Strategies in antihypertensive therapy: Implications of the kidney

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  • Strategies in Antihypertensive Therapy: Implications of the Kidney

    NORMAN K. HOLLENBERG, M.D., Ph.D.

    Boston, Massachusetts

    Because we so rarely know the cause of hypertension, antihyper- tensive therapy remains empiric. However, certain principles of treatment are emerging; one of these concerns the critical role of the kidney in antihypertensive therapy. Whether or not the kidney is primarily responsible for hypertension in a patient, it is the patients renal response to treatment that determines, to a major degree, an agents efficacy. Vasodilators have been a conceptually attractive approach to the treatment of high blood pressure, because they decrease total peripheral resistance, which is considered to be the mechanism responsible for this condition in most patients. Nonspe- cific vasodilators exert a series of actions on the kidney-including profound sodium retention and reactive renin release-that limits therapeutic response. For reasons that are not yet clear, but are apparently related to specific action on calcium entry into vascular smooth muscle, endocrine function, and renal hemodynamics, cal- cium channel blocking agents, such as nifedipine, have an advan- tage in the treatment of hypertension. They cause little or no sodium retention; thus, the addition of a diuretic agent is not required. In fact, there is evidence that sodium loading in certain patients may potentiate the antihypertensive efficacy of these drugs. The renin- angiotensin system seems to be activated to a somewhat lesser degree by calcium channel blocking agents than it is by nonspecific vasodilators; in addition, these agents interfere with the actions of angiotensin on aldosterone release. Moreover, their dilator action on the renal blood supply favors sodium excretion. Nifedipine either has no effect on the renal blood supply or induces an increase in renal blood flow and maintains glomerular filtration rate, both of which combine to support the ensuing natriuresis.

    Despite its empiric basis, our ability to treat high blood pressure effec- tively and safely must be considered one of the great successes of mod- ern day therapeutics. We do not know whether essential hypertension reflects several unrelated diseases or is a single process that has been modified in the individual patient by age, gender] race, inherited traits, such environmental factors as diet and stress, and the duration of the process. A skilled debater could defend either view, marshaling substan- tial evidence to substantiate his/her claim.

    Although we remain ignorant at this fundamental level, a number of relevant therapeutic principles have emerged. Among the more important is the role of the kidney as a determinant of the effectiveness of antihy- pertensive therapy-the subject of this article. In addressing this topic, it is necessary to review the functional abnormalities involving the renal

    From the Departments of Medicine and Radiology, Harvard Medical School and Brigham and Wom- ens Hospital, Boston, Massachusetts. Personal research cited in this article was supported by grants from the National Institutes of Health (HL- 14944, HL-07236, CA-32849, HL-05832, and RR- 00888) and the National Aeronautics and Space Administration (NSG-9078). Requests for reprints should be addressed to Dr. Norman K. Hollenberg, 75 Francis Street, Boston, Massachusetts 02115.

    December 15, 1986 The American Journal of Medicine Volume 81 (suppl 6A) 15

  • SYMPOSIUM ON CALCIUM CHANNEL BLOCKERS-HOLLENBERG

    blood supply in essential hypertension [l--7], the role of potentiated renal vascular response, abnormalities in the newer pharmacologic agents in drug therapy [2,6-l I], renal arteriogram are reversed (often completely) by vas- and both the nature and the extent of reactive responses odilators. that often limit the effectiveness of therapeutic agents [2,9, 12-141. EFFECTS OF ESTABLISHED AGENTS

    RENAL BLOOD SUPPLY

    Direct-acting arteriolar vasodilators have always provided an attractive approach to the treatment of hypertension, because the hemodynamic defect typically responsible for the elevated pressure is an increase in peripheral resist- ance mediated, at least in part, by vasoconstriction [15]. Nonspecific vasodilators, however, have been of limited use, because they activate compensatory systems [12]- e.g., they induce a prominent renal response that involves both sodium retention and increased renin release. This article focuses on some of the practical implications of our increased understanding of the kidneys role in sustaining hypertension, howsoever it is initiated [16].

    There has been considerable interest in the renal blood supply and its effects on the glomerular filtration rate, so- dium handling by the kidney, and renal renin release in hypertension. A reduction in renal blood flow due to renal artery stenosis represents the most common curable form of secondary hypertension, and is still believed by many investigators to contribute to the pathogenesis of essential hypertension in some patients. Whatever factors initiate the onset of hypertension in a patient, it is becoming clear that a renal response must be involved in order to sustain elevated blood pressure [16]. Perhaps most important, it is now evident that the effectiveness of antihypertensive therapy, regardless of the therapeutic agent employed, is largely determined by the renal response. Finally, the con- tinuing damage to the renal microvasculature in uncon- trolled severe hypertension leads to uremia, one of the major complications of this disorder. The sharp reduction in the frequency of occurrence of this complication repre- sents one of the triumphs of modern antihypertensive therapy.

    Renal perfusion is often reduced in patients with essen- tial hypertension. Recent estimates [4] suggest that about two thirds of these patients have an inappropriately re- duced renal blood flow. Multiple lines of evidence now suggest that a functional disturbance-active vasocon- striction-plays a role in the abnormality of renal perfu- sion and glomerular filtration rate in patients with essential hypertension. Renal perfusion varies from moment to moment much more in patients with essential hyperten- sion than it does in normal subjects, an abnormality that must be due to active vasoconstriction [4]. Renal vasomo- tion is also increased in essential hypertension [5]. More- over, renal blood flow increases more dramatically in pa- tients with essential hypertension than it does in normal subjects when nonspecific vasodilators, such as acetyl- choline, are administered into the renal artery [I]. With the

    To the extent that such a functional abnormality involving the renal blood supply plays a role in the etiology of hyper- tension, one might anticipate that individually titrated doses of an appropriate vasodilator would reverse these abnormalities and, thus, improve renal perfusion and fil- tration rate. In fact, the renal response to therapy is condi- tioned largely by the relative influence on the kidney of both the decrease in blood pressure and the direct and indirect effects of the therapeutic agent on the kidney.

    Restriction of sodium intake is the simplest available therapeutic approach to hypertension. However, this maneuver reduces renal blood flow and glomerular filtra- tion rate in animals and in humans. The evidence [3,17] that angiotensin-induced renal vasoconstriction, conse- quent to the reactive increase in renin release, entirely accounts for the renal vasoconstriction is unequivocal. There is now clear evidence [8] that when diuretics are employed as therapy, the reactive increase in plasma renin activity limits the decrease in blood pressure. On the basis of results of studies on the influence of restriction of sodium intake, it is reasonable to conclude that the reduc- tions in renal blood flow and filtration rate induced by a diuretic are also angiotensin-mediated.

    Since activation of the renin-angiotensin system plays such a central role in limiting the response to restriction of sodium intake and diuretics, it is reasonable to suspect that the addition of beta-adrenergic blocking agents- many of which block renin release (to the extent that it is mediated by beta-adrenergic receptors in the juxtaglo- merular apparatus)-would reverse the impact of restric- tion of sodium intake and diuretics on the kidney [I 21. Un- fortunately, propranolol, the most widely studied and uti- lized beta-adrenergic blocking agent, induces renal vaso- constriction directly, apparently through its action on alpha-adrenergic receptors in the kidney [I 81. A propran- olol-induced reduction in renal blood flow, with parallel reductions in glomerular filtration rate, sodium retention, and ability to handle sodium loads, has also been well documented [19] in humans. It was thought that this effect might be due to a decline in cardiac output induced by the negative inotropic and chronotropic actions of beta block- ers on the heart; however, the renal response occurs with doses too small to influence cardiac output [7]. Similar renal responses have been documented [6] for a wide variety of beta-adrenergic blocking agents, including ox- prenolol, pindolol, acebutolol, atenolol, and dichloroiso- proterenol.

    Renal vasoconstriction, however, is not an inevitable outcome of all beta-adrenergic blockade. Nadolol, a long- acting, hydrophilic beta-adrenergic blocking agent, in-

    16 December 15, 1986 The American Journal of Medicine Volume 81 (suppl 6A)

  • SYMPOSIUM ON CALCIUM CHANNEL BLOCKERS-HOLLENBERG

    duces a dose-related increase in renal blood flow at doses exceeding those at which it reduces heart rate and, thus, cardiac output in humans [7]. Further, nadolol does not induce antinatriuresis in the dog: indeed, it appears to in- crease sodium excretion (personal communication from K. Duchin).

    What is the effect of the nonspecific vasodilators on the kidney? This general class includes hydralazine, minoxi- dil, diazoxide, sodium nitroprusside, and trimethaphan, which includes a potent nonspecific vasodilator compo- nent [13,14]. Although these agents produce a broad spectrum of renal responses, certain effects are common to all of them. For example, despite their range of direct actions on the renal blood supply, all of these agents in- duce sodium retention [13,14]; this result often is striking, and can be the limiting factor for therapeutic efficacy 1201. The mechanism of this effect has not yet been completely delineated, but it is clear that the systemic response, es- pecially the drop in blood pressure, plays an important role in such retention. For example, diazoxide is a potent renal vasodilator when infused into the renal artery; this vasodilation is accompanied by a striking natriuresis [21]. When the agent is given intravenously, however, an equally striking antinatriuresis occurs-presumably be- cause of the decrease in blood pressure [6,17,22].

    angiotensin II. Although calcium channel blockers can blunt adrenal aldosterone release, this does not entirely explain the acute natriuresis seen after the administration of nifedipine (Figure 1) [30]. Several lines of evidence [I 7,31-371 suggest that intrarenal angiotensin II concen- trations may actually represent one determinant of renal- sodium handling, presumably through the control of tu- bular-glomerular feedback. One could speculate that the salutary response to calcium channel blockers may reflect their ability to reverse these intrarenal actions of angioten- sin II.

    The impact of systemically administered nonspecific vasodilators on the renal blood supply and renal sodium handling varies widely from patient to patient-from a net increase to a net decrease in renal blood flow. However, antinatriuresis with sodium retention and, typically, a de- crease in glomerular filtration rate occurs with even the most potent direct renal vasodilators in this class-i.e., hydralazine, diazoxide, and minoxidil [13,14,20,23,24]. Thus, to date, the use of a nonspecific vasodilator for its vasodilatory effect on the kidney has not spared patients the negative renal influence of these agents.

    Just as the beta blockers have different effects on the kidney, calcium channel blockers may also have a differ- ent impact. Although no direct comparisons of renal vas- cular responses have been made, data on potentially rele- vant systems do exist. These data indicate that differ- ences may be present-although their precise clinical rel- evance remains somewhat obscure. Zanchetti and Leonetti [28] recently compared the short-term natriuretic response to nifedipine with the response to verapamil. Dosages were adjusted to achieve identical decreases in blood pressure. Although normal subjects did not differ in their response to the two agents, the diuresis and natri- uresis induced by nifedipine in patients with essential hy- pertension were substantially greater than those induced by verapamil (Figure 2).

    EFFECTS OF NEW AGENTS

    The development of angiotensin converting enzyme inhibitors also represents a new approach to antihyper- tensive therapy and a new tool for examining the underly- ing mechanisms of hypertension [38]. These agents have proven to be effective even in severe cases of hyperten- sion, such as those resistant to standard triple-drug ther- apy with a diuretic, hydralazine, and a beta-adrenergic blocking agent [9]. Recent studies [2,6,39] suggest that agents in this class exert an especially beneficial action on the kidney. In patients with essential hypertension, the nonapeptide SQ 20881 induced a potentiated increase in renal blood flow [2] that exceeded by a factor of two the

    Three new classes of agents have been developed that may have special implications for the kidney: calcium channel blocking agents, converting enzyme inhibitors, and dopamine analogues. The first study [I I] demonstrat- ing a short-term renal response to a calcium channel blocking agent used nifedipine and revealed a substantial increase in renal plasma flow, a well-maintained glomeru- lar filtration rate, and a brisk diuresis and natriuresis. Pa- tients with the lowest baseline renal plasma flow and glo- merular filtration rate, presumably reflecting fixed organic renal vascular changes, showed little response. These salutary responses to nifedipine have been amply con- firmed, as have such responses to other agents in this class [25-291.

    In both human and animal studies, calcium channel blockers have been shown to act acutely as antagonists to the pressor, adrenal, and renal vascular responses to

    Figure 7. HelatlonstJlp between Increases In plasma anglo- tensin II levels and changes in plasma aldosterone levels during angiotensin II infusion in patients with mild essential hypertension. Horizontal and vertical bars represent mean k SEM. Reprinted with permission from PO].

    December 15, 1986 The American Journal of Medicine Volume 81 (suppl SA) 17

  • SYMPOSIUM ON CALCIUM CHANNEL BLOCKERS-HOLLENBERG

    Normotensives n = 10 Hypertensives n = 14

    170

    SBP 150 * (mm 40 p 1.005 Iso ** p

  • SYMPOSIUM ON CALCIUM CHANNEL BLOCKERS-HOLLENBERG

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