(Hypertension. 1996;27:1173-1179.)
© 1996 American Heart Association, Inc.
Articles |
Role of Nitric Oxide in Short-term and Prolonged Effects of Angiotensin II on Renal Hemodynamics
From the Division of Nephrology and Hypertension, Georgetown University School of Medicine, Washington, DC.
Correspondence to Christopher S. Wilcox, MD, PhD, Georgetown University Medical Center, Division of Nephrology and Hypertension, 3800 Reservoir Rd NW, PHC F6003, Washington, DC 20007.
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Abstract Short-term infusions of angiotensin II (Ang II) increase renal vascular resistance and thereby endothelial shear stress and nitric oxide (NO) release. Prolonged stimulation of Ang II can decrease the expression of NO synthase isoforms in the macula densa, but prolonged increases in shear stress can increase transcription of endothelial NO synthase. Therefore, we designed these studies to test the hypothesis that Ang II exerts time-dependent effects on renal NO generation as assessed from renal excretion of nitrate and nitrite, percent increases in renal vascular resistance during inhibition of NO synthase with intravenous NG-nitro-L-arginine methyl ester (L-NAME), or decreases in renal vascular resistance during stimulation of endothelial NO synthase with intravenous acetylcholine. Rats were tested during graded short-term (30 to 90 minutes intravenous) or prolonged (5 to 6 days subcutaneous) Ang II infusions that led to dose-dependent increases in blood pressure and renal vascular resistance and reductions in renal blood flow. Captopril was administered for 3 to 4 days to suppress Ang II generation. The renal excretion of nitrate and nitrite was increased during short-term Ang II infusions (from 205±22 to 331±58 pmol·min-1, P<.05) but was unchanged during prolonged Ang II infusion (control group, 197±33 versus Ang II, 245±42 pmol·min-1, P=NS). The percent increase in renal vascular resistance with L-NAME was potentiated dose dependently by short-term but not long-term Ang II infusions. The increase in renal vascular resistance with L-NAME in control rats without Ang II infusions was +150±13%. At an Ang II infusion of 200 ng·kg-1·min-1, the L-NAME–induced percent increase in renal vascular resistance was significantly (P<.01) increased compared with controls in short-term Ang II–infused rats (+369±70%) but was not significantly different in prolonged infused rats (+190±33%). Intravenous acetylcholine caused dose-dependent renal vasodilation that was not significantly changed in rats receiving short-term intravenous Ang II but was significantly (P<.005) potentiated in those receiving prolonged Ang II infusions (change in renal vascular resistance with acetylcholine at 10 µg·kg-1·min-1 versus control, -21.5±5.0%; with short-term Ang II, -24.9±4.5%; with long-term Ang II, -52.1±7.2%). In conclusion, short- and long-term Ang II infusions caused equivalent changes in blood pressure and renal blood flow and hence presumably equivalent increases in endothelial shear stress. However, only short-term Ang II infusions increased NO generation and the dependence of the renal circulation on NO, whereas acetylcholine-induced NO release was enhanced selectively during long-term Ang II infusions. This suggests that during long-term Ang II, renal NO release may become uncoupled from shear stress yet remains highly responsive to receptor-mediated stimulation.
Key Words: nitric oxide • angiotensin II • renal blood flow • vascular resistance • L-NAME
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Introduction |
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Interactions between Ang II and NO in the kidney may have a critical role in governing renal hemodynamics,1 2 3 4 tubular NaCl and fluid reabsorption,5 tubuloglomerular feedback,6 and afferent arteriolar vasoconstriction.7 8 Inhibition of NOS in the rat augments the renal vasoconstrictor responses to short-term infusions of Ang II.4 However, in the conscious dog, no interaction was seen between Ang II and L-NAME on RVR when infused for 5 days.9 In our previous studies, blockade of Ang II generation with an angiotensin-converting enzyme inhibitor or of Ang II type 1 receptors with losartan over 3 days did not alter the renal vasoconstrictor response to L-NAME in rats on a high or low salt diet that provided a wide range of PRA values and presumably of plasma Ang II concentrations.10 With an antibody to a brain-type, constitutive NOS isoform, the immunocytochemical staining of the macula densa of rats is diminished by high salt intake or other maneuvers that suppress the renin-angiotensin system.11 12 The effects of Ang II on endothelial NOS expression have not been studied. However, increased shear stress on endothelial cells, as may occur during Ang II–induced vasoconstriction, enhances NOS transcription.13 Therefore, we tested the hypothesis that the buffering of the renal hemodynamic response to Ang II by NO generation would depend on the duration of Ang II infusion. NO release was assessed indirectly from the excretion of the metabolites NO2 and NO3 and the degree of buffering of the vasoconstrictor response to Ang II from the augmentation of this response by NOS blockade with L-NAME. The effects of Ang II on receptor-mediated endothelium-dependent vasodilation were assessed from the response to graded intravenous infusions of acetylcholine.
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Methods |
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Animal Preparation
Male Sprague-Dawley rats (250 to 350 g) from Harlan Sprague-Dawley, Indianapolis, Ind, were maintained on tap water and a standard rat chow (Teklad, Inc) with a high sodium content of 6 g·100 g-1 for 10 to 14 days before study. The high salt diet was given to suppress endogenous PRA and enhance renal NO generation.10 14 15 All rats received captopril in the drinking water (0.25 mg·mL-1) for 3 to 4 days before study. This was given to reduce endogenous Ang II generation, enhance the range of Ang II levels for studies, and prevent any confounding effects from changes in renin release caused by L-NAME infusion.16 17 18 Rats drank 30 to 50 mL·24 h-1 and therefore consumed 7 to 12 mg·kg-1·24 h-1 of captopril. On the day of the experiment, rats received an additional dose of 10 mg·kg-1 captopril IV with surgery. Pilot studies showed that this captopril dose significantly (P<.05) increased the expected Ang I dose that would raise MAP by 30 mm Hg (ED30) from 0.97±0.13 µg·kg-1 in normal rats (n=7) to 4.9±1.6 µg·kg-1 in captopril-pretreated rats (n=4). On the day of the study, rats were anesthetized with thiobutabarbital (Inactin, 100 mg·kg-1 IP, Research Biochemicals International) and maintained at 37°C on a servo-controlled heated rodent operating table. A tracheostomy was performed with PE-240 tubing. Both external jugular veins were cannulated with PE-50 tubing (Intramedic, Clay Adams). One cannula was used for infusion of albumin (6%, Sigma Chemical Co) dissolved in 0.154 mol/L NaCl infused at 1 mL·h-1. The other was used for infusion of drugs dissolved in 0.154 mol/L NaCl at 1 mL·h-1. The left femoral artery was cannulated for blood sampling and measurement of MAP from the electrically dampened output of a pressure transducer (Statham model P23, Gould Instruments) and recorded on a polygraph (Linearcorder, Type WR 3101, Graphtec Corp). The bladder was catheterized through the suprapubic route for urine collection. The abdomen was opened via a midline incision. The left renal artery was cleaned and a blood flow probe (1RB) was placed around it and connected to a transit-time blood flowmeter (Transonic Systems, Inc). RBF was recorded on the polygraph. This transit-time method has been validated in our laboratory.19 RVR was calculated from MAP factored by RBF. Rats receiving short or prolonged Ang II infusions were prepared identically, except for the implantation of the subcutaneous minipumps 5 days before study in those receiving prolonged Ang II infusions.
Twenty minutes were allowed for equilibration after completion of surgery before any measurements were made. Blood (0.3 mL) for PRA was sampled and replaced with an equal volume of albumin/saline solution. MAP and RBF were measured. Pilot studies demonstrated that changes in MAP and RBF with short-term infusions of Ang II were maximal and stable after 15 to 20 minutes. Therefore, the pre–L-NAME measurements of MAP and RBF in the short-term Ang II study were made 30 minutes after the Ang II infusions were started. Thereafter, rats received an infusion of L-NAME at 11.11 µmol·kg-1·min-1. We had found previously that this dose produced a maximal and stable increase in RVR that peaked at 15 to 20 minutes. Therefore, data for analysis were taken at this time point. At the end of the study, the rats were euthanatized and kidneys removed and weighed.
Chemical Methods
Nitrite and nitrate concentrations in urine were estimated by a
modification of the method of Gilliam et al.20 All
reagents were from Sigma. For determination of nitrate, urine samples
and sodium nitrite standard (0 to 100
µmol·L-1) were reduced by
incubation of 50 µL of the sample or standard with 35 µL NADPH (12
mmol/L), 25 µL flavin adenine dinucleotide (1
mmol/L), and 25 µL nitrate reductase (2
U·mL-1) at room temperature
overnight. Thereafter, 100 µL of supernatant was added to 100 µL of
1% sulfanilamide in 30% acetic acid and 100 µL of 0.1%
N-(1-naphthyl)ethylenediamine dihydrochloride in 60%
acetic acid (Griess reagent). After mixing, the optical density was
read on a spectrophotometer at 570 nm with a microplate reader (MR 600,
Dynatech Laboratories, Inc). The conversions of nitrate to nitrite were
90% to 100% under the experimental conditions used. The
NO2 measured in this way reflects the sum of
NO2 and NO3 in the original sample.
Blood for PRA was drawn into EDTA-containing tubes and the plasma separated and stored at -20°C. For assay, samples were thawed to 4°C, and the rate of Ang I generated over a 90-minute incubation at 37°C was assessed by radioimmunoassay.21
Experimental Protocols
Series 1 (Groups 1 Through 4): Effects of Short-term Ang II
Infusion on the Response to L-NAME
The aim of these studies was to determine the dose-response
relationship for the effects of short-term Ang II infusion on the
hemodynamic response to NOS inhibition. On the day of
the experiment, rats were anesthetized and infused
intravenously with vehicle (group 1, n=8) or Ang II at 10
(group 2, n=7), 40 (group 3, n=10), or 200 (group 4, n=6)
ng·kg-1·min-1.
Thereafter, infusions were maintained, and the
hemodynamic response to L-NAME was assessed.
Series 2 (Groups 5 Through 7): Effects of Prolonged Ang II Infusion
on the Response to L-NAME
The aim of this study was to assess the dose-response
relationships for effects of prolonged infusion of Ang II on the
hemodynamic response to NOS inhibition in groups of
rats with elevations of RVR similar to those in the acute Ang II
series. Rats were anesthetized with inhalation of halothane
(2-bromo-2-chloro-1,1,1-trifluoroethane, Halocarbon). Osmotic minipumps
(Alza Corp) were primed with Ang II to deliver infusions of 10 (group
5, n=9), 200 (group 6, n=6), or 1000 (group 7, n=5)
ng·kg-1·min-1.
The minipumps were inserted subcutaneously via a dorsal incision, and
the rats were allowed to recover. Five to 6 days later, rats were
studied under thiobutabarbital anesthesia with the same
protocol as used for series 1. Rats receiving prolonged Ang II
infusions also received captopril in the drinking water as in series
1.
Additional subgroups of rats were studied to test the effects of captopril pretreatment. Rat groups were prepared similarly to others in series 2 with Ang II infused at 0 (n=10), 10 (n=8), 200 (n=6), or 1000 (n=12) ng·kg-1·min-1 via osmotic minipumps, but these groups did not receive captopril pretreatment. The experiments followed a course similar to that in series 2.
Series 3 (Groups 8 Through 10): Effects of Short- and Long-term
Ang II Infusion on the Response to Acetylcholine
The aim of this series was to study the effects of short- and
long-term Ang II infusion on the response to an
endothelium-dependent vasodilator.22
Control rats (group 8, n=7) or rats receiving short-term
intravenous Ang II at 200
ng·kg-1·min-1
(group 9, n=9) or prolonged subcutaneous Ang II at 1000
ng·kg-1·min-1
(group 10, n=7) were prepared as in series 1 and 2 above. These groups
also received captopril pretreatment to prevent confounding effects
from acetylcholine-induced changes in renin release.16
Under anesthesia, rats received graded doses of
acetylcholine at 0, 0.1, 0.3, 1.0, 3.0, and 10.0
µg-1·kg-1·min-1.
After 20 minutes, stable values for MAP and RBF were obtained.
Therefore, data for analysis were taken at this point.
For groups 1, 4, and 7, urine was collected over 30 minutes with rats under anesthesia for measurement of NO2+NO3 excretion. For group 4, collections were made before and during short-term infusion of Ang II at 200 ng·kg-1·min-1. For groups 1 and 7, collections were made in the basal state, whereas the rats received no Ang II (group 1) or Ang II via osmotic minipump at 1000 ng·kg-1·min-1 (group 7).
Drugs
L-NAME, acetylcholine, captopril, and Ang II were from Sigma
Chemical Co.
Statistics and Data Analysis
The Ang II infusions had significant effects on the baseline
parameters before L-NAME infusion. The absolute changes in
RVR with L-NAME depended on basal RVR (r=.50,
P<.001). However, the fractional changes in RVR with L-NAME
were independent of basal RVR (r=.14, P=NS).
Therefore, we analyzed fractional changes in response to
L-NAME.23 All values are expressed as mean±SE. The
changes in hemodynamics from baseline in each rat group
receiving acetylcholine were analyzed with a within-groups
ANOVA with repeated measures, and when a significant difference was
obtained, a post hoc Dunnett's test was applied. The differences for
parameters measured in the control group that received no
Ang II and the treatment groups were compared by between-groups
ANOVA with post hoc Bonferroni tests applied as appropriate.
Differences were considered significant at a value of
P<.05.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The Table
shows body and kidney weights, rates of
Ang II infusion, and MAP, RBF, RVR, and PRA measured before L-NAME
infusion. Rats receiving the highest dose of Ang II by minipump lost
body weight, although their kidney weights were not significantly
different from those of the control group. Short-term Ang II
infusions led to graded increases in MAP and RVR, with reductions in
RBF. Prolonged Ang II infusions caused a similar pattern of responses
but were slightly less effective in increasing RVR or MAP. Therefore, a
rat group was infused via a minipump at a higher rate (1000
ng·kg-1·min-1)
to provide a range of RVR values that spanned the values obtained at
the highest rate of short-term Ang II infusion. The PRA of the
control group was high, despite the high salt intake, reflecting the
effects of captopril pretreatment. Ang II infusions reduced PRA; this
effect was most prominent in those rats receiving the prolonged Ang II
infusions.
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L-NAME infusion increased MAP and RVR and decreased RBF in all rat
groups studied. We calculated the percent changes in RVR with L-NAME
because these were independent of the baseline values for RVR (see
"Methods"). Fig 1 shows the relationship between
the Ang II infusion rate and the percent changes in MAP, RBF, and RVR
with L-NAME. The percent increase in MAP was similarly reduced at the
highest doses tested for short-term and prolonged Ang II infusions
(Fig 1A). The percent reduction in RBF with L-NAME was significantly
enhanced during short-term but not prolonged Ang II infusions (Fig 1B). The percent reduction in RBF with L-NAME during short-term Ang
II infusion at 200
ng·kg-1·min-1
was significantly (P<.05) greater than that during
prolonged Ang II infusions at either 200 or 1000
ng·kg-1·min-1.
Short-term Ang II infusions led to graded and steep potentiation of
the percent increases in RVR with L-NAME, whereas prolonged Ang II
infusion did not significantly alter the response (Fig 1C). The percent
increase in RVR with L-NAME during short-term Ang II infusions at
200
ng·kg-1·min-1
was significantly greater than that during prolonged Ang II infusions
at either 200 (P<.02) or 1000 (P<.05)
ng·kg-1·min-1.
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) or
prolonged subcutaneous Ang II infusions by osmotic minipump (
).
Mean±SE is shown. *P<.05 compared with control.
Differences between short and prolonged treatment are shown at the two
comparable doses of Ang II at the top of the figures.In separate subgroups, we tested the possibility that captopril pretreatment had altered the response to L-NAME. Four additional rat groups were studied after infusion via an osmotic minipump of vehicle or Ang II at 10, 200, or 1000 ng·kg-1·min-1 in a protocol similar to that of series 2 except that captopril was not administered. The percent changes in MAP, RBF, and RVR with L-NAME infusion did not change significantly comparing data from this group of rats with data from series 2.
We tested the effects of Ang II on the response to an
endothelium-dependent vasodilator in series 3. The
RVR values for control rats and those receiving Ang II were similar to
values in comparable groups in series 1 and 2 (data not shown).
Acetylcholine infusion led to dose-dependent decreases in MAP with
maintained or increased RBF, leading to dose-dependent reductions
in RVR. As shown in Fig 2, although rats receiving
short-term or prolonged Ang II infusions had similar depressor
responses to acetylcholine (Fig 2A), only those receiving prolonged Ang
II infusions had increases in RBF (Fig 2B). Consequently, the decrease
in RVR with acetylcholine was more pronounced in those receiving
prolonged Ang II infusions (Fig 2C). Another group of five rats was
studied after 5 to 6 days of Ang II infusion as in group 7. These
rats were also infused with L-NAME (10
µg·kg-1·min-1)
during the basal period and during testing with acetylcholine (3
µg·kg-1·min-1).
In this group, RVR increased with acetylcholine (29.9±3.0 to 46.6±9.1
mm
Hg·mL-1·min-1·g-1,
P<.05) in contrast to the reduction in RVR seen in rats in
group 7 that received acetylcholine alone. This indicates that the
renal vasodilator response to acetylcholine depended on NOS.
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), rats receiving
short-term Ang II infusions at 200
ng·kg-1·min-1
IV (group 9, n=9,
The renal excretion of nitrate plus nitrite was assessed over 30-minute
clearance periods during the highest rates of short-term and
prolonged Ang II infusions. As shown in Fig 3A,
short-term Ang II infusions increased excretion of
NO2+NO3 significantly, by 65%. In contrast,
NO2+NO3 excretion did not differ significantly
between control rats and those infused with Ang II at 1000
ng·kg-1·min-1
for 5 days (Fig 3B). In another study, 24-hour urine samples from
conscious rats were analyzed for
NO2+NO3. Control rats excreted 1267±207
nmol·24 h-1 (n=11). The results were
not significantly different from those in rats receiving Ang II at 1000
ng·kg-1·min-1
via osmotic minipumps (1407±148 nmol·24
h-1, n=8).
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Discussion |
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Ang II and NO have been assigned major roles as counterbalancing regulatory systems in the kidney. The renin-angiotensin system is stimulated during dietary salt restriction or hypovolemia and coordinates a renal response of vasoconstriction and enhanced tubular fluid reabsorption. In contrast, the L-arginine–NO pathway in the kidney is augmented during salt loading and can buffer vasoconstrictive influences on the renal vessels.10 14 15 However, the details of the interaction between these two systems in the kidney remain incompletely understood. The present study showed that short-term infusions of Ang II into anesthetized rats increase the excretion of NO2 and NO3 and lead to dose-dependent increases in NO action on the renal resistance vessels (as assessed from percent changes in RVR with L-NAME). In contrast, more prolonged infusions of Ang II over 5 days do not increase the excretions of NO2 and NO3 nor the action of NO on the renal resistance vessels. These data indicate that the renal L-arginine–NO pathway is engaged by short-term but not prolonged infusions of Ang II and offsets the effects of Ang II on the kidney. However, acetylcholine led to dose-dependent renal vasodilation that was potentiated by long-term but not short-term Ang II infusion. Therefore, receptor-mediated, endothelium-dependent NO generation and response are well preserved in the kidneys of rats during prolonged Ang II infusions.
NO is metabolized via intermediates to nitrate and nitrite, which are excreted in the urine. Therefore, the excretion of NO2 and NO3 has been used as an index of NO generation.15 However, nitrite clearance by the kidney depends on glomerular filtration rate and urine flow and has been found to be a less-reliable indicator of NO generation in acute studies or under non–steady-state conditions.24 Therefore, the increase in NO2+NO3 excretion found during short-term collection under anesthesia should be interpreted with some caution. On the other hand, NO2+NO3 excretion measured in 24-hour urine samples in control rats and rats with prolonged Ang II infusion should be a more-reliable approximation of NO generation. The results in this group of rats that was not receiving captopril pretreatment also showed no increase in NO2+NO3 excretion in those receiving the high-dose, prolonged Ang II infusion compared with their control group. The finding of increased NO2+NO3 excretion during short-term Ang II infusion but not during prolonged Ang II infusion is consistent with the finding of an increase in the percent rise of RVR with L-NAME during short-term Ang II infusion but not during prolonged Ang II infusion. Together, these data suggest that NO generation is increased only during short-term Ang II infusions.
Glomerular endothelial cells in culture release NO in response to calcium-mobilizing agonists such as bradykinin and acetylcholine. However, Ang II does not increase intracellular calcium or NO release by these cells.25 Therefore, it is likely that the predominant effects of Ang II on endothelial NO generation are mediated indirectly by changes in shear stress. Vessel wall shear stress during physiological states is likely to vary substantially at different segments of the renal vasculature. However, for any given vessel, shear stress increases directly with blood flow and inversely with a power function of vascular radius. At equivalent rates of Ang II infusion at 200 ng·kg-1·min-1, there was a significantly greater reduction in RBF during short-term Ang II infusions. Therefore, a group of rats was studied during prolonged Ang II infusion at a higher rate of 1000 ng·kg-1·min-1. When the physiological data before L-NAME infusion are compared in these two groups, rats infused in the short term at 200 ng·kg-1·min-1 or in the long term at 1000 ng·kg-1·min-1 had the same RBF of 5.2 mL·min-1·g-1, but rats receiving the more-prolonged infusion had a significantly higher blood pressure and RVR. Since RBF was the same yet RVR was increased, this suggests that the diameters of the main renal resistance vessels were less during prolonged compared with short-term Ang II infusions and consequently that endothelial shear stress on these vessels was greater during the prolonged Ang II infusions. Despite this greater stimulus to endothelial NO generation during prolonged Ang II infusions, NO2+NO3 excretion and the percent increase in RVR with L-NAME were increased only during the short-term Ang II infusions. We conclude that shear stress–induced increases in renal NO generation are lost during prolonged Ang II infusions.
Angiotensin-converting enzyme inhibitors can potentiate NO release from vascular endothelium, perhaps by inhibiting kininase II and thereby increasing tissue levels of bradykinin, which is a potent stimulus to NO generation.26 However, in confirmation of our previous studies, 3 days of captopril administration did not alter the RVR response to L-NAME.10 Moreover, during prolonged Ang II infusion, rats had the same increase in baseline RVR and a similar percent increase in RVR with L-NAME regardless of whether they received the 3 days of captopril pretreatment. These data indicate that the captopril pretreatment schedule in rats receiving a high salt diet did not significantly modify the renal L-arginine–NO pathway or its response to Ang II in this experimental setting. Changes in renin release or endogenous Ang II generation were not the major cause of L-NAME–induced increases in RVR in these studies.
In the present study, short-term Ang II infusion did not alter the pressor response to L-NAME except at the highest dose, during which a decreased pressor response was seen. Several previous studies have shown that the short-term pressor response to NOS blockade is largely independent of the renin-angiotensin system.1 2 4 27 28 29 30
Several investigators have manipulated the renin-angiotensin system to determine its role in mediating the renal vasoconstrictor response to NOS inhibition. Ohishi et al8 showed that both afferent and efferent arterioles taken from rats pretreated with Ang II antagonists had a diminished vasoconstrictor response to NG-nitro-L-arginine. Sigmon and colleagues1 2 29 have shown that the increase in RVR with NOS inhibition is blunted in anesthetized rats in which the renin-angiotensin system was severely suppressed by deoxycorticosterone acetate and saline drinking,1 ACE inhibition,29 or losartan administration.2 29 In contrast, Baylis et al31 showed that the renal effects of the endothelium-derived relaxing factor in conscious rats are not mediated by Ang II. Sigmon and Beierwaltes2 attributed the difference between their results and those of Baylis et al to the effect of anesthesia in stimulating Ang II generation. However, we have found no significant effect of 3 days of pretreatment with captopril or losartan on the renal vasoconstrictor response to L-NAME in anesthetized rats adapted to a high or low salt diet.9 In the present study, 3 days of captopril administration did not modify the increase in RVR produced by L-NAME administration to rats studied under anesthesia. The reasons for these discrepant results have not been identified but cannot be ascribed entirely to effects of anesthesia or basal PRA values. Therefore, we studied the role of NO in offsetting Ang II–induced renal vasoconstrictor responses during controlled studies with Ang II infused in the short and long term across a wide range of concentrations.
The present studies with short-term Ang II infusion confirm results of Alberola et al3 and Baylis et al4 that NOS blockade selectively augments the renal vasoconstrictor response to short-term infusions of Ang II in dogs or rats. In the present study, the effect was dose dependent and was seen only at rates of Ang II infusion that increased basal RVR. Further evidence that Ang II was releasing NO in these studies came from the observation that there was a concomitant increase in NO2+NO3 excretion at the highest dose of short-term Ang II infusion studied. Manning et al9 found in the conscious dog only additive effects on RBF of prolonged intravenous infusions of Ang II and L-NAME given over 5 days, although L-NAME did potentiate the fall in glomerular filtration rate. In the present study, the effects of short-term and prolonged Ang II infusions were contrasted in rats at similar levels of dietary salt intake, anesthesia, and surgical manipulation. As in the studies of Manning et al in the dog, prolonged Ang II infusions did not potentiate the fall in RBF with L-NAME. Moreover, the renal excretion of NO2+NO3 did not increase.
The reason for the failure of prolonged Ang II infusion to increase NO generation has not been established. Previous studies have shown that Ang II can inhibit the induction of NOS by cytokines in vascular smooth muscle cells.32 Moreover, stimulation of the renin-angiotensin system by a low salt intake decreases the immunocytochemical staining of macula densa cells with an antibody to brain-type constitutive NOS.11 12 On the other hand, NOS transcription in endothelial cells in culture is stimulated by prolonged increases in endothelial shear stress, as should occur during prolonged infusions of Ang II.13 Therefore, we tested the response to graded acetylcholine infusions to determine whether endothelial NOS was depleted or inactivated during prolonged Ang II infusions to account for the diminished response seen previously to L-NAME infusions. Acetylcholine leads to renal vasodilation that depends on NOS because it is blocked by monomethyl-L-arginine.22 The present results in the intact kidney showed that the renal vasodilator response to acetylcholine was dose dependent and potentiated during prolonged but not during short-term Ang II infusions. These data show that prolonged Ang II infusions selectively blunt the coupling between shear stress–induced activation of NOS in the renal vascular endothelium but potentiate receptor-mediated activation of the response. These changes are presumably not a response to high blood pressure itself because the response of blood vessels to acetylcholine and NOS blockade in essential hypertension has been found to be depressed in most, but not all, studies.33
An increase in NO generation during short-term increases in Ang II buffers the renal circulation against abrupt changes in renin release and Ang II generation. On the other hand, the loss of this buffering action on blood pressure and renal hemodynamics during more-prolonged increases in Ang II should enhance its actions on the circulation. This could contribute to the "slow pressor effect," whereby prolonged infusions of Ang II lead to progressive increases in blood pressure rather than to tachyphylaxis.34 Moreover, a failure of long-term Ang II infusion to enhance NO generation could contribute to some of the other important long-term effects of Ang II on vascular growth and remodeling.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by a grant from the National Institutes of Health to Christopher S. Wilcox (RO1, DK49870) and from the George E. Schreiner Chair of Nephrology.
Received August 21, 1995; first decision September 19, 1995; accepted December 19, 1995.
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References |
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