Role of nitric oxide in the control of blood pressure in young and adult spontaneously hypertensive rats

Sodium nitroprusside attenuates hyperproliferation of vascular smooth muscle cells from spontaneously hypertensive rats through the inhibition of overexpression of AT1 receptor, cell cycle proteins, and c-Src/growth factor receptor signaling pathways

We recently showed that sodium nitroprusside (SNP), a NO donor, attenuated hypertension in spontaneously hypertensive rats (SHR). Since hypertension is associated with enhanced proliferation and hypertrophy of vascular smooth muscle cells (VSMC), the present study examines whether in vivo treatment of SHR with SNP could also inhibit the augmented proliferation of VSMC and explore the signaling mechanisms. Treatment of 8 week old SHR and Wistar Kyoto rats with SNP twice a week for 2 weeks inhibited the enhanced proliferation of VSMC from SHR, the enhanced expression of angiotensin II type 1 (AT1) receptor, and enhanced activation of c-Src and growth factor receptors and ERK1/2 signaling pathways. In addition, SNP also inhibited the overexpression of cell cycle proteins including cyclins D1, Cdk4, and phosphorylated pRB and restored the downregulated Cdk inhibitors p21Cip1 and p27Kip1 expression towards control levels. Furthermore, SNP-induced inhibition of enhanced levels of the AT1 receptor and enhanced proliferation was reversed by L-NAME, an inhibitor of nitric oxide synthase. These results suggest that the SNP-induced antiproliferative effect may be mediated through the inhibition of enhanced expression of the AT1 receptor, cell cycle proteins and activation of c-Src, growth factor receptors, and MAP kinase signaling.

Inhibition of overexpression of Giα proteins and nitroxidative stress contribute to sodium nitroprusside-induced attenuation of high blood pressure in SHR

We earlier showed that vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit enhanced expression of Giα proteins which was attributed to the decreased levels of nitric oxide (NO), because elevation of the intracellular levels of NO by NO donors; sodium nitroprusside (SNP) and S-Nitroso-N-acetyl-DL-penicillamine (SNAP), attenuated the enhanced expression of Giα proteins. Since the enhanced expression of Giα proteins is implicated in the pathogenesis of hypertension, the present study was undertaken to investigate if treatment of SHR with SNP could also attenuate the development of high blood pressure (BP) and explore the underlying molecular mechanisms. Intraperitoneal injection of SNP at a concentration of 0.5 mg/kg body weight twice a week for 2 weeks into SHR attenuated the high blood pressure by about 80 mmHg without affecting the BP in WKY rats. SNP treatment also attenuated the enhanced levels of superoxide anion (O₂^- ), hydrogen peroxide (H₂ O₂ ), peroxynitrite (ONOO^- ), and NADPH oxidase activity in VSMC from SHR to control levels. In addition, the overexpression of different subunits of NADPH oxidase; Nox-1, Nox-2, Nox-4, P^22phox , and P^47phox , and Giα proteins in VSMC from SHR were also attenuated by SNP treatment. On the other hand, SNP treatment augmented the decreased levels of intracellular NO, eNOS, and cGMP in VSMC from SHR. These results suggest that SNP treatment attenuates the development of high BP in SHR through the elevation of intracellular levels of cGMP and inhibition of the enhanced levels of Giα proteins and nitroxidative stress.

Chronic hydrogen-rich saline treatment attenuates vascular dysfunction in spontaneous hypertensive rats

In hypertensive patients, increased oxidative stress is thought to be one important cause of vascular dysfunction. Recently, it has been suggested that hydrogen exerts a therapeutic antioxidant activity by selectively reducing hydroxyl radical and peroxynitrite, the most cytotoxic chemicals of reactive oxygen species (ROS). Herein, we investigated the protective effect of chronic treatment with hydrogen-rich saline (HRS) against vascular dysfunction in SHR and the underlying mechanism. The 8-week-old spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY) were randomized into HRS-treated (6ml/kg/day for 3 months, i.p.) and vehicle treated group. Treatment with HRS ameliorated vascular dysfunction including aortic hypertrophy and endothelial function in SHR. Treatment with HRS had no significant effect on blood pressure, but it significantly improved baroreflex function in SHR. Treatment with HRS abated oxidative stress, restored antioxidant enzymes including superoxide dismutase, glutathione peroxidase, and catalase, and suppressed NADPH oxidase. Furthermore, treatment with HRS depressed pro-inflammatory cytokines expression including IL-6 and IL-1β and suppressed NF-κB activation, restored mitochondrial function including ATP formation and membrane integrity. In addition, although treatment with HRS had no significant effect on nitric oxide amount in circulating or aorta, it suppressed endothelial nitric oxide synthase expression and upregulated dimethylarginine dimethylaminohydrolase 2 expression in SHR. In conclusion, treatment with HRS alleviates vascular dysfunction through abating oxidative stress, restoring baroreflex function, suppressing inflammation, preserving mitochondrial function, and enhancing nitric oxide bioavailability.

Vascular system: role of nitric oxide in cardiovascular diseases

In contrast with the short research history of the enzymatic synthesis of nitric oxide (NO), the introduction of nitrate-containing compounds for medicinal purposes marked its 150th anniversary in 1997. Glyceryl trinitrate (nitroglycerin) is the first compound of this category. On October 12, 1998, the Nobel Assembly awarded the Nobel Prize in Medicine or Physiology to scientists Robert Furchgott, Louis Ignarro, and Ferid Murad for their discoveries concerning NO as a signaling molecule in the cardiovascular system. NO-mediated signaling is a recognized component in various physiologic processes (eg, smooth muscle relaxation, inhibition of platelet and leukocyte aggregation, attenuation of vascular smooth muscle cell proliferation, neurotransmission, and immune defense), to name only a few. NO has also been implicated in the pathology of many inflammatory diseases, including arthritis, myocarditis, colitis, and nephritis and a large number of pathologic conditions such as amyotrophic lateral sclerosis, cancer, diabetes, and neurodegenerative diseases. Some of these processes (eg, smooth muscle relaxation, platelet aggregation, and neurotransmission) require only a brief production of NO at low nanomolar concentrations and are dependent on the recruitment of cyclic guanosine monophosphate (cGMP)-dependent signaling. Other processes are associated with direct interaction of NO or reactive nitrogen species derived from it with target proteins and requires a more sustained production of NO at higher concentrations but do not involve the cGMP pathway.

The expression and activity of renal nitric oxide synthase and circulating nitric oxide in polycystic kidney disease rats

Nitric oxide (NO) influences tubular fluid and electrolyte transport, and hence possibly also fluid accumulation in renal cysts. The expression and activity of intrarenal constitutive NO synthase (cNOS) [neuronal NOS, nNOS and endothelial NOS, eNOS] and inducible NOS (iNOS) and plasma nitrite/nitrate (PNOx) concentration were assessed in homozygous Han:SPRD polycystic kidney disease (PKD) rats (cy/cy), heterozygous Han:SPRD PKD rats (cy/+), homozygous normal Han:SPRD littermates (+/+) and Sprague Dawley rats (sd). The results showed: 1) nNOS expression was decreased in proximal tubules and thick ascending limbs of the loop of Henle in cy/cy and cy/+ rats compared to +/+ and sd rats (p<0.05). nNOS was weakly expressed in the epithelium of small cysts and unexpressed in epithelium of large cysts. 2) iNOS expression was increased in proximal tubular epithelial cells in cy/+ rats compared to +/+ rats and sd rats (p<0.01). iNOS expression in cyst epithelium was decreased in cy/+ rats (p<0.05) and absent in cy/cy rats. 3) eNOS expression was similar in the endothelium of intrarenal arteries in all groups. 4) The activity of renal cNOS was decreased in cy/cy and cy/+ rats; the activity of iNOS was decreased only in cy/cy rats, with no significant difference among the other three groups. 5) PNOx concentration was higher in cy/cy rats than in the other three groups, and correlated positively with plasma creatinine and urea. In conclusion, NOS expression and activity decreased as cysts developed, suggesting that NO downregulation is involved in the pathogenesis of PKD.

Endothelial dysfunction and reduced nitric oxide in resistance arteries in autosomal-dominant polycystic kidney disease

BACKGROUND

Patients with autosomal-dominant polycystic kidney disease (ADPKD) have defective endothelium-dependent relaxation (EDR). We investigated the relationship between endothelial dysfunction and nitric oxide generation in hypertension and chronic renal insufficiency (CRI) in ADPKD.

METHODS

We contrasted acetylcholine (ACh)-induced EDR, 3-morphollinosydnonimine (SIN-1)-induced endothelium-independent relaxation (EIDR) and constitutive nitric oxide synthase (cNOS) activity in subcutaneous resistance vessels and plasma levels and excretion of NO2-/NO3- (NOX) in normal, control (N = 10) patients with ADPKD or essential hypertension.

RESULTS

EDR was decreased significantly in normotensive ADPKD (N = 9), but more severely in hypertensive ADPKD (N = 6), or those with CRI (N = 5) and in essential hypertension (N = 9). The increases in EDR with l-arginine and decreases with LG-nitro-l-arginine methyl ester (L-NAME) were lost in all groups of patients with ADPKD and in essential hypertension except for a modest effect of L-NAME in normotensive ADPKD. EIDR was unimpaired throughout. Vascular cNOS activity and renal NOX excretion were reduced profoundly in patients with all categories of ADPKD and especially in those with hypertension.

CONCLUSION

EDR in resistance vessels from patients with ADPKD is impaired even in the absence of hypertension or CRI, but becomes more marked as hypertension develops. Patients with ADPKD have defective nitric oxide generation from diminished cNOS activity. Endothelial dysfunction and impaired cNOS activity in ADPKD may predispose to hypertension whose occurrence is accompanied by a further sharp deterioration in EDR.

Evaluation of the involvement of nitric oxide and substance P in reducing baroreflex gain in the genetically hypertensive (GH) rat

The attenuation of baroreflex gain associated with hereditary hypertension could involve abnormal signalling by nitric oxide or substance P. Baroreflex gain was measured in age-matched male genetically hypertensive (GH) and nonnotensive (N) anaesthetised rats from heart rate changes in response to i.v. phenylephrine or sodium nitroprusside. In subgroups of these animals, nitric oxide synthesis was inhibited using NG-nitro-L-arginine methyl ester (L-NAME, 30 mg x kg(-1) i.v.), substance P transmission was blocked using the antagonist SR 140333 (360 nmoles x kg(-1) i.v.) or substance P release was inhibited with resiniferatoxin (4 doses of 0.3 microg x kg(-1) i.v. at 4 min intervals). Baroreflex gain was markedly reduced in GH compared to N animals (N -0.37 +/- 0.04 beat x min(-1) x mm Hg(-1), GH -0.17 +/- 0.02 beat x min(-1) x mm Hg(-1), p < 0.0001). Inhibition of nitric oxide synthase increased baroreflex gain in each strain, but the inter-strain difference in gain persisted (post-treatment N -0.57 +/- 0.07 beat x min(-1) x mm Hg(-1), GH -0.24 +/- 0.05 beat x min(-1) x mm Hg(-1) (p < 0.001). Blockade of receptors or inhibition of substance P release did not affect gain in either strain. Nitric oxide, but not substance P, appears to play an inhibitory role in the rat arterial baroreflex. Impairment of baroreflex gain in GH rats is not secondary to altered nitric oxide signaling.

Association of renal injury with nitric oxide deficiency in aged SHR: prevention by hypertension control with AT1 blockade

BACKGROUND

Aged spontaneously hypertensive rats (SHR) develop end-stage renal disease resembling that of uncontrolled essential hypertension in humans. Nitric oxide (NO) and angiotensin II (Ang II) play an important role in the regulation of blood pressure and the growth of vascular smooth muscle and renal mesangial cells. The relationship between renal NO system, Ang II activity and renal injury in aged SHR is not fully understood.

METHODS

The 8-week-old SHR were randomized into losartan-treated (30 mg/kg/day for 55 weeks) and vehicle treated groups. The age-matched Wistar-Kyoto rats (WKY) served as controls. Renal histology and tissue expressions of endothelial and inducible NO synthases (eNOS and iNOS) and nitrotyrosine were examined at 63-weeks of age.

RESULTS

Compared to the WKY group, untreated SHR showed severe hypertension, proteinuria, renal insufficiency, a twofold decrease in renal tissue eNOS and iNOS expressions and massive nitrotyrosine accumulation. This was associated with severe glomerulosclerosis, tubular atrophy and interstitial fibrosis. Losartan therapy normalized blood pressure, prevented proteinuria and renal insufficiency, abrogated the fall in renal eNOS and iNOS protein contents, mitigated renal nitrotyrosine accumulation, and prevented the histological abnormalities found in the untreated SHR.

CONCLUSIONS

Aged SHR exhibit severe renal lesions with acquired NO deficiency that are prevented by hypertension control with AT1 blockade. These findings point to the possible role of NO deficiency in the pathogenesis of renal lesions in aged SHR.

EDHF contributes to strain-related differences in pulmonary arterial relaxation in rats

Pulmonary arteries from the Madison (M) strain relax more in response to acetylcholine (ACh) than those from the Hilltop (H) strain of Sprague-Dawley rats. We hypothesized that differences in endothelial nitric oxide (NO) synthase (eNOS) expression and function, metabolism of ACh by cholinesterases, release of prostacyclin, or endothelium-derived hyperpolarizing factor(s) (EDHF) from the endothelium would explain the differences in the relaxation response to ACh in isolated pulmonary arteries. eNOS mRNA and protein levels as well as the NO-dependent relaxation responses to thapsigargin in phenylephrine (10(-6) M)-precontracted pulmonary arteries from the M and H strains were identical. The greater relaxation response to ACh in M compared with H rats was also observed with carbachol, a cholinesterase-resistant analog of ACh, a response that was not modified by pretreatment with meclofenamate (10(-5) M). N(omega)-nitro-L-arginine (10(-4) M) completely abolished carbachol-induced relaxation in H rat pulmonary arteries but not in M rat pulmonary arteries. Precontraction with KCl (20 mM) blunted the relaxation response to carbachol in M rat pulmonary arteries and eliminated differences between the M and H rat pulmonary arteries. NO-independent relaxation present in the M rat pulmonary arteries was significantly reduced by 17-octadecynoic acid (2 microM) and was completely abolished by charybdotoxin plus apamin (100 nM each). These findings suggest that EDHF, but not NO, contributes to the strain-related differences in pulmonary artery reactivity. Also, EDHF may be a metabolite of cytochrome P-450 that activates Ca(2+)-dependent K(+) channels.

Effect of severe aortic banding above the renal arteries on nitric oxide synthase isotype expression

BACKGROUND

Severe aortic stenosis above the renal arteries leads to a reduction in renal perfusion, increased renin secretion, and elevation of arterial blood pressure above the stenotic site. Nitric oxide (NO) plays an important role in regulation of renal and systemic vascular resistance, renal blood flow, and Na(+) handling. Abdominal aortic banding provides an excellent model for simultaneous testing of the effects of increased and decreased pressure, flow, and shear stress in the same animal.

METHODS

We studied protein expressions of endothelial NO synthase (eNOS), inducible NOS (iNOS), and neuroneal NOS (nNOS) isotypes in the renal cortex, renal medulla, heart, brain, and aorta segments above and below the stenosis site three weeks after abdominal aortic banding above the renal arteries. The results were compared with those obtained in the sham-operated controls. NOS isotype proteins were measured by Western blot.

RESULTS

Compared with the control group, the banded group showed significant up-regulations of eNOS, iNOS, and nNOS in renal cortex and medulla. Likewise, heart eNOS, brain nNOS, and thoracic aorta eNOS proteins were significantly increased in the banded group. However, eNOS and iNOS expressions were unchanged in the aorta segment below the stenotic site. Likewise, iNOS expression in the heart and thoracic aorta remained unchanged in the banded animals. No significant difference was found in creatinine clearance or urinary protein excretion between the two groups.

CONCLUSIONS

These findings clearly demonstrate the up-regulatory action of increased pressure on eNOS expression in the thoracic aorta and heart and of nNOS expression in the brain. These data further show up-regulation of all NOS isotypes in the kidney, which must have helped to mitigate the associated hypoperfusion.

Dexamethasone worsens nitric oxide inhibition-induced hypertension and renal dysfunction

Chronic nitric oxide (NO) inhibition with Nomega-nitro-L-arginine methyl ester (L-NAME) has previously been reported to produce systemic hypertension, renal vasoconstriction, and renal damage. In this study we investigated whether a compensatory restoration of NO synthesis occurs in chronic L-NAME hypertension and whether chronic treatment with dexamethasone (Dex) (which inhibits inducible NO synthase [iNOS]) can influence the course of the hypertension. We found that in the conscious chronically L-NAME-treated (approximately =10 mg/kg/24 h) hypertensive rats, acute systemic NOS inhibition elicited a further increase in blood pressure (BP), indicating partial restoration of NO production. Chronic Dex in a dose previously reported to inhibit iNOS (5 microg/24 h), amplified the hypertension (within 2 days), renal vasoconstriction, and reduction in glomerular filtration rate because of L-NAME. In contrast, chronic Dex alone had no effects on renal hemodynamics or BP during the first week, although by the end of week 2 a small increase in BP (approximately =10 mm Hg) was evident. These results show that BP continues to increase with chronic L-NAME despite partial restoration of NO production. An iNOS, which might be stimulated and escaped inhibition by L-NAME, may be responsible for the compensatory restoration of NO synthesis, serving to attenuate the development of hypertension and renal dysfunction.

Effects of chronic inhibition of nitric oxide synthase in the genetically hypertensive rat

1. The effects of graded inhibition of nitric oxide synthase (NOS) on blood pressure in the genetically hypertensive (GH) rat strain and NOS activity in regions of the brain (cerebellum, striatum, hippocampus, frontal cortex and medulla oblongata) as a measure of body NOS inhibition were studied. 2. Male GH and normotensive (N) rats (n = 7-10 per group) were given N(G)-nitro-L-arginine methyl ester (L-NAME; 2, 5, 10 or 20 mg/kg per day in drinking water) from age 7 weeks. Age- and weight-matched controls received water only. Systolic blood pressure (SBP) was measured weekly by the tail-cuff method from age 6 weeks. By age 10 weeks, rats were killed and NOS activity was measured. 3. Some GH rats that received over 5 mg/kg per day L-NAME developed stroke-like symptoms and were killed before the end of the treatment period. 4. No difference in NOS activity was found between untreated N and GH strains but, in those that received treatment, a graded inhibition was observed with increasing L-NAME dose levels. The frontal cortex in the GH strain given 20 mg/kg per day L-NAME had NOS inhibition of 90% where the N strain had 73% inhibition. Similar results were seen in the other areas of the brain. 5. Left ventricular mass, weight related, was significantly greater in the GH compared with N and was further elevated by treatment with L-NAME. 6. The SBP at 10 weeks was significantly elevated in GH rats by NOS inhibition with L-NAME in a dose-dependent manner; 25% for 2 mg/kg per day, 31% for 20 mg/kg per day (P < 0.001). There was a non-significant increase in BP in the N-treated groups (average change of 7.5%). 7. Nitric oxide synthase inhibition causing increased SBP in GH rats suggests an abnormality in the nitric oxide-L-arginine pathway in this strain.

Cardiovascular effects of chronic nitric oxide synthase inhibition in genetically hypertensive rats

1. The possible role of an endothelial defect in the hypertension of the New Zealand genetically hypertensive (GH) rat strain was assessed by examining cardiovascular responses to the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) and the endothelium-dependent depressor agent acetylcholine (ACh). The vascular sensitivity of the hindquarter to nitric oxide (NO) was examined using the NO donor sodium nitroprusside (SNP). 2. NG-Nitro-L-arginine methyl ester (10 mg/kg per day in drinking water) was given to GH and normotensive (N) rats from age 7-9 weeks, with GH and N untreated control groups. Systolic blood pressure (tail-cuff) was monitored weekly from age 5-9 weeks. At age 9 weeks, pressure responses to various vasoactive agents were measured in vivo and in the rat isolated hindquarter. Left ventricular (LV) mass was measured at the time of death. 3. NG-Nitro-L-arginine methyl ester induced a greater hypertensive effect in GH (P < 0.001) compared with N (P < 0.05) rats and caused a significant increase in hindquarter perfusion pressure in GH rats only (P < 0.01). 4. Genetically hypertensive rats had LV hypertrophy that was exacerbated by L-NAME (P < 0.01). Left ventricular hypertrophy was not induced by L-NAME in N rats. 5. The normalized response to ACh did not differ between GH and N control rats and was unaffected by L-NAME treatment in vivo and in vitro except at the highest ACh dose (3 micrograms/kg) in GH hindquarters (P < 0.01). The response to SNP was similar in GH and N hindquarters and enhanced by L-NAME in GH (0.1 microgram; P < 0.05) and N rats (0.01 microgram, P < 0.01; 0.01 microgram, P < 0.001). 6. These results suggest that the L-arginine/NO system is not deficient in GH rats and that endothelial function in the GH hindquarter is preserved. They confirm that NO is involved in mediating blood pressure in GH and N rats and raise the possibility that a non-NO-mediated mechanism may underlie ACh-induced vasodilation in GH and N.

Enhanced nitric oxide synthesis reverses salt-induced alterations in blood flow and cGMP levels

To understand the role of nitric oxide in salt-induced hypertension, we evaluated cardiovascular, hemodynamic and biochemical parameters in Dahl salt-sensitive rats fed low (0.3%) and high (8.0%) sodium diets. Two high salt groups received 1.25 and 2.5 g/L l-arginine in their drinking water. After three weeks of treatment, blood pressure was greater in the high salt groups. l-arginine did not modify salt-induced hypertension. Eicosapentaenoic acid (EPA) caused a smaller depressor response compared to normotensive rats. The increase in blood pressure was associated with decreases in aortic and renal blood flows. In renal artery, the reduction was counteracted by both l-arginine doses; whereas in the aorta, only the higher l-arginine one restored blood flow. The salt-induced reduction in aortic cyclic GMP level was only overcome by the higher l-arginine treatment. These data suggest that at the dose levels tested, nitric oxide reverses the reduction in cGMP and blood flow, but not the blood pressure changes associated with salt-induced hypertension.

Regional renal nitric oxide release in stroke-prone spontaneously hypertensive rats

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 omega-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.

Dietary calcium decreases blood pressure without decreasing renal vascular resistance or altering the response to NO blockade

Many vasoactive elements are involved in the elevation of blood pressure in spontaneously hypertensive rats (SHR). Elevated dietary calcium has been observed to reduce blood pressure in SHR. This study investigates interactions among dietary calcium, renal vascular resistance (RVR), elevation of blood pressure and effects of norepinephrine and nitric oxide synthesis. We completed a series of experiments on two groups each (fed low, 0.1% and high, 2.0% dietary calcium, respectively) of 9-week-old Wistar Kyoto (WKY), 9-week-old and 6-week-old SHR. Although 9-week-old SHR had elevated baseline blood pressure compared to 9-week-old WKY and also compared to 6 week-old SHR, there was no corresponding elevation in baseline RVR. All SHR fed high calcium diets had lower blood pressure compared to low calcium diets, and there was no corresponding reduction in RVR. WKY controls' blood pressure and RVR were unaffected by dietary calcium levels. In all hypertensive rats the blood pressure and renal vascular resistance were elevated by N(G)-nitro-L-arginine methylester (L-NAME), but the dietary differences were sustained. Blood pressure of WKY was unaffected by the low dose of L-NAME. The increase in RVR to L-NAME was greater in SHR than in controls. The renal vascular response to norepinephrine was related to diet in older SHR, but smaller and unrelated to diet in younger SHR. Following L-NAME, WKY had greater responses to norepinephrine than 9-week-old SHR. We conclude that noradrenergic vasoconstriction is enhanced in the adult SHR, especially in the absence of high calcium diet. Alterations in NO synthesis may effect the norepinephrine response.