The deficiency of Npr1 [genetic determinant of natriuretic peptide receptor A (NPRA)] increases arterial pressures and causes hypertensive heart disease in mice similar to those seen in untreated human hypertensive patients. However, the quantitative role of NPRA in mediating the renal responses to blood volume expansion remains uncertain. To determine the specific contribution of NPRA in mediating the signaling mechanisms responsible for natriuretic and diuretic responses to nondilutional intravascular expansion, we administered whole blood to anesthetized Npr1 homozygous null mutant (0-copy), wild-type (2-copy), and gene-duplicated (4-copy) mice. In wild-type (2-copy) animals, urinary flow (microl x min-1 x g kidney wt-1) increased from 4.9 +/- 1.0 to 14.4 +/- 1.8 and sodium excretion (microeq x min-1 x g kidney wt-1) from 1.15 +/- 0.22 to 3.11 +/- 0.60, associated with a rise in glomerular filtration rate (GFR; ml x min-1 x g kidney wt-1) from 0.63 +/- 0.03 to 0.82 +/- 0.09 and renal plasma flow (RPF; ml x min-1. g kidney wt-1) from 2.96 +/- 0.17 to 4.36 +/- 0.41, whereas arterial pressure did not significantly increase. After volume expansion, 0-copy mice showed significantly lesser increases in urinary flow (P < 0.001) and sodium excretory (P < 0.001) responses even though the increases in arterial pressures were greater (P < 0.001) compared with 2-copy mice. The 4-copy mice showed augmented responses in urinary flow (P < 0.01) and sodium excretion (P < 0.001) along with rises in both GFR (P < 0.01) and RPF (P < 0.01) compared with 2-copy wild-type mice. These results establish that NPRA activation is the predominant mechanism mediating the natriuretic, diuretic, and renal hemodynamic responses to acute blood volume expansion.

Chronic infusions of initially subpressor doses of angiotensin II (ANG II) lead to progressive hypertension over a 2-wk period and to augmented intrarenal ANG II levels. The present study was performed to investigate total renal blood flow (RBF) and medullary blood flow (MBF) autoregulatory behavior and pressure-natriuresis in ANG II-infused hypertensive rats and how these are modified by concomitant treatment with an ANG II AT(1) receptor antagonist. ANG II-infused rats (n = 27) were prepared by administration of ANG II at 60 ng/min via osmotic minipump for 13 days. Twelve of the ANG II-infused hypertensive rats were treated with losartan in the drinking water (30 mg. kg.(-1) day(-1)). Rats were anesthetized with pentobarbital sodium (50 mg/kg, ip) and prepared for renal function measurements. An aortic clamp was placed above the junction of the left renal artery to reduce renal arterial pressure. Autoregulatory responses for renal plasma flow, overall RBF, and glomerular filtration rate were impaired in ANG II-infused hypertensive rats; however, MBF autoregulation was not disrupted. Most strikingly, pressure-natriuresis was markedly suppressed in ANG II-infused hypertensive rats. Chronic treatment with losartan prevented the impairment of the pressure-natriuresis relationship caused by chronic ANG II infusion. These findings demonstrate that chronic ANG II infusion leads to marked impairment of sodium excretion and suppression of the pressure-natriuresis relationship, which may contribute to the progressive hypertension that occurs in this model. These renal effects are prevented by simultaneous treatment with an AT(1) receptor blocker.

Abstract-Recent studies have indicated that changes in intrarenal nitric oxide (NO) production participate in mediating arterial pressure-induced changes in urinary sodium excretion. Until recently, however, the means to measure changes in intrarenal NO activity in vivo have not been available. For the present study, changes in renal tissue NO activities were assessed directly using an NO-selective microelectrode inserted into the cortical tissue of anesthetized dogs. Control studies demonstrated that the electrode was responsive to intrarenal bolus injections of acetylcholine and to the NO donor S-nitroso-acetylpenicillamine (SNAP). Intrarenal nitro-L-arginine (50 g ⅐ kg Ϫ1 ⅐ min Ϫ1) decreased renal tissue NO concentration by 593Ϯ127 nmol/L (PϽ0.05; nϭ7). Infusions of SNAP (1, 2, and 3 g ⅐ kg Ϫ1 ⅐ min Ϫ1 for 25 minutes) in nitro-L-arginine-treated dogs (nϭ5) resulted in dose-dependent increases in renal tissue NO activity, which showed a positive correlation with changes in urinary excretion rates of NO metabolites, nitrates and nitrites, (rϭ0.62, PϽ0.05) and sodium (rϭ0.78, PϽ0.01). During graded reductions of renal arterial pressure within the autoregulatory range (144Ϯ3 to 73Ϯ2 mm Hg; nϭ10), there were decreases in tissue NO activity that were positively correlated with changes in renal arterial pressure (rϭ0.45; PϽ0.05), urinary nitrate/nitrite excretion (rϭ0.64, PϽ0.005), and urinary sodium excretion (rϭ0.46; PϽ0.05). These data support the hypothesis that acute changes in renal arterial pressure result in alterations in intrarenal NO activity, which may be responsible for the associated changes in sodium excretion. (Hypertension. 1998;32:266-272.) Key Words: nitric oxide electrode Ⅲ sodium excretion Ⅲ nitrate excretion Ⅲ pressure-diuresis N itric oxide is known to play a substantive role in the regulation of renal hemodynamics and renal excretory function.1-3 Previous studies have suggested that renal production of NO is important in the acute and long-term regulation of sodium and water excretion. [4][5][6][7] In particular, the changes in urinary sodium excretion in response to acute alterations in RAP appear to be closely associated with changes in the intrarenal production rate of NO. 4,5,8 -11 In support of this hypothesis, previous studies have demonstrated a positive relationship between RAP and urinary excretion rate of NO metabolites, nitrates and nitrites (NO 3 Ϫ / NO 2 Ϫ ), in anesthetized dogs. 12 However, direct in vivo assessment of changes in intrarenal NO activity during changes in RAP has not yet been performed. To understand further the role of NO in the pressure-natriuresis phenomenon, it is important to determine the dynamic changes in intrarenal NO activity during acute changes in RAP. However, the continuous in vivo assessment of intrarenal NO activity has not been possible until recently. [12][13][14] NO-sensing microelectrodes that provide real-time monitoring of NO concentration in biological tissues during both in vitro and in vivo conditions have been developed recently. [15][1...

Experiments were performed in anesthetized male Sprague-Dawley rats to determine whether increased nitric oxide (NO) activity during the development of hypertension exerts a protective effect on renal cortical blood flow (CBF) and medullary blood flow (MBF). The effects of acute NO synthase inhibition on renal function and on CBF and MBF, measured by laser-Doppler flow probes, were evaluated in control and ANG II-infused hypertensive rats, prepared by the infusion of ANG II at a rate of 65 ng/min via osmotic minipumps implanted subcutaneously for 13 days. In normotensive rats ( n = 8), intravenous infusion of N ω-nitro-l-arginine (NLA; 20 μg ⋅ 100 g−1 ⋅ min−1) decreased CBF by 21 ± 4% and MBF by 49 ± 8% and increased blood pressure from 118 ± 1 to 140 ± 2 mmHg. In ANG II-infused rats ( n = 7), CBF and MBF decreased by 46 ± 5% and 25 ± 6%, respectively, during infusion of NLA. Arterial pressure increased from 160 ± 5 to 197 ± 7 mmHg, which was a greater absolute increase than in normotensive controls. Basal renal blood flow (RBF), estimated from p-aminohippurate clearance and hematocrit, was similar in both the control (6.0 ± 0.5 ml ⋅ min−1 ⋅ g−1) and hypertensive (6.0 ± 0.6 ml ⋅ min−1 ⋅ g−1) rats. However, NLA-induced reductions in RBF averaged 60 ± 5% in the hypertensive rats, compared with 31 ± 9% observed in control rats. GFR in control (0.97 ± 0.03 ml ⋅ min−1 ⋅ g−1) and hypertensive rats (0.78 ± 0.12 ml ⋅ min−1 ⋅ g−1) decreased to a similar extent during the first 30-min period of NLA infusion. GFR returned toward control levels in control rats; in contrast, GFR remained significantly decreased in the ANG II-infused rats (0.58 ± 0.11 ml ⋅ min−1 ⋅ g−1). Basal urinary sodium excretion (0.2 ± 0.08 μeq ⋅ min−1 ⋅ g−1), fractional excretion of sodium (0.3 ± 0.13%), and urine flow (4.9 ± 0.39 μl ⋅ min−1 ⋅ g−1) in hypertensive rats did not increase significantly after NLA treatment as occurred in normotensive controls. These data suggest that a compensatory increase in nitric oxide activity partially counteracts the vasoconstrictor influence of elevated ANG II levels to regulate renal hemodynamics and maintain cortical perfusion in the renal circulation.

We have previously demonstrated that nitric oxide (NO) exerts a greater modulatory influence on renal cortical blood flow in ANG II-infused hypertensive rats compared with normotensive rats. In the present study, we determined nitric oxide synthase (NOS) activities and protein levels in the renal cortex and medulla of normotensive and ANG II-infused hypertensive rats. Enzyme activity was determined by measuring the rate of formation of L-[(14)C]citrulline from L-[(14)C]arginine. Western blot analysis was performed to determine the regional expression of endothelial (eNOS), neuronal (nNOS), and inducible (iNOS) isoforms in the renal cortex and medulla of control and ANG II-infused rats. Male Sprague-Dawley rats were prepared by the infusion of ANG II at a rate of 65 ng/min via osmotic minipumps implanted subcutaneously for 13 days and compared with sham-operated rats. Systolic arterial pressures were 127 +/- 2 and 182 +/- 3 mmHg in control (n = 13) and ANG II-infused rats (n = 13), respectively. The Ca(2+)-dependent NOS activity, expressed as picomoles of citrulline formed per minute per gram wet weight, was higher in the renal cortex of ANG II-infused rats (91 +/- 11) than in control rats (42 +/- 12). Likewise, both eNOS and nNOS were markedly elevated in the renal cortex of the ANG II-treated rats. In both groups of rats, Ca(2+)-dependent NOS activity was higher in the renal medulla than in the cortex; however, no differences in medullary NOS activity were observed between the groups. Also, no differences in medullary eNOS levels were observed between the groups; however, medullary nNOS was decreased by 45% in the ANG II-infused rats. For the Ca(2+)-independent NOS activities, the renal cortex exhibited a greater activity in the control rats (174 +/- 23) than in ANG II-infused rats (101 +/- 10). Similarly, cortical iNOS was greater by 47% in the control rats than in ANG II-treated rats. No differences in the activity were found for the renal medulla between the groups. There was no detectable signal for iNOS in the renal medulla for both groups. These data indicate that there is a differential distribution of NOS activity, with the Ca(2+)-dependent activity and protein expression higher in the renal cortex of ANG II-infused rats compared with control rats, and support the hypothesis that increased constitutive NOS activity exerts a protective effect in ANG II-induced hypertension to maintain adequate renal cortical blood flow.