Lack of evidence of blood pressure-independent protection by
renin-angiotensin system blockade after renal ablationVASCULAR
BIOLOGY – HEMODYNAMICS – HYPERTENSION
Lack of evidence of blood pressure-independent protection by
renin-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 insufficient for an
accurate assessment of the fluctuating ambient BP pro- to explain
its superior renoprotective ability, which has files. 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 in the 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 reduction ment, 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 of 10, 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 that 7 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 all trophy, 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-derived untreated rats
(0.7 6 0.14) was not significantly altered by growth factor and an
increased matrix accumulation with either 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 the provides
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 pressure burden, 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 intense However, BP
characteristically exhibits rapid, spontaneous,investigation and
debate [1–8]. Data obtained in several and large BP fluctuations in
conscious, unrestrained ratsexperimental models and in human renal
disease have (and other species), particularly when hypertension is
present [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 overall pressure
burden with sufficient accuracy to allow defini-Received for
publication June 11, 1999 tive 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
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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 at approximately 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
METHODS samples. 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 described
synchronized to a 12:12-hour light (6:00 to 18:00 h) and previously
[27–31]. At the conclusion of the surgery, a dark (18:00 to 6:00 h)
cycle. All rats received food 150 mmol/L NaCl bolus equal to 1% of
body weight was and water ad libitum throughout the study.
administered. Two 20-minute clearances of inulin were
obtained. Blood samples were obtained at the midpoint
Radiotelemetry
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 one posterior extrarenal branch of the left renal artery),
and Morphology and morphometrics prepared for telemetric monitoring
of BP (Data Sciences Transverse sections of the kidney through the
papilla International, St. Paul, MN, USA) at the time of the were
fixed in situ by perfusion for five minutes at the renal ablation
surgery as previously described [27–31]. measured BP with 1.25%
glutaraldehyde in 0.1 mol/L Each rat had a BP sensor inserted
intraperitoneally. The cacodylate buffer. Sections were cut at a
thickness of 2 mm sensor’s catheter was inserted into the aorta
below the and stained with hematoxylin and eosin and periodic level
of the renal arteries, and the radio frequency trans- acid-Schiff.
Sections were evaluated systemically in each mitter was fixed to
the peritoneum. The rats were housed kidney for glomerular injury
(segmental sclerosis and/or individually in plastic cages that were
placed on top of necrosis) in a blinded fashion by standard
morphologic the receiver. The signals from the pressure sensor were
methods [4, 27–31]. At least 100 glomeruli in each animal
converted, temperature compensated, and sent via the were
evaluated, and the severity of GS was expressed radio 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 results matrix that transmitted the information to the Lab
Pro that are essentially identical to an alternative method data
acquisition system. Systolic BP in each animal was that uses a
morphometric scoring method to estimate GS recorded at 10-minute
intervals throughout the course [28, 32] in terms of correlation
with proteinuria and/or of approximately seven weeks, with each
reading being BP parameters. Glomerular volume (VG) was measured
the 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). The days for the measurement of serum
creatinine as an cross-sectional area (AG) of 75 consecutive
glomerular index of the degree of renal mass reduction [29]. At
profiles contained in one kidney section for each animal
approximately 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 from doses of the ACE inhibitor benazepril (25, 50,
100 mg/L) the respective mean AG as VG 5 b/k (AG
3/2), where b 5 or the AT1 receptor antagonist losartan (50, 120,
180 mg/L) 1.38 is the size distribution coefficient and k 5 1.1 is
the of 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 sodium pentobarbital (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 a scribed 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 GFR N 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
2.660.3 Benazepril 50 12 25464 0.3860.03 3.360.5 0.8360.05 45069
35.562.6ab 3.260.3 Benazepril 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
2.860.3 Losartan 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. Untreated bP , 0.05 vs.
Benazepril 25 cP , 0.05 vs. Losartan 50
The glomerular filtration rate (GFR) was calculated us- highest
dose of losartan (180 mg/L). The differences in the 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 the average 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 in the 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 systolic BP of all
treated animals during the final six weeks wasbetween the groups
using the Minitab Software package significantly lower than their
respective average systolic(Minitab, Inc., PA, USA) [34]. A P value
of , 0.05 was BP 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 substantial
variability of the average individual systolic BP values
RESULTS within each of the subgroups receiving either the lowest
Table 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 not groups were not
significantly different from each other. statistically different
from that of the untreated control Similarly, the serum creatinine
at three days was not rats. By contrast, the average posttreatment
systolic BP significantly 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 and different 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 of 50
mg/L, or received losartan 180 mg/L, and it shows seven weeks and
the percentage of glomeruli exhibiting the persistence of
significant BP lability despite a marked GS in the remnant kidneys
of these seven groups are reduction in systolic BP. Figure 2
illustrates the course presented in Figure 4. Graded reductions in
proteinuria of weekly averages of systolic BP after ,5/6 renal
abla- and GS were seen in the benazepril- and losartan-treated tion
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
systolic average BP during the first seven days after renal abla-
BP values, statistical significance was only achieved for tion and
before the initiation of antihypertensive therapy reductions in
proteinuria and GS in the groups receiving was 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 and The 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 renal
ablation from (A) an untreated control rat, (B) a rat receiving 50
mg/L in drinking water of benazepril, and (C ) a rat receiving 180
mg/L in drinking water of losartan, showing the per- sistence of
significant BP lability despite a marked 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. An excellent 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 for all animals
combined (r 5 0.72).
Figure 6 shows the effects of RAS blockade on rem- nant kidney and
glomerular growth. The remnant kidney weight factored for body
weight was not significantly different between the treated groups
or in comparison to untreated rats. Similarly, glomerular volume
was not significantly different between the treated groups or as
compared with the untreated group. The correlations between
glomerular volume and GS were not significant within the individual
groups (untreated controls, r 5 0.36; 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
approximately were 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 either benazepril at a dose of 25
(r; N 5 10), 50 (d; N 5 12), or 100 (solid star; 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/L of 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 few exceptions, 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 subject of 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 first seven days (A) and
during the subsequent ap- proximately six weeks (B) in 5/6 renal
ablated rats that, after the seventh day, were: left un- treated
(h); received benazepril at 25, 50, or 100 mg/L of drinking H2O
(j); or received losartan at 50, 120, or 180 mg/L of drinking water
( ). 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 group during 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 renal
ablation 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 untreated controls.
nevertheless, the superiority of RAS blockade has been adequate to
ensure “equivalent BP control,” a critical prerequisite 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 when chronic 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 and their average systolic BP during the final six
weeks (the mean of all approximately 6000 BP readings in each rat).
After the first seven days, the rats 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 the relationship
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 for all 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 did not 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 results are 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 inhibitor enalapril 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 reduction
with 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 accounted for 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, in that 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 relationship between 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 pathogenesis of 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 systemic and/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 effect of 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 weeks in 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 the first 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 entry through
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 animals with 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 determinant of 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 the impaired 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 exert their
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 on the
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 in autoregulation [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 rats with 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 potential explanation 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 the present 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 arteriolar dilation 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 may not 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 lower but 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 of RAS 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 cellular effects 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 receptor antagonists 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 additional beneficial 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 inhibition and 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 blockade may 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 RAS blockade 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 extensive documentation 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 GS in 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 these rats. 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 may be 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 RAS blockade 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 of glomerular hypertension for complete
expression. In- ACKNOWLEDGMENTS deed, 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 suggested for 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 Medical Center,
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
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