Kidney International, Vol. 57 (2000), pp. 1651–1661

VASCULAR BIOLOGY – HEMODYNAMICS – HYPERTENSION

Lack of evidence of blood pressure-independent protection byrenin-angiotensin system blockade after renal ablation

ANIL K. BIDANI, KAREN A. GRIFFIN, GEORGE BAKRIS, and MARIA M. PICKEN

Departments of Medicine and Pathology, Loyola University Medical Center and Hines Veterans Administration Hospital,Maywood; and Department of Preventive Medicine, Rush-Presbyterian-St. Luke’s Medical Center, Chicago, Illinois, USA

Lack of evidence of blood pressure-independent protection by indicated that although antihypertensive agents in gen-renin-angiotensin system blockade after renal ablation. eral tend to retard this progression, the most consistent

Background. The superiority of renin-angiotensin system and uniform renoprotection is achieved with agents that(RAS) blockade in providing renoprotection has been attrib-produce a blockade of the renin-angiotensin systemuted to class-specific blood pressure “(BP)-independent” mecha-(RAS) [9–15]. It has been postulated that the antihyper-nisms. However, the conventional BP measurement methodol-

ogy on which such conclusions are based is inherently limited tensive effects of RAS blockade per se are insufficientfor an accurate assessment of the fluctuating ambient BP pro- to explain its superior renoprotective ability, which hasfiles. The present studies were undertaken to rigorously exam- therefore been attributed to class-specific, blood pressureine the relationship of renoprotection to the antihypertensive

(BP)-independent effects. Such BP-independent effectseffects of RAS blockade using chronic BP radiotelemetry inthe 5/6 renal ablation model. include: (1) a proportionately greater reduction in glo-

Methods. Rats with 5/6 renal ablation received either no treat- merular capillary pressure (PGC) for any given reductionment, the angiotensin-converting enzyme inhibitor benazepril in systemic BP as a consequence of the preferential effer-at a dose of 25, 50, and 100 mg/L; or the angiotensin receptor

ent arteriolar dilation produced by RAS blockade [5, 9,antagonist losartan at a dose of 50, 120, and 180 mg/L of10, 12, 16]; and/or (2) a blockade of specific angiotensindrinking H2O; and were followed for seven weeks.

Results. Glomerulosclerosis (GS) at sacrifice (approximately II-mediated pathogenetic effects on glomerular cells that7 weeks) demonstrated a close correlation with the average favor the development of progressive GS. These effects,systolic BP in untreated (r 5 0.76, N 5 20), benazepril-treated mediated through the activation of the angiotensin II(r 5 0.80, N 5 33), losartan-treated (r 5 0.83, N 5 32), or

type 1 receptor (AT1) [17, 18], include glomerular hyper-all animals combined (r 5 0.81, N 5 85, P , 0.0001 for alltrophy, an increased expression of growth factors suchcorrelations). The slope of the relationship between GS and

BP (percentage of increase in GS/mm Hg increase in BP) in as transforming growth factor-b and platelet-deriveduntreated rats (0.7 6 0.14) was not significantly altered by growth factor and an increased matrix accumulation witheither benazepril (0.96 6 0.13) or losartan (0.60 6 0.08), indicat-

eventual GS [3, 18–23].ing that RAS blockade, by either agent, resulted in renoprotec-These widely accepted conclusions as to the contribu-tion that was proportionate to the achieved BP reductions.

Conclusions. These data demonstrate that RAS blockade tion and importance of BP-independent pathways in theprovides renoprotection in the rat remnant kidney model of renoprotection provided by RAS blockade are, however,progressive GS, primarily through “BP-dependent” and not critically dependent on an accurate and adequate assess-“BP-independent” mechanisms.

ment of the ambient BP profiles and glomerular pressureburden, and the assumption that conventional BP mea-surements are sufficient to separate the “BP-dependent”

The mechanisms responsible for the progressive na-and “BP-independent” effects of therapeutic interventions.ture of chronic renal disease remain the subject of intenseHowever, BP characteristically exhibits rapid, spontaneous,investigation and debate [1–8]. Data obtained in severaland large BP fluctuations in conscious, unrestrained ratsexperimental models and in human renal disease have(and other species), particularly when hypertension ispresent [24–28]. Such BP lability makes it extremely un-likely that the conventional methodology for BP measure-Key words: hypertension, glomerulosclerosis, AT1 receptor antagonist,

angiotensin-converting enzyme inhibitor, progressive renal disease. ments used in such investigations can assess the overallpressure burden with sufficient accuracy to allow defini-Received for publication June 11, 1999tive conclusions. Therefore, the present studies were un-and in revised form October 20, 1999

Accepted for publication November 2, 1999 dertaken to rigorously examine the dose-response rela-tionship of glomeruloprotection to the antihypertensive 2000 by the International Society of Nephrology

1651

Bidani et al: RAS blockade and GS1652

effects of RAS blockade using BP radiotelemetry in the sure. A femoral vein was cannulated with PE-50 tubing,and a priming dose of inulin in 150 mmol/L NaCl wasremnant kidney model of 5/6 renal ablation, in which

conventional BP measurements have indicated the protec- administered, followed by a continuous maintenance in-fusion of 150 mmol/L NaCl containing inulin at 0.055tion by both angiotensin-converting enzyme (ACE) inhib-

itors and AT1 receptor antagonists to be “BP indepen- mL/min to maintain the plasma concentration of inulin atapproximately 50 mg/dL, and for replacement of surgicaldent” [3, 9, 12, 19].and ongoing fluid losses. The left ureter was then cannu-lated with polyethylene tubing for the collection of urine

METHODSsamples. A 1.0 mm R series flow probe (Transonic Sys-

Studies were conducted in male Sprague-Dawley rats tems, Inc., Ithaca, NY, USA) was placed around the left(body weight 225 to 300 g) that were fed a standard renal artery for measurement of renal blood flow (RBF)(24%) protein diet (Purina, St. Louis, MO, USA) and by a flowmeter (Transonic Systems, Inc.), as describedsynchronized to a 12:12-hour light (6:00 to 18:00 h) and previously [27–31]. At the conclusion of the surgery, adark (18:00 to 6:00 h) cycle. All rats received food 150 mmol/L NaCl bolus equal to 1% of body weight wasand water ad libitum throughout the study. administered. Two 20-minute clearances of inulin were

obtained. Blood samples were obtained at the midpointRadiotelemetry

of each urine collection. At the conclusion of these stud-The rats were anesthetized with sodium pentobarbital ies, the rats were killed, and the kidneys were harvested

(45 mg/kg intraperitoneally), subjected to ,5/6 renal for morphological studies.ablation (right nephrectomy and ligation of all but oneposterior extrarenal branch of the left renal artery), and Morphology and morphometricsprepared for telemetric monitoring of BP (Data Sciences Transverse sections of the kidney through the papillaInternational, St. Paul, MN, USA) at the time of the were fixed in situ by perfusion for five minutes at therenal ablation surgery as previously described [27–31]. measured BP with 1.25% glutaraldehyde in 0.1 mol/LEach rat had a BP sensor inserted intraperitoneally. The cacodylate buffer. Sections were cut at a thickness of 2 mmsensor’s catheter was inserted into the aorta below the and stained with hematoxylin and eosin and periodiclevel of the renal arteries, and the radio frequency trans- acid-Schiff. Sections were evaluated systemically in eachmitter was fixed to the peritoneum. The rats were housed kidney for glomerular injury (segmental sclerosis and/orindividually in plastic cages that were placed on top of necrosis) in a blinded fashion by standard morphologicthe receiver. The signals from the pressure sensor were methods [4, 27–31]. At least 100 glomeruli in each animalconverted, temperature compensated, and sent via the were evaluated, and the severity of GS was expressedradio frequency transmitter to the telemetry receiver. as the percentage of glomeruli exhibiting such injury.The receiver was connected to a BCM-100 consolidation We have previously shown that this method yields resultsmatrix that transmitted the information to the Lab Pro that are essentially identical to an alternative methoddata acquisition system. Systolic BP in each animal was that uses a morphometric scoring method to estimate GSrecorded at 10-minute intervals throughout the course [28, 32] in terms of correlation with proteinuria and/orof approximately seven weeks, with each reading being BP parameters. Glomerular volume (VG) was measuredthe average value obtained during a 10-second sampling by area perimeter analysis (Bioquant System IV soft-period. Tail vein blood samples were obtained at three ware; R&M Biometrics, Inc., Nashville, TN, USA). Thedays for the measurement of serum creatinine as an cross-sectional area (AG) of 75 consecutive glomerularindex of the degree of renal mass reduction [29]. At profiles contained in one kidney section for each animalapproximately seven days, the rats were randomly as- was measured using a digitizing pad as described pre-signed to the untreated group or received one of three viously [27–31]. The mean VG was then calculated fromdoses of the ACE inhibitor benazepril (25, 50, 100 mg/L) the respective mean AG as VG 5 b/k (AG

3/2), where b 5or the AT1 receptor antagonist losartan (50, 120, 180 mg/L) 1.38 is the size distribution coefficient and k 5 1.1 is theof drinking water. After seven weeks, 24-hour urine col- shape coefficient for glomeruli idealized as spheres [33].lections were obtained to measure the protein excretion.

Analyses, calculations, and statisticsThe rats were then anesthetized with intravenous sodiumpentobarbital (40 mg/kg). A tracheostomy was per- Urinary protein was measured by the quantitative sul-formed using polyethylene (PE-200) tubing, and the rats fosalicylic acid method, with human serum albumin serv-were surgically prepared for clearance studies as de- ing as the standard. Creatinine was measured using ascribed previously [27–31]. In brief, a carotid artery was creatinine analyzer (Beckman Instruments, Inc., Fuller-cannulated with PE-50 tubing and connected to a Windo- ton, CA, USA) [29–31]. Inulin in urine and plasma fil-graf (model 40-8474; Gould Inc., Glen Burnie, MD, trates was determined spectrophotometrically by the di-

phenylamine method as described previously [27–31].USA) for continuous recording of the mean arterial pres-

Bidani et al: RAS blockade and GS 1653

Table 1. Renal function data after RAS blockade

Initial 3 days Final

Body weight SCr 24 h Protein SCr Body weight RBF GFRN g mg/dL mg/24 h mg/dL g mL/min/kg mL/min/kg

Untreated 20 25765 0.3360.01 3.560.4 0.8260.3 426610 24.262.3 2.560.3

Benazepril 25 10 26264 0.3560.02 2.760.2 0.8660.06 427614 25.263.3 2.660.3Benazepril 50 12 25464 0.3860.03 3.360.5 0.8360.05 45069 35.562.6ab 3.260.3Benazepril 100 11 24867 0.2960.04 2.860.3 0.8060.04 43269 36.062.7ab 3.860.3a

Losartan 50 9 25167 0.3060.02 3.060.7 0.8860.06 432612 30.463.5 2.860.3Losartan 120 12 25864 0.3260.03 3.760.7 0.8360.04 428613 34.262.6a 3.760.3a

Losartan 180 11 24766 0.3260.02 4.260.6 0.8460.04 40867 41.362.3ab 4.460.5abc

SCr is serum creatinine.aP , 0.05 vs. UntreatedbP , 0.05 vs. Benazepril 25cP , 0.05 vs. Losartan 50

The glomerular filtration rate (GFR) was calculated us- highest dose of losartan (180 mg/L). The differences inthe overall “pressure load” between the seven groupsing standard formulae. Linear regression analysis was

used to calculate the slopes and intercepts of the relation- are summarized and illustrated by a comparison of theaverage systolic BP during the first week before the initi-ship between BP and GS in each group. Statistical analy-

sis was performed using analysis of variance followed by ation of treatment and during the following approxi-mately six weeks (Fig. 3). The average BP during theStudent-Newman-Keuls test or by Kruskal-Wallis non-

parametric analysis of variance followed by Dunn’s mul- final six weeks was modestly but significantly higher inthe untreated control rats as compared with their respec-tiple comparison test as appropriate [34]. Analysis of

covariance was used to compare the slopes and intercepts tive first-week values. By contrast, the average systolicBP of all treated animals during the final six weeks wasbetween the groups using the Minitab Software packagesignificantly lower than their respective average systolic(Minitab, Inc., PA, USA) [34]. A P value of , 0.05 wasBP during the first week before the initiation of treat-considered statistically significant. All results are ex-ment, when the paired t-test was used for comparisonpressed as mean 6 SE.within each group. However, because of the substantialvariability of the average individual systolic BP values

RESULTS within each of the subgroups receiving either the lowestTable 1 shows that the initial body weight, serum creat- dose of benazepril (25 mg/L) or the two lower doses

inine, and 24-hour urine protein excretion for the seven of losartan (50 and 120 mg/L), these values were notgroups were not significantly different from each other. statistically different from that of the untreated controlSimilarly, the serum creatinine at three days was not rats. By contrast, the average posttreatment systolic BPsignificantly different between the groups, indicating of the groups receiving the two higher doses of benaze-comparable renal mass reduction in all groups. The final pril (50 and 100 mg/L) and the group receiving the high-body weight at seven weeks was also not significantly est dose of losartan (180 mg/L) was comparably anddifferent between the five groups. significantly reduced as compared with the untreated

Figure 1 illustrates the systolic BP recordings from control group (P , 0.01).individual rats that were untreated, received benazepril The protein excretion rate (mg/24 h) at the end of50 mg/L, or received losartan 180 mg/L, and it shows seven weeks and the percentage of glomeruli exhibitingthe persistence of significant BP lability despite a marked GS in the remnant kidneys of these seven groups arereduction in systolic BP. Figure 2 illustrates the course presented in Figure 4. Graded reductions in proteinuriaof weekly averages of systolic BP after ,5/6 renal abla- and GS were seen in the benazepril- and losartan-treatedtion in untreated animals and the three groups each groups, which paralleled the reductions in their respec-treated with the different dosages of the ACE inhibitor tive BP as compared with the untreated rats. However,benazepril or the AT1 receptor antagonist losartan. The as was the case for the average post-treatment systolicaverage BP during the first seven days after renal abla- BP values, statistical significance was only achieved fortion and before the initiation of antihypertensive therapy reductions in proteinuria and GS in the groups receivingwas comparable in the seven groups. An initiation of the two higher doses of benazepril (50 and 100 mg/L)RAS blockade resulted in BP reductions in all six groups. and the highest dose of losartan (180 mg/L). RBF andThe BP reduction was the most pronounced for the GFR measurements obtained in these rats before sacri-

fice similarly indicated better preservation of renal func-groups receiving 50 or 100 mg/L of benazepril and the

Bidani et al: RAS blockade and GS1654

Fig. 1. Course of systolic blood pressure(BP) recorded every 10 minutes for approxi-mately seven weeks in a rat with ,5/6 renalablation from (A) an untreated control rat,(B) a rat receiving 50 mg/L in drinking waterof benazepril, and (C ) a rat receiving 180 mg/Lin drinking water of losartan, showing the per-sistence of significant BP lability despite amarked reduction in systolic BP. The initia-tion of therapy is indicated by the arrows (↓).

tion in these same groups of rats receiving the higher 0.76, P , 0.001), benazepril-treated rats (r 5 0.80, P ,

0.001), or the losartan-treated rats (r 5 0.83, P , 0.001).doses of benazepril or losartan (Table 1).Figure 5A shows the correlation of average systolic Figure 5B shows the separately calculated regression

lines for the untreated, benazepril-, or losartan-treatedBP and GS in all of the individual rats in the study. Anexcellent direct correlation was observed (r 5 0.81, P , rats and shows that the slopes and the intercepts of the

relationship between BP and GS in the benazepril- and0.0001). Similar strong correlations were observed be-tween the average systolic BP and GS when data were losartan-treated groups were not significantly different

from untreated rats.separately analyzed for the untreated control rats (r 5

Bidani et al: RAS blockade and GS 1655

Fig. 1. (Continued).

proteinuria was less strong (untreated, r 5 0.67; benaze-pril treated, r 5 0.58; losartan treated, r 5 0.77) or forall animals combined (r 5 0.72).

Figure 6 shows the effects of RAS blockade on rem-nant kidney and glomerular growth. The remnant kidneyweight factored for body weight was not significantlydifferent between the treated groups or in comparisonto untreated rats. Similarly, glomerular volume was notsignificantly different between the treated groups or ascompared with the untreated group. The correlationsbetween glomerular volume and GS were not significantwithin the individual groups (untreated controls, r 50.36; benazepril, r 5 0.11; and losartan, r 5 0.21, P .0.05 for all correlations). However, when the groupsFig. 2. Course of systolic BP (24 hour averages) over approximatelywere combined, a significant but weak correlation wasseven weeks in the seven groups. All rats underwent ,5/6 renal ablation

(right nephrectomy 1 infarction of ,2/3 of the left kidney). After observed (r 5 0.29, P , 0.01).seven days, the rats were left untreated (h; N 5 20) or received eitherbenazepril at a dose of 25 (r; N 5 10), 50 (d; N 5 12), or 100 (solidstar; N 5 11) mg/L of drinking H2O, or losartan at a dose of 50 (open

DISCUSSIONdiamond; N 5 9), 120 (s; N 5 12), or 180 (open star; N 5 20) mg/Lof drinking water. BP was radiotelemetrically recorded continuously at Numerous previous studies have compared the salu-10-minute intervals.

tary effects of RAS blockade with that of other antihy-pertensive regimens on the progression of both experi-mental and clinical renal disease [5, 9–15]. With fewexceptions, such studies have concluded that RAS block-An excellent correlation was also observed between

histologic glomerular injury (% GS) and its functional ade is more consistently effective in providing renopro-tection as compared with other therapeutic interven-correlate, proteinuria, whether the analysis was per-

formed for all animals combined (r 5 0.85) or whether tions, but the responsible mechanisms remain the subjectof controversy [2, 3, 5, 6, 9–12, 16, 18–23]. Although suchindividual groups were separately analyzed (untreated,

r 5 0.73; benazepril treated, r 5 0.87; or losartan treated, RAS blockade-conferred protection has almost uni-formly been associated with significant BP reductions,r 5 0.81). However, the correlation between BP and

Bidani et al: RAS blockade and GS1656

Fig. 3. Overall average BP during the firstseven days (A) and during the subsequent ap-proximately six weeks (B) in 5/6 renal ablatedrats that, after the seventh day, were: left un-treated (h); received benazepril at 25, 50, or100 mg/L of drinking H2O (j); or receivedlosartan at 50, 120, or 180 mg/L of drinkingwater ( ). BP was radiotelemetrically re-corded continuously at 10-minute intervals.*P , 0.001 compared with untreated controls;*P , 0.05 maximum compared to the respec-tive average systolic BP of the same groupduring the first week.

Fig. 4. Proteinuria (A) and percentage glo-merulosclerosis (B) at the end of approxi-mately seven weeks in the rats with ,5/6 renalablation that were: left untreated (h); re-ceived benazepril at 25, 50, or 100 mg/L (j);or received losartan at 50, 120, or 180 mg/L( ) of drinking water after the first week.*P , 0.01 maximum compared with untreatedcontrols.

nevertheless, the superiority of RAS blockade has been adequate to ensure “equivalent BP control,” a criticalprerequisite for the valid interpretation of the observedattributed to “BP-independent” mechanisms, as equiva-

lent BP reductions achieved with other antihypertensive differences between antihypertensive regimens. The ne-cessity of an accurate assessment of the ambient BPclasses have often failed to provide comparable protec-

tion. Such evidence, however, is less than definitive. Con- profiles and the pressure burden on the kidneys is dem-onstrated by the exceedingly close linear relationshipventional BP measurements are obtained at intervals

ranging from one to several weeks, when, in fact, BP between the overall average systolic BP and GS whenchronic BP radiotelemetry is used to measure BP contin-displays substantial moment-to-moment lability [24–31].

This fundamental time-dependent BP variability, which uously in untreated individual rats after ,5/6 renal abla-tion, consistent with the hypertensive pathogenesis ofpersists during treatment with antihypertensive agents

[28, 30, 31], renders conventional BP measurements in- GS in this model [27]. The alternative interpretation—

Bidani et al: RAS blockade and GS 1657

Fig. 5. (A) Correlation of the percentage of glomeruli with sclerosis at approximately seven weeks in individual rats with 5/6 renal ablation andtheir average systolic BP during the final six weeks (the mean of all approximately 6000 BP readings in each rat). After the first seven days, therats had received: no treatment (h; N 5 20); benazepril at 25 (r; N 5 10), 50 (d; N 5 12), or 100 (solid star; N 5 11) mg/L; or losartan at 50(open diamond; N 5 9), 120 (s; N 5 12), or 180 (open star; N 5 11) mg/L in drinking water. (B) Linear regression analysis of the slopes of therelationship between the average BP during the final six weeks and % GS for all rats combined and separately for the untreated (h; N 5 20),benazepril-treated (d; N 5 33), and losartan-treated (s; N 5 32) rats. Untreated (N 5 20): r 5 0.76, slope 0.71 6 0.14, x intercept 136 mm Hg;benazepril (N 5 37): r 5 0.80, slope 0.96 6 0.14, x intercept 125 mm Hg; losartan (N 5 32): r 5 0.83, slope 0.60 6 0.08, intercept 124 mm Hg.The slopes and intercepts for the individual groups were not significantly different from each other or from the combined slope and intercept forall animals in the study (N 5 85, r 5 0.81, slope 0.72 6 0.05, x intercept 123.7 mm Hg).

that GS is the primary determinant and hypertension is variability in GS in individual animals. Although we didnot directly compare other antihypertensive regimens tothe dependent variable [35]—is rendered less tenable

because hypertension antedates the development of GS RAS blockade in the present study, the present resultsare consistent with the results obtained in an earlier studyin the model [27]. Moreover, if such an interpretation

was valid, a progressive increase in the severity of hyper- in which a single-dose regimen of the ACE inhibitorenalapril was compared with two different dosages of atension would be expected in untreated animals after

the third week with worsening GS, and such is not seen combined triple-therapy regimen of hydralazine, hydro-chlorothiazide, and reserpine [28]. Renoprotection wasin the majority of untreated rats over the observed time

course [27]. found to be proportional to the achieved BP reductionwith all antihypertensive regimens, and the superior re-Given this close linear relationship between BP and

GS, the impact of an antihypertensive agent on the slope noprotection provided by enalapril could be accountedfor by its superior antihypertensive efficacy. However,of this relationship (increase in percentage GS/mm Hg

increase in BP) may provide a more valid method to because the single dose (50 mg/L) of enalapril used, inthat as well as another study [30], was very effective inseparate the agent’s “BP-dependent” and “BP-indepen-

dent” effects on GS [28, 30, 31]. If a given antihyperten- reducing BP (,135 to 140 mm Hg), the relationshipbetween BP and GS after RAS blockade could only besive agent reduces both BP and GS, but does not signifi-

cantly alter the slope of the relationship between BP examined within a narrow BP range. By contrast, the pres-ent studies show that the protection by RAS blockadeand GS from that observed in untreated animals, the

glomeruloprotection can be ascribed primarily to its BP- is BP dependent across the entire BP range in this model.The antihypertensive effectiveness of RAS blockadedependent (antihypertensive) effects [28]. A significant

contribution of “BP-independent” mechanisms to the in the 5/6 renal ablation model suggests that hyperten-sion in this model may be largely angiotensin II depen-glomeruloprotection by a therapeutic intervention would

be expected to reduce the slope and/or to shift it to the dent. Such an interpretation regarding the pathogenesisof hypertension in the 5/6 renal ablation model is consis-right. Such an effect was not observed with any of the

agents or dosages used for RAS blockade. Nevertheless, tent with the well-characterized effects of both systemicand/or locally generated angiotensin II in the develop-GS at sacrifice in individual animals showed an excellent

correlation with their average systolic BP within each ment and maintenance of hypertension [16]. Of note,although the mean BP data of the various subgroups isgroup treated with either the ACE inhibitor or the AT1

receptor antagonist. These data indicate that GS in both consistent with a dose-dependent antihypertensive effectof both ACE inhibitors and AT1 receptor blockade, suchuntreated and treated animals with remnant kidneys is

primarily BP dependent, with differences in the overall an interpretation is in some sense misleading. An exami-nation of the individual animal data demonstrates sig-BP burden possibly accounting for 60 to 65% of the

Bidani et al: RAS blockade and GS1658

Fig. 6. Kidney weight and glomerular vol-ume at the end of approximately seven weeksin the rats with ,5/6 renal ablation that were:left untreated (h); received benazepril 25, 50,or 100 mg/L (j); or received losartan 50, 120,or 180 mg/L ( ) of drinking water after thefirst week. There were no significant differ-ences between the groups.

nificant heterogeneity within the subgroups receiving the sodic or sustained) from being transmitted to the glomer-ular capillaries [38–40]. This preglomerular autoregula-lower doses of each agent. These data indicate that, while

the lower dose of either agent is sufficient to produce tory vasoconstriction is dependent on calcium entrythrough voltage-gated calcium channels and is conse-normotension in a certain proportion of rats with 5/6

ablation, an increase in dosage results in an increase in quently impaired by calcium channel blockers in animalswith intact kidneys [40–42]. The observation that thethe number of individual rats that achieve normotension

within a subgroup. However, a further increase in the dihydropyridine calcium channel blockers cause an addi-tional impairment of renal autoregulation in this modeldose, as for instance of benazepril from 50 to 100 mg/L

in drinking water, did not result in further BP reductions and also have a predictable adverse effect on the rela-tionship between BP and GS emphasizes the importanceand, of significance, did not provide greater renoprotec-

tion. Thus, the present data do not provide support for of the preglomerular resistance as a major determinantof the susceptibility to GS for any given BP elevationthe concept that dosages of RAS blockade higher than

that required to produce normotension may provide ad- [30, 31]. ACE inhibitors, by contrast, do not alter theimpaired autoregulatory responses of the preglomerularditional “BP-independent” renoprotection [11, 18]. The

significant individual variability within a subgroup fur- vasculature in this model [30], but are thought to exerttheir superior renoprotective effects through blockadether emphasizes the limitations of group comparisons,

and suggests that additional individual animal data anal- of the tonic vasoconstrictor effects of angiotensin II onthe postglomerular efferent arteriole [5, 9, 10, 12, 16].yses as performed in the present study may provide truer

insights into the antihypertensive effects of RAS block- Although the efferent arteriole does not participate inautoregulation [38, 39, 43], it is, nevertheless, an impor-ade as well as the relationship between BP and GS in

this model. tant determinant of the ambient PGC [16, 43]. Thus, con-comitant relative efferent arteriolar dilation by RASGlomerulosclerosis is expected and postulated to be a

consequence of an increase in local glomerular pressures, blockade would be postulated to result in a dispropor-tionate reduction in PGC for any given reduction in sys-rather than systemic BP per se; therefore, the close corre-

lation observed between BP and GS indicates that glo- temic BP [5, 9, 10, 12, 16]. Micropuncture studies in ratswith 5/6 renal ablation, comparing RAS blockade tomerular capillary pressure (PGC) profiles parallel the

fluctuating systemic pressures in individual animals with other antihypertensives, are consistent with such a postu-late [5, 9, 10, 12, 19]. However, the results of the present5/6 renal ablation. Such an interpretation is consistent

with studies that have demonstrated that the renal auto- study do not provide support for a significant contribu-tion of such a mechanism to the overall glomeruloprotec-regulatory mechanisms are impaired in rats with 5/6 abla-

tion [36, 37]. Normally, the autoregulatory vasoconstric- tion provided by chronic RAS blockade. A potentialexplanation for this seeming dissociation between thetor responses of the preglomerular vasculature, most

prominently the afferent arteriole, provide the primary results predicted by the micropuncture studies and thepresent data may stem from the inherent limitations ofprotection against increases in systemic pressure (epi-

Bidani et al: RAS blockade and GS 1659

the micropuncture methodology. PGC measurements are, to be a mediator of the preferential efferent arteriolardilation and PGC reduction observed with ACE inhibitorsby necessity, obtained at a single time point in anesthe-

tized rats, and anesthesia activates both the neurohormo- [18, 56, 57]. Conversely, chronic ACE inhibition maynot eliminate the continued production of angiotensinnal systems and the RAS [44, 45], which can indepen-

dently alter segmental vascular resistances and magnify II through ACE-independent pathways, albeit at lowerbut still potentially pathogenic levels [18, 55]. Therefore,the contribution of efferent arteriolar resistance to the

observed PGC, given the potential for enhanced renin it has been suggested that the more distal blockade ofRAS by AT1 receptor antagonist may be more effectiverelease in this model [46]. Additionally, isolated PGC mea-

surements are likely to have limitations similar to that in blocking the AT1 receptor-mediated adverse cellulareffects of angiotensin II as compared with ACE inhibi-of isolated BP measurements [27, 28, 30].

“Blood pressure-independent” protection by RAS tors [18, 55]. It has also been suggested that AT1 receptorantagonists may, in fact, enhance AT2 receptor activationblockade is also postulated to occur via a blockade of

the nonhemodynamic adverse effects of angiotensin II by angiotensin II, which has the potential for additionalbeneficial cellular effects [18, 55, 58]. No differences wereon glomerular growth, mesangial proliferation, increased

expression of transforming growth factor-b and platelet- observed in the present study between ACE inhibitionand AT1 receptor antagonism, suggesting that the describedderived growth factor, increased production and accumu-

lation of extracellular matrix, and eventual GS [18–23]. differences between these two methods of RAS blockademay not play a significant role during chronic therapy,Of these potential pathogenetic mechanisms, only the

issue of glomerular growth is addressed directly by the consistent with previous results in this model [54].Thus, in summary, the present studies show that atpresent data, which show that differences in GS between

individual animals treated with RAS blockade are not least in the renal ablation model, renoprotection by RASblockade is “BP dependent.” No evidence of “BP-inde-explained by differences in glomerular growth. Although

the cellular and molecular basis for the structural com- pendent” protection was observed, despite the extensivedocumentation of the potential “BP-independent” ad-pensatory hypertrophy response remains controversial

at present, the predominant stimulus seems to be renal verse effects of angiotensin II in the pathogenesis of GSin this model. These data, however, do not exclude themass reduction per se. For instance, compared with con-

trols, equivalent increases in glomerular volume are ob- unexamined possibility that dose-dependent and “BP-independent” beneficial effects of RAS blockade mayserved after 5/6 renal mass reduction regardless of whether

the rats are hypertensive with increased RAS activity be observed after substantially longer follow-up in theserats. Similarly, it is possible that in states such as diabetes,(the infarction model) or normotensive with relative

RAS suppression (the surgical excision model) [29, 47]. where hypertension may be less angiotensin II depen-dent or the mechanisms of renal injury may be less “BPSuch an interpretation is also consistent with the variable

effects of RAS blockade on glomerular hypertrophy seen dependent” [59], the “BP-independent” pathways asso-ciated with RAS blockade may play a more significantin this model [19, 48, 49]. Although the present data do

not address the potential individual contributions of the role [13, 14, 18, 20, 23, 59]. Such models therefore maybe more appropriate than the 5/6 ablation model for theother local cellular pathways that mediate the eventual

development of GS, they strongly suggest that local baro- investigation of “BP-independent” effects of antihyper-tensive agents. Nevertheless, it is of note that the demon-trauma and/or the glomerular capillary stretch conse-

quent to the increased PGC, rather than angiotensin II stration of “BP-independent” beneficial effects of RASblockade in these disease states has also to date beenper se, may be the primary initiating mechanism for these

pathways [50, 51]. Alternatively, such deleterious cellular based on only conventional BP measurements.effects of angiotensin II may require the presence ofglomerular hypertension for complete expression. In- ACKNOWLEDGMENTSdeed, recent studies of left ventricular hypertrophy in This research was supported by a National Institutes of Health grant

DK-40426. The authors thank Ms. Jennifer Orr and Ms. Shilpa GudeAT1 receptor knockout mouse models have suggestedfor technical assistance and Ms. Martha Prado for secretarial assistance.that the presence of angiotensin II is not an obligate

requirement for the complete phenotypic expression of Reprint requests to Anil K. Bidani, M.D., Loyola University MedicalCenter, Division of Nephrology, 2160 South First Avenue, Maywood,pressure-induced target organ damage [52, 53].Illinois 60153, USA.In this context, it has also been suggested that although

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