ca 2+ channel agonist or caffeine in SHR (Traub and Webb, 1993). Moreover, thiazide therapy has been found to improve aortic distensibility in SHR (Chillon et al., 1992), and hydrochlorothiazide to directly relax human subcutaneous resistance arteries (Calder et a/., 1991). The hydrochlorothiazide-induced acute relaxation in blood vessels of the human, rat, and guinea-pig appears to involve Ca2+-activated K + channels (Calder et al., 1992, 1993, 1994). Thiazides have thus been reported to reduce peripheral arterial resistance in humans, and to alter the responsiveness in vitro of vascular preparations from several experimental animals after short-term administration. The long-term effects of diuretic treatment on arterial function, however, remain largely unknown.

In our investigations, trichlormethiazide treatment moderately augmented the relaxation to ACh in SHR, the response still remaining slightly impaired when compared with WKY. Furthermore, the ability to sequester Ca 2+ into cellular stores, and the function of Na+,K +- ATPase were augmented in vascular smooth muscle of trichlormethiazide-treated SHR (see Fig. 4). These results suggest that the long-term blood pressure-lowering action of trichlormethiazide in SHR is also accompanied by improved arterial relaxation and augmented endothelial function (K~ih6nen et al., 1995a).

O t h e r a n t i h y p e r t e n $ i v e d r u g s

The knowledge about the effects of other antihypertensive agents, like calcium channel antagonists, on arterial function also remains rather limited. Most reports have focused on the direct acute effects of drugs on vascular preparations in vitro. Nevertheless, diminished forearm arterial responsiveness to the NO synthesis inhibitor L-NMMA in newly diagnosed patients with essential hypertension returned to normal after treatment by both enalapril and the Ca 2+ channel blocker amlodipine (Lyons et al., 1994). In addition, 12-week-long felodipine treatment in humans was found to decrease pulse-wave velocity and to increase brachial arterial diameter and compliance, while hydrochlorothiazide was without effect on these variables, suggesting that felodipine had beneficial effects on the arterial system of hypertensive subjects (Asmar et al., 1993).

The impaired endothelium-dependent relaxations in mesenteric resistance vessels and coronary arteries from SHR have been reported to recover after prolonged antihypertensive treatment with nifedipine and Ang II blockade with CGP48369 or valsartan (Dohi et al., 1992b; Tschudi et al., 1994) as well as after treatment with the Ca 2+ channel blockers verapamil in SHRSP aorta (Novosel eta/., 1994) and nitrendipine in SHR mesenteric artery (Rizzoni et al., 1994). Enhanced contrac-

tile responses of the aorta of one-kidney/one-clip renal hypertensive rats to NA were normalized by treatments with either captopril or enalapril, but not with nicardipine, the findings of which were attributed to the effective restoration of endothelial function by ACE inhibition (Hoshino et al., 1994). In contrast, long-term treatment of SHR with nisoldipine at effective antihypertensive doses, which also alleviated cardiac and aortic hypertrophy, was found to decrease contractile force generation of arteries to KCI, improve vascular relaxation, and suppress the development ofpostcontraction tone characteristics of SHR arteries, suggesting normalization of the function of Ca 2+ channels in these animals (Godfraind et al., 1991). Long-term therapy with isradipine decreased peripheral resistance and aortic medial hypertrophy, and increased cardiac output, left ventricular contractility, and arterial compliance in SHR (Levy et al., 1994). Thus, most of the reports on long-term CaZ+-channel antagonism have suggested that this class of compounds has beneficial effects on the vasculature.

In contrast, therapy with the vasodilator hydralazine has been reported to be ineffective in correcting the vascular disturbances in experimental hypertension: 7-day-long administration was without effect on abnormal pressor responses to phenylephrine, KC1, or phorbol ester (Major et al., 1993); 20-week treatment failed to alter the impaired endothelium-dependent and -independent relaxations (Clozel et al., 1990); and 4-week therapy had no influence on Ca2÷-channel agonist- or caffeine-induced contractile force development in SHR (Traub and Webb, 1993). These negative results obtained with hydralazine suggest that blood pressure reduction per se does not always have a positive influence on the abnormalities of arterial and endothelial function in hypertension.

C o n c l u s i o n s

Several reports have shown that vascular constrictor responses to various agents are reduced by ACE inhibition (Arvola et a/., 1993a; Clozel et al., 1990; K/ih6nen et al., 1995d), while atenolol (K~ih6nen et al., 1994b), trichlormethiazide (K/ih6nen et al., 1995a), and hydralazine therapies (Clozel et al., 1990) were practically without effect on arterial contractions. Even though the knowledge about the effects ofantihyper- tensive drugs other than ACE inhibitors on vascular function is limited, the blood pressure-lowering effect of a compound per se does not necessarily explain all alterations in arterial responses in experimental hypertension, since the effects of different drugs on arterial responses vary considerably.

In our studies, atenolol (K~ih6nen et al., 1994b), quinapril (Arvola et al., 1993a; K~ih6nen et al., 1995b), and trichlormethiazide (K~ih6nen et al., 1995a) therapies, as well as nonpharmacological lowering of blood pressure with calcium supplementation (M/ikynen et al., 1995), promoted endothelium-dependent agonist-induced relaxations (see Figs. 4 and 5). However, Panza et al. (1993c) have reported that clinically effective antihypertensive therapy did not restore the impaired endothelium-dependent vascular relaxation of patients with essential hypertension, suggesting that normalization of blood pressure itself does not always restore endothelial function in humans. Since several different kinds of drugs were used in the study, it remained unclear whether a specific type of therapy might be able to reverse endothelial dysfunction in hypertensive patients (Panza et al., 1993c). Nevertheless, the fact that diverse antihypertensive agents can all improve endothelium- dependent arterial relaxations in SHR suggests that long-term blood pressure reduction may indeed be an important contributor to this effect. This is supported by the finding that long-term administration of Ca 2+ channel blocker, nitrendipine, in SHR improved mesenteric arterial relaxation to ACh and reduced constriction to NA as long as the reductions of blood pressure and media-lumen ratio were maintained by

Antihyper tensive Therapy and Arterial Funct ion 233

the therapy (Rizzoni et al., 1994). Moreover, Gohlke et al. (I 993a, 1993b) have reported tha t long-term low-dose adminis t ra t ion of the ACE inhibitors, perindopril, ramipril, and zabicipril, which did not affect blood pressure, were capable of enhanc ing the relaxation response to A C h and reducing the contractile response to N A in SHR. Taken together, o ther mechanisms in addit ion to lowered blood pressure seem to play a role in the long-term effects of A C E inhibi t ion on arterial and endothelial function. Nevertheless, whether augmenta t ion of vascular function by ant ihypertensive therapy in hypertension is caused by decreased blood pressure or by a mechanism distinct from it, enhanced endothel ium-mediated vasodilation probably plays an impor tan t role in the maintenance of reduced peripheral resistance and the prevent ion of cardiovascular diseases.

This work was supported by the Emil Aaltonen Foundation, the University of Tampere, the Kalle Kaihari Fund and Finnish Cultural Foundation (Pirkanmaa Fund).

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antihypertensive therapy and arterial function in experimental hypertension

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