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(Hypertension. 1997;30:1479-1486.)
© 1997 American Heart Association, Inc.
Articles |
Regional Renal Nitric Oxide Release in Stroke-Prone Spontaneously Hypertensive Rats
From the Departments of Pediatrics (A.Z.), Pathology (P.N.C.), and Pharmacology (C.T.S.), New York Medical College, Valhalla, NY, and the Department of Medicine (G.A.Z.), Veterans Administration Hospital, Bronx, NY.
Correspondence to Andrea Zuckerman, MD, Department of Pediatrics, New York Medical College, Valhalla, NY 10595.
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Abstract |
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Abstract Diminished nitric oxide (NO) production has been implicated in the pathogenesis of salt-sensitive hypertension. We questioned whether such a defect is responsible for the malignant hypertension and nephrosclerosis in stroke-prone spontaneously hypertensive rats (SHRSP) fed a high-salt/stroke-prone diet (S) versus a regular diet (R). NO release from 30-minute incubates of cortex and outer and inner medulla were studied in SHRSP at 10, 12, and 16 weeks of age on the S diet versus R diet. SHRSP-S (n=16) exhibited a marked age-dependent increase in NO release, especially in the cortex. Increases were only modest in SHRSP-R (n=21). At 16 weeks, cortical NO was 93±25 versus 6±1 pmol/mg tissue in SHRSP-S versus SHRSP-R (P<.001). Immunohistochemical staining increased mostly for neuronal, slightly for endothelial, and negligibly for inducible isoforms of NO synthase and was predominantly in the cortex of SHRSP-S versus SHRSP-R. Despite similar hypertension in SHRSP-S versus SHRSP-R (mean arterial pressure, 174±7 versus 177±2 mm Hg), malignant nephrosclerosis was seen only in SHRSP-S, affecting 22±6% of glomeruli and 23±4 vessels per 100 glomeruli by 16 weeks. N
-nitro-L-arginine
(15 mg/kg per day) in SHRSP-S (n=6) abrogated the increase in cortical
NO but further augmented the hypertension and accelerated lesion
development. Wistar-Kyoto rats at 16 weeks on the R diet (n=8) had NO
levels similar to those of SHRSP-R, showed increased cortical NO to
only 28±10 pmol/mg on the S diet (n=9) (P<.05 versus
SHRSP-S), but remained normotensive and lesion-free. We conclude that
hypertension and lesion development in SHRSP are not due to deficient
renal NO. Accelerated onset of malignant
nephrosclerosis by NO synthase inhibition suggests that
NO is protective in these animals, mitigating the effects of
hypertension and S diet on renal pathology.
Key Words: rats, inbred SHR • rats, Wistar-Kyoto • malignant nephrosclerosis • sodium • N-nitro-L-arginine • nitric oxide synthase
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Introduction |
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Several studies have suggested that enhanced NO synthesis in response to increased sodium intake is important in the regulation of vascular tone and maintenance of sodium homeostasis.1 2 3 4 Defects in NO production have been postulated to play a key role in the development of salt-sensitive hypertension.5 Whether a defect in endogenous NO production contributes to hypertensive end-organ damage has not been clearly established.
SHRSP are known for their propensity to develop severe hypertension, with stroke, and malignant nephrosclerosis, which is not apparent before 20 weeks of age when fed a standard rat chow. The evolution of end-organ damage is accelerated by high dietary salt intake, especially in animals fed stroke-prone rodent chow. These animals typically exhibit proteinuria by 11 weeks of age, with frank lesions of malignant nephrosclerosis by 16 weeks of age.6 Further acceleration can be achieved by the addition of the NOS inhibitor L-NNA starting at 8 weeks of age. These animals develop proteinuria within 24 hours of initiation of treatment and renal lesions 1 to 2 weeks later.7 8 Whether the detrimental effects of NO blockade in that study were due to further reductions of an already inadequate NO response to the high-salt/stroke-prone diet in these animals or to greater dependence on the vascular protective functions of NO is not known.
To determine whether lesion development in SHRSP is due to a diminished ability of the kidney to produce NO, we compared renal NO release in SHRSP fed the high-salt/stroke-prone diet versus standard Purina chow and water. Comparisons were also made with rats of the progenitor strain, WKY, since these animals remain normotensive and lesion-free even on the high-salt/stroke-prone diet.9 Immunohistochemical staining of the renal tissue for three isoforms of NOS was performed to determine whether variations in NOS exist at specific sites in the nephron or vasculature. Finally, NO release was measured in SHRSP on the high-salt/stroke-prone diet plus L-NNA to further understand the importance of NO production in renal lesion development.
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Methods |
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Animals
Studies were conducted in accordance with institutional guidelines using male SHRSP/A3N (generation F73–76), n=37, and WKY/N (generations F60–61), n=17, from our local colonies. All animals were initially allowed free access to tap water and Purina Laboratory Chow 5001 (Ralston-Purina). At 8 weeks of age, some animals (denoted SHRSP-S) were given the stroke-prone rodent diet #39-288 (Zeigler Brothers, Inc) and 1% NaCl in the drinking water. This diet has a lower potassium and protein content than standard chow, and in conjunction with high salt accelerates the development of hypertensive end-organ pathology, as described previously.10 Littermate control animals, which continued to receive the regular (R) diet of Purina Laboratory Chow and water throughout the study period were designated SHRSP-R. SHRSP were killed at 10, 12, and 16 weeks of age, corresponding to pre-, early and late stages of renal lesion development. Six additional SHRSP (designated SHRSP-SL) received the S diet plus L-NNA (Sigma), 7.5 mg/kg twice daily by gavage starting at 8 weeks of age, and were pair-sacrificed at 10 weeks with littermate controls. WKY, given the R or S diet by the same protocol, were killed at 16 weeks of age for comparative purposes.
Surgical Preparation of Animals
At the end of the study, the animals were weighed and
anesthetized with Inactin (Lockwood) 100 mg/kg IP. Body
temperature was maintained at 37°C using a heating lamp connected to
a temperature regulator and a rectal thermistor probe. The trachea was
cannulated with PE-240 tubing (Clay-Adams), and the animal was allowed
to breathe spontaneously. The abdominal aorta was exposed through a
midline incision and cannulated with a blunt 19-gauge butterfly needle
connected to tubing containing heparinized saline (30 IU/mL) for
arterial blood pressure measurement. Mean
arterial blood pressure was monitored using a COBE CDX III
fixed-dome transducer connected to a DIGI-Med blood pressure
analyzer (Micro-Med, Inc), which in turn was connected to a
DPU-411 thermal printer. Loose sutures were placed around the left and
right renal hilar vessels and the aorta superior to the right kidney in
preparation for the subsequent perfusion. The abdominal incision was
closed, and the blood pressure and heart rate were allowed to
stabilize. A preterminal measurement of arterial blood
pressure was obtained over the subsequent 30-minute period.
Preparation of Kidney
After the last pressure measurement was obtained, the abdomen
was reopened, and 0.1 mL of sodium heparin (1000 IU/mL) was injected
into the left renal vein. The sutures around the aorta and the right
renal hilar vessels were then tightened, and the right kidney was
excised. The left kidney was perfused in a retrograde fashion through
the abdominal aorta with 30 mL of mKRB composed of (mmol/L) NaCl
118, dextrose 11, KCl 4.7, KH2PO4 1.2,
NaHCO3 25, and CaCl2 2.5, pH 7.4,11
and warmed to 37°C. The blood-free left kidney was removed and placed
immediately into ice-cold mKRB before sectioning for NO generation
studies. Midcoronal sections of the right kidney and one from the left
kidney were fixed in 10% neutral-buffered formalin for
histopathological and immunohistochemical analysis. The left
kidney was decapsulated and sectioned while on ice. Approximately 150
mg each of renal cortex and outer medulla and 80 mg of inner medulla
were dissected from 2-mm tissue sections, diced, and placed in separate
vials containing 1 mL of ice-cold mKRB.
Tissue Incubation
Vials containing tissue were covered with parafilm and placed in
a 37°C bath, and the incubation medium was gassed with a mixture of
95% O2 and 5% CO2. After 30 minutes, the
tissues were removed, blotted dry, and weighed, and the incubation
medium was frozen at -20°C until the time of NO assay.
NO Assay
NO released by tissues in vitro into aqueous,
oxygenated solutions at neutral pH in the absence of
hemoglobin is converted rapidly, and virtually completely, to
nitrite.12 Therefore, NO release by the kidney was measured
as nitrite concentration in the incubation medium using the method of
Bush and coworkers.13 Briefly, acidic vanadium chloride was
heated to 95°C in a purging chamber (Radical Purger, Sievers). The
sample was added through a septum into the reaction chamber. The
released NO was transported by a flow of pure nitrogen into the
chemiluminescence chamber of the NO analyzer (model 270B,
Sievers). All samples were run in duplicate. A Hewlett-Packard
integrator (model IIP3396) on-line with the NO analyzer was
used to analyze analog signals from the detector. The standard
curve for nitrite was linear over the range of 0 to 6
µmol/L, and all samples fell within this range. NO release was
recorded in picomoles per milligram of tissue.
Histopathology
Kidney slices were embedded in paraffin according to standard
techniques, and 2- to 3-µm sections were cut for histopathology and
immunohistochemical staining. For histopathology, sections were stained
with hematoxylin and eosin and examined for lesions of thrombotic
microangiopathy in glomeruli and blood vessels. Histopathology was
evaluated in a blinded fashion from a single
representative midcoronal section from each of the 60
animals studied. An average of 192 glomeruli per midcoronal section
(range, 170 to 239) was counted. Glomerular pathology and
vascular pathology were expressed as the number of lesional glomeruli
and vessels, respectively, per 100 glomeruli examined.
Immunohistochemistry
Deparaffinized cut sections from 16-week-old animals were
stained for neuronal NOS (nNOS), endothelial
cell–derived NOS (ecNOS), and macrophage-type iNOS using the
avidin-biotin-horseradish-peroxidase complex technique (Vectastain ABC
kit, Vector). As specified by Alexis Corp (San Diego, Calif) on the
basis of Western blotting, the antiserum to nNOS detects a 160-kD band
from porcine brain; the antiserum to bovine ecNOS detects a 140-kD band
in rat, human, and mouse samples; the antiserum to mouse
macrophage iNOS detects a 130-kDa band in rat, human, and mouse
tissues; and there is no cross-reactivity between the three antisera.
The nNOS antiserum has been shown to stain the macula densa of rat
kidneys, and the specificity has been additionally verified by
preabsorption with purified nNOS from porcine cerebellum, which
abolished the immunoreactivity.14
The sections were first incubated with blocking serum (horse) for 20 minutes before staining with rabbit antiserum to nNOS (1:1000 dilution), ecNOS, or macrophage-type iNOS (1:50 dilution) overnight at 4°C. A step-section on each slide was similarly treated with horse serum alone as a negative control. After a washing with PBS, sections were incubated with biotinylated goat anti-rabbit antibody (1:50 dilution; Transduction) for 30 minutes. The sections were again washed and then quenched with 3% H2O2 for 30 minutes to remove nonspecific peroxidases. After washing, the Vectastain ABC reagent was overlaid on the section for 30 minutes. The sections were rinsed and then incubated with the peroxidase substrate 3'3-diaminobenzidine in urea hydrogen peroxide (Sigma), and the staining was enhanced with incubation in osmium (Stevens Metallurgical). The sections were counterstained with periodic acid-Schiff and hematoxylin.
Statistical Analysis
Data were analyzed for differences between groups by
ANOVA using version 1.1 of the BMDP New System software package (BMDP
Statistical Software, Inc). Values of P<.05 were considered
statistically significant. Data were reported as mean±SE.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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NO Release
Fig 1
shows the cortical, outer
medullary, and inner medullary NO release by SHRSP-R and SHRSP-S at 10,
12, and 16 weeks of age. Cortical NO was significantly higher in
SHRSP-S compared with SHRSP-R at all ages. SHRSP-S exhibited a marked
age-dependent increase in cortical NO (P<.05 for 10 versus
16 and 12 versus 16 weeks of age), whereas SHRSP-R showed only modest
increases with time (P<.01 for 10 versus 12 and 10 versus
16 weeks of age). As a consequence, cortical NO was markedly different
between the groups at 16 weeks. Significant differences between SHRSP-R
and SHRSP-S were also seen for outer medullary NO release at 12 and 16
weeks of age. There was a tendency for outer and inner medullary NO
release to increase with age in both groups, with significant
differences in outer medullary NO between 10 and 12 weeks for SHRSP-S
(P<.05) and in inner medullary NO release between 10 and 16
weeks in SHRSP-R (P<.01).
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P<.01, and 
P<.001 for S diet vs R diet at
the same age.
Cortical and outer and inner medullary NO in WKY-R and WKY-S were
obtained at 16 weeks of age (Fig 2).
Relative to WKY-R, WKY-S showed a significant increase only in cortical
NO, although there was a trend toward increased outer medullary NO as
well. Compared with SHRSP at 16 weeks of age, there was no difference
in NO release between WKY-R and SHRSP-R for each of the three regions.
The increase in cortical NO in WKY-S was substantially less than that
seen in SHRSP-S (P<.05).
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Mean Arterial Pressure
Mean arterial pressure did not change over the study
period for SHRSP-R and SHRSP-S and did not differ between the two
groups at any time point (Fig 3A). Mean
arterial pressure in WKY was substantially lower than
SHRSP(P<.001) and was not affected by diet (88±2
mm Hg in WKY-R versus 98±6 mm Hg in WKY-S).
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Histopathology
Fig 3B
and 3C summarizes the renal histopathologic findings in
SHRSP. No significant renal pathology was noted in SHRSP-S kidneys at
10 weeks of age or in SHRSP-R kidneys through 16 weeks of age. In
contrast, cortical lesions, involving both glomeruli and vessels, were
present in SHRSP-S by 12 weeks of age. Although there was a trend
toward increased glomerular and vascular pathology in the
16- versus 12-week-old SHRSP-S, the differences did not reach
statistical significance. There was no correlation between the level of
cortical NO production and the degree of glomerular
or vascular pathology in the 12- and 16-week-old SHRSP-S
(P>.7, Spearman's rank order correlation).
Representative photomicrographs of renal cortex from
WKY-R, WKY-S, and SHRSP-R at 16 weeks of age are shown in Fig 4A, 4B and 4C, respectively. Lesions of
malignant nephrosclerosis were absent in all of the
sections obtained from these three groups. Lesions of thrombotic
microangiography characteristic of malignant
nephrosclerosis were well-developed in the cortex of
SHRSP-S (Fig 4D). These consisted primarily of ischemic
retraction or thrombosis/necrosis of glomerular capillary
tufts, glomerular endocapillary or extracapillary cellular
swelling and proliferation, fibrinoid necrosis or thrombosis of small
arteries and arterioles (microvessels), and concentric proliferative
arteriolopathy. In the vasculature, proliferative lesions showed slight
predominance over necrotizing and thrombotic lesions in small arteries
and arterioles. Fibrinoid necrosis and focal myointimal fibroplasia of
arcuate-size arteries were only occasionally seen. No
appreciable leukocytic infiltrate was observed in glomeruli or
interstitium. The histopathologic analysis of the left kidney
sections confirmed that the perfused kidney was virtually free of
blood.
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Immunohistochemistry
The most impressive alteration in immunostaining
among the four groups of animals was for nNOS in SHRSP-S, especially in
the cortex (Fig 5). Staining for nNOS in
WKY-R, WKY-S, and SHRSP-R was moderately intense in the cortical distal
tubules and collecting ducts and in outer medullary thick ascending
limbs. Staining was also intense in the macula densa but tended to be
reduced in WKY-S. There was minimal focal staining in the media of
blood vessels, with consistent but mild staining evident in the
endothelium of arcuate and larger arteries.
Virtually no staining for nNOS was seen in proximal tubules or
glomeruli. In contrast, SHRSP-S displayed marked de novo expression in
proximal tubules and glomeruli, prominently in visceral epithelial
cells, but also in many parietal epithelial cells. Focal endocapillary
staining was also seen in glomeruli with thromboproliferative lesions.
Distal nephrons displayed marked increases in nNOS, especially at the
site of the macula densa. The most prominent increase, however, was
noted in blood vessels, especially in those with concentric
proliferative arteriopathy, where proliferating myointimal cells
stained intensely. A similar staining pattern was seen in the few
viable myocytes in small vessels with fibrinoid necrosis. The
endothelium was not discernible in the lesional
vessels; however, markedly increased endothelial
staining was noted in smaller nonlesional vessels and arcuate
and larger-size arteries. Many of the large vessels also revealed
medial and occasionally adventitial staining.
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Immunostaining for ecNOS was comparable in WKY-R, WKY-S, and SHRSP-R. Staining was most intense in the endothelium of large interlobular, arcuate, and interlobar arteries. Focal medial and adventitial staining was also noted. Moderate staining was seen in cortical distal tubules and collecting ducts and in outer medullary thick ascending limbs. Staining was absent in the macula densa. Proximal tubules and glomeruli showed minimal staining, the latter more prominent in visceral epithelial cells rather than microvascular endothelium. SHRSP-S showed a less dramatic increase in ecNOS staining compared with that of nNOS. The most obvious increase was seen in small-vessel endothelium and focally in the media and adventitia, especially in vessels with proliferative arteriopathy. Increases in the already intense endothelial staining of larger vessels could not be discerned. There was a slight increase in glomerular and proximal tubular staining.
Staining for iNOS was similar for both SHRSP and WKY and did not vary with diet. Large vessels showed focal moderate endothelial staining. Weak or focal staining was occasionally present in cortical collecting ducts, distal tubules, outer medullary thick ascending limbs, and inner medullary tubules. Staining was absent or trivial in the interstitium. Staining was absent in all step sections used as negative controls where the specific primary antisera had been omitted.
L-NNA Treatment
Data for SHRSP-SL were obtained at 10 weeks of age and were
compared with data for age-matched SHRSP-S and SHRSP-R. SHRSP-SL had a
significantly abrogated cortical NO response to the S diet (3.3±.6
pmol/mg tissue, P<.05 versus SHRSP-S,
P=NS versus SHRSP-R). Outer medullary and inner medullary NO
findings (5.7±2.5 and 1.8±1.0 pmol/mg tissue, respectively)
did not differ from those for SHRSP-S or SHRSP-R. Mean
arterial pressure in SHRSP-SL (213±10 mm Hg) was
higher than that of SHRSP-S or SHRSP-R (P<.05). Thrombotic
microangiopathy affecting glomeruli (9±2%) and blood vessels (10±2
vessels per 100 glomeruli) was present in SHRSP-SL at 10 weeks of
age. Glomerular lesions were equally distributed between
ischemic and thrombonecrotic/proliferative. Vascular lesions
were predominantly thrombonecrotic with mild proliferative
arteriopathy. Many of the vessels unaffected by thrombonecrotic
microangiopathy displayed mural thickening.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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In this study, we used a sensitive and reproducible technique for directly measuring NO as nitrite from three regions of the kidney to examine whether deficient renal NO production contributes to the development of lesions of malignant hypertension in SHRSP. For this purpose, comparisons were made between SHRSP of different ages given a diet known to accentuate renal lesion development in these rats. WKY were also examined because these animals do not develop lesions.9 We found that regional NO release from the kidney was not different between 16-week-old SHRSP and WKY on a regular diet. This is consistent with the observations that aortic NO production15 and acetylcholine-induced NO release from isolated perfused kidneys16 are similar in WKY and SHR maintained on a regular diet. If SHR, the parent strain of SHRSP, were hypertensive due to deficient NO production, blocking NO formation with L-NAME might be expected to elicit a diminished pressor response. On the contrary, SHR have been reported to exhibit a greater maximal pressor response to L-NAME than do WKY, suggesting that SHR are more dependent on basal NO production than WKY.16 17 In our study, mean arterial pressure was significantly greater in 16-week-old SHRSP than in WKY on a regular diet. Given that regional NO release from the kidney was not different in SHRSP-R compared with WKY-R, the hypertension seen in these animals does not appear to be due to altered NO production.
Previous studies have shown an increase in serum and urinary nitrate and nitrite within 2 weeks of initiation of a high-salt diet in Sprague-Dawley rats.3 Similarly, we found that renal NO formation was increased in both WKY and SHRSP on a high-salt diet but that the increment was substantially greater in SHRSP-S than WKY-S. In contrast, in response to a high-salt diet, Dahl S rats develop hypertension and exhibit an impaired ability to form NO relative to Dahl R rats, which remain normotensive.1 In further support of a role for deficient NO production in the hypertension of Dahl S rats, administration of L-arginine (a precursor of NO formation) was able to lower blood pressure, and inhibition of NO production with L-NMMA produced a diminished pressor response in these animals.18 Failure of dietary L-arginine supplementation to lower blood pressure in SHRSP-S19 is consistent with our observation that NO levels are not deficient in these animals.
Spontaneous lesion development was seen only in SHRSP-S, despite similar levels of mean arterial pressure in SHRSP-S and SHRSP-R at 12 and 16 weeks of age. These findings suggest that lesion development cannot be attributed solely to the magnitude of the hypertension. It is interesting to note that a modest elevation in the level of renal NO was seen before the development of overt kidney lesions in 10-week-old SHRSP-S, which might suggest a relationship between lesions and NO overproduction. However, inhibition of NO production in SHRSP-S with L-NNA at this early time point led to acceleration rather than abrogation of lesion development. This would suggest that accentuated NO levels have renal protective rather than deleterious effects, delaying but not preventing the ultimate development of end-organ damage. Similarly, the facts that (1) marked increases in NO were seen after established lesions were present, (2) hypertension did not increase further with continued lesion development, and (3) L-NNA treatment induced more severe hypertension suggest a compensatory role for NO in the regulation of blood pressure. NO has been suggested to play an important role in countering the effects of progressive glomerulosclerosis in aging through mediation of age-related vasodilation20 and to play a compensatory role in the maintenance of coronary blood flow in dogs with congestive heart failure.21 In contrast to SHRSP-S, WKY-S did not develop renal vascular lesions and remained normotensive. These findings are consistent with the concept that elevated arterial pressure in SHRSP-S is necessary, but is not sufficient alone, for the development of renal lesions.
We also examined tissue specimens with immunohistochemical staining for NOS, the enzyme responsible for NO formation. Because measurements of NO were made in tissues containing multiple cell types, we sought information relevant to the derivation of the NO formation in SHRSP-R and SHRSP-S. Staining for NOS was present in several cell types. Noteworthy were the increases in nNOS observed in glomeruli, proximal tubules, and the macula densa in SHRSP-S. Since nNOS is typically not considered to be inducible and typically is not observed in glomeruli or proximal tubular elements, this probably reflects de novo formation in these animals.22 In comparison with findings in Sprague-Dawley rats on a control diet, macula densa mRNA for neuronal NOS decreased with high-salt intake and increased with low-salt intake and has led to the suggestion that nNOS expression in macula densa cells is inversely related to salt intake.23 In support of this proposal, we found a tendency for diminished macula densa immunostaining for nNOS in WKY-S versus WKY-R. In SHRSP-S, however, we observed a prominent increase in macula densa immunostaining for nNOS. We previously reported that plasma renin activity is markedly suppressed by the S diet in WKY, whereas SHRSP exhibit a paradoxical increase.6 These observations are consistent with the concept that NO synthesized in macula densa cells is an important stimulus for renin secretion.23 24 Additional studies will be needed to determine the relationship between increases in nNOS and renin in SHRSP-S, but it is tempting to hypothesize that an abnormal sensing or response of the macula densa to distal sodium delivery in SHRSP may be the stimulus for the paradoxical increase in macula densa nNOS and activation of the renin-angiotensin system in these animals. The stimulus for increased nNOS expression in glomeruli, proximal tubules, and vessels remains unclear, since little is known at present about factors that enhance expression of this isozyme.
We also observed a mild increase in immunostaining for ecNOS in cortical microvessels in SHRSP-S. Heightened NOS expression in these vessels may reflect the known stimulatory effects of ischemia25 or endothelial shear stress,26 perhaps directly or indirectly exacerbated by angiotensin-induced vasoconstriction. The increase in ecNOS does not appear to be due to an effect of salt per se, since our observations in WKY and those of others23 27 indicate no changes in cortical ecNOS expression.
Whereas weak to minimal iNOS expression has been found in the rat renal cortex, considerable iNOS expression has been demonstrated in the renal medulla in Sprague-Dawley rats.27 28 Expression of iNOS has been shown to increase dramatically after immunological stimulation.28 29 We found similar immunostaining for iNOS in SHRSP and WKY, which, unlike nNOS or ecNOS, did not increase in SHRSP-S. The lack of leukocytic infiltrate and unchanged iNOS expression in SHRSP-S suggests that the increase in NO production in these animals is not due to an immunoactivated state.
Whether the increased NOS expression observed is responsible for the increased NO production in SHRSP-S cannot be determined from the present studies. Likewise, the relative contribution of specific structures to overall NO production cannot be ascertained from these studies. However, our immunohistochemical findings fail to reveal reduced NOS expression at specific tubular or vascular sites in the kidneys of SHRSP-S. Taken together, the results of immunohistochemistry, albeit semiquantitative, corroborate the NO release data and suggest that glomerular and vascular lesion development is not due to deficient NO production.
In summary, by measurement of NO release and supported by immunohistochemical staining, our data show that the kidneys of SHRSP are capable of heightened NO generation in response to a high-salt/stroke-prone diet. Thus, a deficiency of NO production does not appear to be the cause of spontaneous lesion development in SHRSP-S. The increased NO production is most likely a compensatory response to the combined effects of severe hypertension and the high-salt/stroke-prone diet. Whether this compensatory increase plays a role in limiting further glomerular or vascular damage in SHRSP-S remains to be determined. However, it may be proposed that the earlier induction of malignant nephrosclerosis when these animals are treated with L-NNA is related to attenuation of this compensatory response.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was supported by US Public Health Service grant HL-35522. The authors wish to thank Saramma George-Matthew for expert histological processing of the renal tissue; Mark Pantoja for skillful processing of the photomicrographs; and Newton Y.-T. Fan, James Fink, and Jessica Burstein for their expert technical assistance.
Received May 9, 1997; first decision June 16, 1997; accepted July 30, 1997.
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