Kidney International, Vol. 46 (1994), pp. 1010—1018

Radiotelemetric BP monitoring, antihypertensives andglomeruloprotection in remnant kidney model

KAREN A. GRIFFIN, MARIA PIcIN, and ANIL K. BIDANI

Departments of Medicine and Pathology Loyola University Medical Center and Hines Veterans' Administration Hospital Maywood, illinois, USA

Radiotelemetric BP monitoring, antihypertensives and glomerulopro-tection in remnant kidney model. The mechanisms by which antihyper-tensives exert a glomeruloprotective effect in the remnant kidney modelremain controversial. Based on periodic tail-cuff BP measurements, thevariable glomeruloprotective efficacy of antihypertensive agents has beenascribed to mechanisms other than or in addition to their ability to lowerBP. To more precisely define the relationship between BP control andglomeruloprotection, systolic BP was continuously monitored radiotele-metrically at 10-minute intervals for —65 days in rats after —5/6 renalablation. Rats with remnant kidneys received either no therapy or one ofthree antihypertensive regimens in their drinking water after the firstweek: enalapril, a triple therapy regimen (reserpine, hydralazine, hydro-chlorothiazide); or a high dose triple therapy regimen. Although allantihypertensive regimens significantly lowered BP, considerable inter-animal variability was observed. Additionally, marked lability of BP waspresent in both untreated and treated rats. Glomerular injury in individualanimals (N = 34) was very strongly correlated with their overall averagedsystolic BP during the final eight weeks (r = 0.91) and with the frequencyof systolic BP readings >150 mm Hg (r = 0.89). These data do not provideevidence of a therapeutic advantage for any of the regimens independentof their antihypertensive effects but indicate that the glomeruloprotectiveefficacy of these antihypertensive regimens is directly proportional to theirantihypertensive efficiency.

The phenomenon of the progressive nature of human renaldisease has been extensively investigated using the rat remnantkidney (RK) model. This model produced by right nephrectomyand segmental infarction of two-thirds of the left kidney ischaracterized by adaptive increases in size and function of theremnant glomeruli [1—5]. With time these RK rats developproteinuria, hypertension and progressive glomerulosclerosis, thepathogenesis of which remains controversial. Although hyperten-sive mechanisms have been postulated to play a major pathoge-netic role [6—111, antihypertensive therapies have not been uni-formly glomeruloprotective despite adequate and equivalent BPcontrol as judged by periodic tail-cuff BP measurements [12—17].This has led to the concept that the glomeruloprotective efficacyof antihypertensive agents depends on mechanisms other than orin addition to their ability to lower systemic blood pressure. It hasbeen suggested that antihypertensives differ in their ability tolower intraglomerular pressure (PGC) despite comparable reduc-tions in systemic blood pressure and that antihypertensive regi-

Received for publication September 28, 1993and in revised form April 6, 1994Accepted for publication May 10, 1994

© 1994 by the International Society of Nephrology

mens that do not lower P0 are not glomeruloprotective [13].Others have suggested that glomeruloprotection with antihyper-tensive agents is independent of Gc but depends on their abilityto inhibit the glomerular hypertrophic response after renal massreduction [15]. Our recent studies using the radiotelemetrictechnique of continuous blood pressure monitoring [18] in thismodel have demonstrated marked spontaneous lability of bloodpressures and have suggested that the tail-cuff method may not beadequate to assess completely the blood pressure profiles in theseanimals [19]. The present studies were undertaken to examinemore critically the relationship of blood pressure control withdifferent antihypertensive regimens to glomerular injury in indi-vidual RK rats utilizing this radiotelemetric technique.

Methods

The studies were performed on male Sprague-Dawley rats(body wt 200 to 300 g) who were fed a standard 22% protein dietand synchronized to a 12:12 hour light (6:00 to 18:00 hours) anddark (18:00 to 6:00 hours) cycle. After baseline tail-cuff systolicblood pressure measurements (Apollo 179 Analyzer, IITC/LifeScience Instruments, California, USA) in unanesthetized re-strained rats the animals were anesthetized with sodium pento-barbital (45 mg/kg i.p.) and subjected to —5/6 renal ablation (rightnephrectomy and ligation of all but one posterior extrarenalbranch of the left renal artery) [7, 11]. At the time of surgery, ratswere prepared for telemetric monitoring of blood pressure (DataSciences, Inc., Minnesota, USA) [18, 19]. Each rat had a bloodpressure sensor (model TA1 1PA-C40) inserted intraperitoneally.The sensor's catheter was inserted into the aorta below the levelof the renal arteries, and the radio-frequency transmitter was fixedto the peritoneum. The rats were housed individually in plasticcages which were placed on top of the receiver. The signals fromthe pressure sensor were converted, temperature compensatedand sent via the radio-frequency transmitter to the telemetryreceiver. The receiver was connected to a BCM-100 consolidationmatrix which transmitted the information to the Dataquest IVacquisition system. Systolic blood pressure in each animal wascontinuously recorded at 10-minute intervals for the duration(—65 days) of the study. All rats received food and water adlibitum throughout the study.

In most rats, tail vein blood samples were obtained at three daysfor measurement of serum creatinine (Sr) as an index of thedegree of renal mass reduction. At —7 days, randomly selectedrats with —5/6 renal ablation received either no treatment (NT) orone of the following regimens in drinking water: enalapril (E) 50

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Griffin et al: Antihypertensives and glomeruloprotection 1011

mg/liter; triple therapy (TT) with reserpine 5 mg/liter, hydralazine80 mg/liter and hydrochlorothiazide 25 mg/liter [14]; or high dosetriple therapy (HDTF) with reserpine 10 mg/liter, hydralazine 200mg/liter, and hydrochlorothiazide 50 mg/liter [15]. The treatmentregimens were continued through the remainder of the animals'course. During the course of the study, one HDTT and two NTanimals died. The data for these animals were not included in thisanalysis. The data for an additional rat treated with the HD'lTregimen were excluded due to the discovery of obstructive hydro-nephrosis at the time of the clearance studies. The numbers ofanimals for each group available for final analysis were: NT (N =8); E (N = 8); iT (N = 12), and HDTT (N = 6). At the end ofthe study (—65 days), 24 hour urine collections for proteinexcretion were obtained. The rats were then anesthetized withintravenous sodium pentobarbital (40 mg/kg); a tracheostomy wasperformed using polyethylene (PE-200) tubing and they weresurgically prepared for clearance studies as previously described[7, 11]. A carotid artery was cannulated with PE-50 tubing andconnected to a Gould Windograf (Model 40-8474) for continuousrecording of mean systemic pressure. A femoral vein was cannu-lated with polyethylene tubing (PE-50) and a priming dose ofinulin in 150 mM NaC1 was administered, followed by a continuousmaintenance infusion of 150 mM NaCI containing inulin 0,055ml/min to maintain the plasma concentration at —50 mg/dl andfor replacement of surgical and ongoing fluid losses. The leftureter was then cannulated with polyethylene tubing for collectionof urine samples. A 1.0 mm Transonic R series flow probe wasplaced around the left renal artery for measurement of renalblood flow (RBF) by the Transonic flowmeter (Transonic Sys-tems, New York, USA), as previously described [20]. At theconclusion of the surgery, a 150 mM NaCl bolus equal to 1% ofbody weight was administered. Two 20-minute clearances ofinulin were obtained. Blood samples were obtained at the mid-point of each urine collection.

At the conclusion of these studies the rats were sacrificed andthe kidneys were harvested for morphological studies. A trans-verse section of the kidney through the papilla was fixed in 10%neutral buffered formalin and embedded in paraffin. Sectionswere cut at 4 tm thickness and stained with hematoxylin andeosin (H&E) and Periodic acid-Schiff (PAS) stains. Sections wereevaluated systematically in each kidney for glomerular injury(segmental sclerosis and/or necrosis) in a blinded fashion bystandard morphologic methods. At least 100 glomeruli in eachanimal were evaluated and the severity of glomerular injury wasexpressed as the percent of glomeruli exhibiting such injury.Additionally, semiquantitative glomerular injury scores for indi-vidual kidneys were also derived based on the percentage ofglomerular surface area involved [21]. A score of 1 was given for0 to 25%; 2 for 25 to 50%; 3 for 50 to 75%; and 4 for 75 to 100%glomerular surface area involvement with a maximum obtainablescore of 400.

Analyses, calculations, and statistics

Urinary protein was measured by the quantitative sulfosalicyclicacid method with human serum albumin serving as standard.Serum creatinine was measured using a Beckman creatinineanalyzer [7, 11]. Inulin in urine and plasma filtrates was deter-mined spectrophotometrically by the diphenylamine method aspreviously described [7, 11]. Glomerular filtration rate was calcu-lated using standard formulae.

0 10 20 30 40 50 60 70

Days post 5/6 renal ablation

Fig. 1. The course of systolic blood pressure after —5/6 renal ablation in thefour groups of rats. BP was radiotelemetrically recorded continuously at10-minute intervals for the duration of the study. The 24 hour averages ofBP at 5 day intervals are shown. After the first 7 days, the rats receivedeither no treatment (D, N 8) or one of the following regimens indrinking water: enalapril (,N = 8) 50 mg/liter; triple therapy (V, N12) with reserpine 5 mg/liter hydralazine 80 mg/liter and hydrochlorothi-azide 25 mg/liter; or high dose triple therapy (A, N = 6) with reserpine 10mg/liter, hydralazine 200 mg/liter and hydrochlorothiazide 50 mg/liter.

The results are expressed as mean SE. Statistical analysis wasperformed using analysis of variance (ANOVA) followed byStudent-Newman-Keuls test. Differences within a group wereanalyzed by the Student's paired t-test. A P value of <0.05 wasconsidered statistically significant.

ResultsFigure 1 illustrates the course of BP after ——5/6 renal ablation

and the effects of the different antihypertensive regimens over—65 days of the study. The 24 hour averages of systolic BP at fiveday intervals are plotted for convenience of illustration. All threetreatment regimens significantly reduced BP when compared tothe untreated group, but HDTT and E were more successful thanTi' in maintaining BP control. This is supported by the data inTable 1 which present the overall averages of systolic bloodpressures for the first 10 days and the final —eight weeks in eachof the study groups. The systolic blood pressure averages were notsignificantly different between the untreated and treated groupsduring the first 10 days except for the HDTT rats which weresignificantly higher (P < 0.05) than the other groups for unclearreasons. A significant antihypertensive effect was observed with allthree regimens as indicated by the lower overall SBP averagesduring the final —eight weeks as compared to their systolic BPduring the first 10 days (P < 0.001), paired t-test. In contrast, theaverage systolic BP during the final eight weeks in the untreatedgroup was not significantly different from that during the first 10days. Similarly, when the overall BP averages of the groups duringthe final —eight weeks are compared the E (119 5.3 mm Hg),HDTT (127 6.7 mm Hg) and TT (133 7.5 mm Hg) groupswere significantly lower (P < 0.05) compared to the untreated NT(158 9 mm Hg) group. Figure 2 A through D illustratesrepresentative 12 hour recordings of systolic blood pressure at10-minute intervals in a rat from each of the four experimental

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1012 Griffin et at: Antihypertensives and glomeruloprotection

Table 1. Blood pressure averages (mm Hg)

Final —8 weeks

groups. These figures graphically display the spontaneous variabil-ity and fluctuations in systolic blood pressure in this model. Thefluctuations in BP observed in the untreated control grouppersisted in the treated animals. This is also indicated by the lackof any significant differences in the standard deviations of theoverall BP averages in the four groups (Table 1). However, adifferential effect of the antihypertensive regimens on the diurnalrhythm was noted. The nighttime pressures were modestly butsignificantly higher in the untreated (NT) rats. This circadianrhythm was preserved in the E group, but was lost in the Ti' andHDTT groups.

Although Sr at three days was not obtained in all animals dueto technical difficulties, no significant differences were presentbetween the groups based on the measurements that were ob-tained in a majority of animals in each group suggesting compa-rable renal mass reduction (0.9 0.07 mg/dl in 6/8 NT; 0.850.05 mg/dl in 5/8 E; 0.89 0.05 mg/dl in 9/12 Ti'; and 0.94 0.07mg/dl in 5/6 HDTT). Table 2 presents the data for the final bodyweight, 24 hour urine protein excretion, renal blood flow (RBF),and GFR in the four groups. There were no significant differences

between body weights, RBF (mI/mm/kg), and GFR (mi/mm/kg)between the four groups. There was a trend to reduction inurinary protein excretion (mg/day) with blood pressure controlbut this reached statistical significance only in the E group (227.8 mg/24 hr) as compared to the untreated rats (66 10.6 mg/24hr; P < 0.05). Data for glomerular injury in the four groups arealso presented in Table 2. The glomerular injury is expressed bothas percent of glomeruli exhibiting injury as well as the calculatedglomerular injury score.

Figure 3 A through D illustrates the range of glomerularpathology observed in the present study and that was used tocalculate the glomerular injury scores in individual animals.Although there was a trend toward a decrease in the total percentof glomeruli exhibiting injury and in the glomerular injury score,paralleling the decrease in BP (glomerular injury was lowest in theE group and highest in the NT group, Table 2), the differences didnot reach statistical significance in group comparisons due to thescatter within the groups (ANOVA). However, when glomerularinjury was correlated with the parameters of telemetrically mon-itored systolic blood pressures in individual RK rats, there was an

First 10 days Overall Std Dev

NT (N = 8) 146 4.5 158.4 9.0 22.7 3.2 155.6 8.8 161.2 9.2cE (N = 8)Ti' (N = 12)HDTF (N = 6)

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Rats with remnant kidneys underwent continuous radiotelemetric monitoring of systolic BP at 10-minute intervals for 65 days. After the first 7 days,rats received either no treatment (NT) or one of three antihypertensive regimens in drinking water: enalapril (E), triple therapy (TT) regimen(reserpine, hydralazine and hydrochiorothiazide), or high dose triple therapy (HDTT) regimen. See text for dosages.

a P < 0.05 compared to NT, ANOVA; h <0.0005 compared to first 10 days; paired t-test: Cp <0.001 compared to daytime, paired t-test. Day time6 am. to 6 p.m.: nighttime 6 p.m. to 6 am.

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Fig. 2. illustrations of systolic blood pressurerecordings evety 10 minutes over 12 hours in a rat

___________________________ from each of the four groups: (a) untreated (NT);(b) enalapril (E); (c) triple therapy (TT) regimen(reseipine, hydralazine, hydrochiorothiazide); (d)high dose triple therapy (HDTT) regimen. See textfor details of dosage regimens.

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Fig. 3. An illustration of the range of glomerular pathology used to calculate the glomerular injuFy in the present study. (A) Less than 25% of glomerulartuft involvement with a segmental increase in mesangial matrix and collapse of the adjacent glomerular capillaries. There is also a small adhesion withBowman's capsule (Score 1) H&E x 625. (B) Between 25 to 50% of glomerular tuft involvement with segmental collapse of glomerular capillaries,increase in mesangial matrix and broad adhesions with Bowman's capsule (Score 2) H&E >< 625.

Fig. 3. Continued. (C) Fifty to 75% of tuft involvement with segmental sclerosis and hyalinosis (Score 3). PAS)< 625. (D) Grade IV: greater than 75%of the glomerular tuft is obliterated by sclerosis. There are also broad adhesions with Bowman's capsule (Score 4). PAS x 625.

Griffin et al: Antihypertensives and glomeruloprotection 1015

Table 2. Final functional and morphologic data

Body wtg

Proteinuriamg/24 hr

RBFmi/mm/kg

GFRmi/mm/kg

Glomerular iniur% Score

NT (N = 8) 508 18 66.4 10.6 18.9 1.9 2.0 0.3 47.8 9.6 99.3 36.9E (N = 8) 467 31 22.6 4.8 23.1 1.6 2.9 0.4 18.0 6.1 27.0 9.2YF (N = 12) 491 24 57.6 10.5 22.2 1.6 2.3 0.2 36.1 6.6 70.6 28.9HDTF (N = 6) 415 30 37.5 12.0 21.3 1.7 2.4 0.5 27.3 10.6 56.8 35.4

Rats with remnant kidneys underwent continuous radiotelemetric monitoring of systolic BP at 10-minute intervals for —65 days. After the first 7 days,rats received either no treatment (NT) or one of three antihypertensive regimens in drinking water: enalapril (E), triple therapy (TI') regimen(reserpine, hydralazine and hydrochlorothiazide), or high dose triple therapy (HDTF) regimen. See text for dosages.

P < 0.05 compared to NT, ANOVA

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Overall averaged systolic BP, mm Hg(Final 8 weeks)

Fig. 4. Con-elation of percentage of glomendi with injuly (focal and segmen-tal sclerosis and/or necrosis) in individual rats from all four groups with theiroverall averaged systolic BP for the final —8 weeks. Systolic BP wastelemetrically recorded every 10 minutes for the duration of the study.After the 7th day, the rats had received either no treatment (; N = 8) orone of these regimens in drinking water: enalapril (U, N = 8); tripletherapy (A, N = 12); or high dose triple therapy (A, N = 6). See text fordetails of dosage regimens. r = 0.91.

Overall averaged systolic BP, mm Hg(Final 8 weeks)

Fig. 5. Con-elation of the glomerular injury score derived from the percentarea of the giomendus exhibiting injury by the method of Raij et a! [20] inindividual rats with their overall averaged systolic BPfor the final —8 weeks.Systolic BP was telemetrically recorded every 10 minutes for the durationof the study. After the 7th day, the rats had received either no treatment(0, N 8) or one of these regimens in drinking water: enalapril (U, N =8); triple therapy (A, N = 12); or high dose triple therapy (A, N = 6). Seetext for details of dosage regimens. r = 0.84.

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excellent correlation of percent glomerular injury and overallmean systolic blood pressure during the final —eight weeks inindividual rats (r = 0.91, P < 0.0001; Fig. 4). Similar excellentcorrelations between these two parameters were observed withineach group: NT (N = 8),r = 0.96, P < 0.0002; E (N 8), r = 0.94,P < 0.0005; TF (N = 12), r = 0.93, P < 0.0001; and HDTT (N =6), r = 0.88, P < 0.02. The slopes of the relationship betweenoverall averaged systolic BP for the final eight weeks and glomer-ular injury in the four groups were not significantly different fromeach other, NT, 1.03 0.13; E, 1.09 0,16; TT, 0.82 0.10 andHDTT, 1.41 0.37, or from the slope for all animals combined(0.88 0.07, Fig. 4). When glomerular injury was expressed in theform of calculated glomerular injury scores based on the percentglomerular surface area exhibiting injury, a similar strong corre-lation was observed with the overall averaged systolic bloodpressure for the final eight weeks (r =0.84,P < 0.0001; Fig. 5). Asimilar excellent correlation was also observed between percentglomerular injury and frequency with which a systolic blood

pressure reading of >150 mm Hg was obtained as a percentage ofthe total readings in individual rats after the first 10 days (r =0.89,P < 0.0001; Fig. 6). The correlations of overall averaged systolicBP with other non-histologic parameters at the end of the studyshowed a weaker but significant correlation with proteinuria (r =0.51, P < 0.005; Fig. 7) and GFR (r = 0.55, P < 0.001). Thecorrelation with RBF was not statistically significant (r =0.33,P > 0.05). Similarly, while both proteinuria and GFR weresignificantly correlated with glomerular injury, the correlations inindividual animals were not very strong: proteinuria (r = 0.53,P <0.002); GFR (r = 0.64, P < 0.0001). In contrast, no significantcorrelation was observed between RBF and glomerular injury inindividual animals (r = 0.33, P > 0.05).

Discussion

The spontaneous variability of blood pressure in the non-anesthetized state has been documented in several species includ-ing rats and humans [18, 22—25]. Using the radiotelemetric

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Fig. 6. Correlation of percentage of glomeruli with injury in individual ratsfrom all four groups with the frequency with which a systolic BP reading of>150mm Hg was recorded as a percentage of total readings for the final —8weeks. Systolic BP was telemetrically recorded every 10 minutes for theduration of the study. After the 7th day, the rats had received either notreatment (E, N = 8) or one of these regimens in drinking water: enalapril(U,N = 8); triple therapy (A, N = 12); or high dose triple therapy (A, N= 6). See text for details of dosage regimens. r 0.89.

technique of continuous HP monitoring, we have recently docu-mented a marked increase in the spontaneous lability of bloodpressure after —5/6 renal ablation as compared to control shamoperated rats [19]. The present studies demonstrate that thisspontaneous lability persists during antihypertensive therapy andmay have significant implications for BP measurements in studiesof this model. The present studies also show that antihypertensiveregimens may have divergent effects on circadian rhythms. Thenormal diurnal variation in BP observed in the untreated groupwas maintained in rats treated with E. In contrast both the HDTTor TI' regimens abolished this circadian rhythm. The specificagent(s) and mechanism(s) responsible for this loss of diurnalvariation remain to be determined.

An excellent correlation was observed between glomerularinjury and the average of —-8000 fluctuating BP readings inindividual untreated and treated rats after —5/6 renal ablation inthe present study. The results are consistent with a predominantrole for hypertensive mechanisms in the pathogenesis of glomer-ular injury. The glomeruloprotection which was achieved paral-leled the antihypertensive effects in these rats, within and betweengroups. Although the present study was not designed to establishthe relative merits of these therapeutic regimens in this model bygroup comparisons, evidence of glomerulprotective ability, inde-pendent of their antihypertensive effects, was not observed for anyof the therapeutic regimens used in the present study. This isindicated by the lack of significant differences between the slopesof the relationship between blood pressure and glomerular injuryin the four groups. However, in the dosages used, the convertingenzyme inhibitor, enalapril, was the most consistently effective.The HD1T regimen was almost as effective and the TI' regimenwas the least effective. Our data suggest that such differences

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Overall averaged systolic BP, mm Hg(Final 8 weeks)

Fig. 7. Correlation ofproteinuria in individual rats from all four groups withthe overall averaged systolic BP for the final 8 weeks. Systolic HP wastelemetrically recorded every 10 minutes for the duration of the study.After the 7th day, the rats had received either no treatment (D, N = 8) orone of these regimens in drinking water: enalapril (U, N = 8); tripletherapy (A, N = 12); or high dose triple therapy (A, N = 6). See text fordetails of dosage regimens. r = 0.51.

between the groups are, at least for the most part, a function ofthe effectiveness of these regimens in controlling systemic hyper-tension in individual animals and that the superior glomerulopro-tective ability of enalapril is largely a consequence of its superiorconsistency as an antihypertensive in this model. However, addi-tional beneficial effects of converting enzyme inhibitors cannotdefinitively be excluded.

The results of the present study indicating that the glomerulo-protective efficacy of these therapeutic regimens is directly pro-portional to their antihypertensive efficiency, are at seemingvariance with the conclusions of most of the previous studies ofthese antihypertensive agents in this model [12, 13—15]. Anderson,Rennke, and Brenner compared triple therapy to enalapril indosage regimens identical to those used in the present study andfound that both regimens resulted in similar BP control asdetermined by tail-cuff BP measurements [13]. Glomeruloprotec-tion was however, only observed in the enalapril group, suggestingthat control of systemic HP alone, as obtained with the tripletherapy (fl) regimen, was not sufficient to prevent glomerulo-sclerosis in this model. Kakinuma et al also documented a similarineffectiveness of the TI' regimen in halting the progression ofglomerulosclerosis in this model as compared to angiotensinconverting enzyme inhibitors despite the attainment of equivalentsystemic blood pressure using the tail-cuff method [14]. Similarly,Purkerson, Hoffsten, and Klahr concluded that only partial pro-tection could be obtained in this model with afl regimen despitethe achievement of adequate and satisfactory control of hyperten-sion [12]. By contrast, Yoshida et al were able to achieve a degreeof glomeruloprotection comparable to enalapril using a higherdose triple therapy regimen identical to the HDTI' in the presentstudy [15]. The tail-cuff pressures achieved with this regimen byYoshida et al were comparable to enalapril and no different than

Gnjfin et al: Antihypertensives and glomendoprotection 1017

that reported in other studies with the standard TF regimen.Although these studies differed in the postulated mechanismsof protection, they reached the similar conclusion that glomeru-loprotective effects of these therapeutic regimens were, at least inpart, independent of their effects on the systemic BP. Theinterpretations in these studies are greatly dependent on theadequacy of the periodic tail-cuff BP measurements to reflect theambient blood pressure profiles in both treated and untreatedanimals. The significant spontaneous lability of BP observed byradiotelemetry in both treated and untreated rats in the presentstudy suggests that the tail-cuff method may not be adequate tofully assess ambient blood pressures in this model. Moreover,considerable variability in the BP response to each of the antihy-pertensive regimens was noted in individual animals in the presentstudy. It is possible that similar undetected variability in BPresponses may account for the variable and often conflictingresults that have been obtained with similar therapeutic regimensby different investigators in this model [12—17].

The mechanism of protection, when protection has beenachieved in these studies, is also the subject of significant dispute[13, 15, 16, 26]. Anderson, Rennke, and Brenner attributed theglomeruloprotective efficacy of enalapril to its ability to lower P0and the failure of the IT regimen to glomeruloprotect to itsinability to similarly lower Gc [13]. This was ascribed to thepreferential efferent arteriolar dilation achieved by enalapril.Yoshida et al also found that enalapril, but not HDTT, loweredGc [15]. However, in contrast to the results of Anderson,Rennke, and Brenner, glomeruloprotection was seen in bothgroups. Therefore, Yoshida et al concluded that a decrease in P0,was not necessary for protection and that glomeruloprotectionwas mediated by the ability of both enalapril and HDTF to reducethe hypertrophic response after —5/6 renal mass reduction. Theseseemingly irreconcilable observations about the pathogeneticsignificance of P0 in this model are both dependent on theassumption that isolated measurements of Gc obtained underanesthesia are an accurate and adequate index of ambient gb-merular capillary pressures. However, the limitations that havebeen demonstrated for periodic tail-cuff blood pressure measure-ments in the present study are probably also true for P0measurements. The interpretations of Gcmeasurements in theseand other studies in the RK model have been made with theassumption that glomerular capillary pressures remain relativelystable with little variability [27—29]. For this inference to be valid,it is necessary that either the systemic BP not exhibit significantlability or that the autoregulatory mechanisms that normallyprevent the transmissions of fluctuations in systemic pressures tothe glomerular capillaries be intact [30]. By contrast, the RKmodel is characterized by both marked lability of BP [18, presentstudy as well as impaired autoregulation, as has been demon-strated by us and others [7, 11, 31, 32]. Micropuncture studieshave indeed demonstrated parallel changes in P0ç in response tochanges in arterial pressures, produced either mechanically [31,32] or pharmacologically [33, 34]. Therefore, it is likely thatpressures in the glomerular capillaries exhibit qualitatively similarlability in parallel with the systemic pressures. Episodes of acutelyincreased systemic pressures, even if of limited duration, would beexpected to be communicated to the glomerular capillaries.Therefore, the transmission of hypertension to glomerular capil-laries in this model is likely to be a much more dynamic processthen can be appreciated by isolated measurements of steady-state

systemic and/or glomerular pressures. The exceedingly closecorrelations of glomerular injury in the present study with overallaveraged BP over a 65 day period and with the frequency of BPmeasurements >150 mm Hg, regardless of treatment modality,provide strong support for such an interpretation. Given thepotential lability of glomerular capillary pressures, the lack ofconsistent correlations of isolated P0. measurements with glomer-ular injury is not entirely unexpected [15, 28, 29]. Alternatively, itis possible that both systemic hypertension and glomerular injuryare separate but parallel consequences of some other underlyingpathogenetic mechanism or that they are very closely linkedtogether, but by some mechanism other than that of glomerulartransmission. However, there is no evidence at present to supportsuch postulates.

Regardless of the precise mechanisms of glomerular injury, orthe mechanism by which antihypertensive agents are protective,the present studies demonstrate that the degree of glomerulopro-tection achieved is very closely correlated with the degree ofreduction in systemic BP, provided the systemic pressures areprecisely measured. Additionally, these data indicate that such areduction in hypertension may be necessary for glomeruloprotec-tion regardless of the therapeutic regimen that is used. Thepersistence of the spontaneous lability of BP despite antihyper-tensive treatment and the documentation of similar lability inhypertensive humans [24, 25] suggest a need for more rigorous BPmonitoring in evaluating antihypertensive regimens in patientswith renal disease. This may be particularly important in themanagement of patients in whom an enhanced glomerular trans-mission of systemic hypertension is thought to occur such as inpatients with renal insufficiency and/or diabetes [3, 8, 35, 36]. It isof interest that recent studies utilizing ambulatory BP in humanshave also suggested that end organ damage in hypertension isrelated to 24 hour BP variations [25, 37]. Finally, these dataemphasize the limitations of occasional measurements of systemicand/or glomerular pressures in the assessment of the role ofhypertensive mechanisms in the pathogenesis of glomerular injuryafter renal mass reduction.

Acknowledgments

This research was supported by National Institutes of Health grant#DK40426. This work was presented in part at the 25th Annual Meetingof the American Society of Nephrology, Baltimore, Maryland, 1992 andpublished in abstract form in JAm Soc Nephrol 3:739A, 1992. The authorsthank Wanda Plott, Jue Ouyang, Zafer Jawich and Lisa Kelly for theirtechnical assistance and Martha Prado for secretarial assistance.

Reprint requests to Anil K Bidani MD., Loyola University MedicalCenter, 2160 South First Avenue, Maywood, illinois 60153, USA.

References

1. KAUFMAN JM, DIMEOLA HJ, SiEGEL NJ, LY1-rON B, KASHGARIAN M,HAYSLETIT JP: Compensatory adaptation of structure and functionfollowing progressive renal ablation. Kidney mt 6:10—17, 1974

2. HAYSLErr JP: Functional adapation to reduction in renal mass.Physiol Rev (59) 1:137—164, 1979

3. BRENNER BM: Nephron adaptation to renal injury or ablation. Am JPhysiol 249:F324—F337, 1985

4. FINE L: The biology of renal hypertrophy. Kidney mt 29:619—634, 19865. HOSTETrER TH, OLSON JL, RENNKE HG, VENKATACHALAM MA,

BRENNER BM: Hyperfiltration in remnant nephrons: A potentiallyadverse response to renal ablation. Am J Physiol 241:F85—F93, 1981

1018 Griffin et al: Antihypertensives and glomeruloprotection

6. ANDERSON S. MEYER TW, RENNKE HG, BRENNER BM: Control ofglomerular hypertension limits glomerular injuiy in rats with reducedrenal mass. J Clin Invest 76:612—619, 1985

7. BIDANI AK, SCHWARTZ MM, LEwis El: Renal autoregulation andvulnerability to hypertensive injury in remnant kidney. Am J Physiol252:F1003—F1010, 1987

8. BALDWIN DS, NEUGARTEN J: Hypertension and renal disease. Am JKidney Disease 10:186—191, 1987

9. Os.soi'i JL, HEPTINSTALL RH: Non-immunologic mechanisms of gb-merular injury. Lab Invest 59:564—578, 1988

10. TOLINS JP, SHULTZ P, RAil L: Mechanisms of hypertensive gbomerularinjury. Am J Cardiol 62:54G—58G, 1988

11. Biii&Ni AK, MITCHELL KD, SCHWARTZ MM, NAVAR LG, LEWIS EJ:Absence of glomerular injury or nephron loss in a normotensive ratremnant kidney model. Kidney mt 38:28—38, 1990

12. PURKERSON ML, HOFFSTEN PE, Kii-w S: Pathogenesis of the gb-merulopathy associated with renal infarction in rats. Kidney mt9:407—417, 1976

13. ANDERSON S, RENNKE HG, BRENNER BM: Therapeutic advantage ofconverting enzyme inhibitors in arresting progressive renal diseaseassociated with systemic hypertension in the rat. J Clin Invest 77:1993—2000, 1986

14. KAK]NUMA Y, KAWAMURA T, BILLS T, YosHIoIt, T, ICHIKAWA I,FoGo A: Blood pressure-independent effect of angiotensin inhibitionon vascular lesions of chronic renal failure. Kidney mt 42:46—55, 1992

15. YOSHIDA Y, KAWAMAURA T, IKOMA M, FoGo A, ICHIKAwA I: Effects

of antihypertensive drugs on glomerular morphology. Kidney mt36:626—635, 1989

16. TOLINS JP, RAIJ L: Angiotensin converting enzyme inhibitors andprogression of chronic renal failure. Kidney mt 38:S-118—S-122, 1990

17. JACKSON B, DEBREVI L, CUBELA R, WnrvrY M, JOHNSTON CI:Preservation of renal function in the rat remnant kidney model ofchronic failure by blood pressure reduction. Gun Exp PharmacolPhysiol 13:319—323, 1986

18. BROCKWAY BP, MILLS PA, AZAR SH: A new method for continuouschronic measurement and recording of blood pressure, heart rate andactivity in the rat via radiotelemetry. Clin Exp Hypertens A13:885—895,1991

19. BIDANI AK, GRIFFIN KA, PICKEN M, LsKY DM: Continuoustelemetric BP monitoring and glomerular injury in the rat remnantkidney model. Am J Physiol 265:F391—F398, 1993

20. GRIFFIN KA, BIDAr4I AK, PICKEN M, ELLIS VR, CHURCHILL PC:Prostaglandins do not mediate impaired autoregulation or increasedrenin secretion in remnant rat kidneys. Am J Physiol 263:F1057—F1062, 1992

21. RA.u L, AZAR S. KEANE W: Mesangial immune injury, hypertension,and progressive glomerular damage in DahI rats. Kidney mt 26:137—143, 1984

22. HE J, WAGNER AJ, HOLSTEIN-RATHLOU NH, MARSH DJ: Spectralanalysis of blood pressure fluctuations in 6-day records telemeteredfrom rats. (abstract) FASEB J 6:A1254, 1992

23. MArTES A, LEMMER B: Effects of Ambodipine on circadian rhythms inblood pressure, heart rate, and motility: A telemetric study in rats.Chronobiol Int 8:526—538, 1991

24. MANCIA G, ZANCHETFI A: Blood pressure variability, in Handbook ofHypertension: Pathophysiology of Hypertension, Cardiovascular Aspects(vol 7), edited by ZANCHETrI A, TARAZI RC, Amsterdam, Elsevier/North Holland, 1986, pp. 125—152

25. PARATI G, OMB0NI 5, Di RIENZO M, F1trroLA A, ALBINI F, MANCIAU: Twenty-four hour blood pressure variability: Clinical implications.Kidney Int 41:S-24—S-28, 1992

26. KI.1lR S, SCHREINER G, IcHIKAwA I: The progression of renal disease.NEng1J Med 318:1657—1666, 1988

27. LAX DS, BENSTEIN JA, TOLBERT E, DWORKIN LD: Effects of saltrestriction on renal growth and glomerular injury in rats with remnantkidneys. Kidney Int 41:1527—1534, 1992

28. YOSHIDA Y, FoGo A, SHIRAGA H, GLICK AD, ICHIKAWA I: Serialmicropuncture analysis of single nephron function in subtotal renalablation. Kidney mt 33:855—867, 1988

29. Y0SHIDA Y, Focio A, IcHIKAWA I: Glomerular hemodynamic changesvs. hypertrophy in experimental glomerular sclerosis. Kidney Int35:654—660, 1989

30. OFSTAD J, AUKLAND K: Renal circulation, The Kidney, Physiology andPathophysiology, edited by SELDIN DW, GIEBISCH G New York,Raven, 1985, pp. 471—496

31. HOSTETrER TEl, KREN SF: Autoregulation of the remnant nephroncirculation. (abstract) Clin Res 33:859A, 1985

32. PELAYO JC, WESTCOTF JY: Impaired autoregulation of glomerularcapillary hydrostatic pressure in the rat remnant nephron. J Clin lnvest88:101—105, 1991

33. Yosi-iioic& T, SHIRAGE H, YOSHIDA Y, FoGo A, GLICK AD, DEENWM, HOYER JR, ICHIKAWA I: "Intact nephrons" as the primary originof proteinuria in chronic renal disease. J Gun Invest 82:1614—1623,1988

34. ANDERSON S: Renal hemodynamic effects of calcium antagonists inrats with reduced renal mass. 1-lypertension 17:288—295, 1991

35. ANDERSON 5, RENNKE HG, GARCIA DL, BRENNER BM: Short andlong term effects of antihypertensive therapy in the diabetic rat.Kidney Int 36:525—536, 1989

36. ZArz R, MEYER TW, RENNKE HG, BRENNER BM: Predominance ofhemodynamic rather than metabolic factors in the pathogenesis ofdiabetic glomerulopathy. Proc NatlAcad Sci USA 82:5963—5967, 1985

37. PARATI G, POMIDOSsI G, ALBINI F, MALASPINA D, MANCIA C:Relationship of 24-hour blood pressure mean and variability toseverity of target-organ damage in hypertension. J Hypertens 5:93—98,1987