Abstract-In the present studies, the influence of inducible nitric oxide synthase (NOS) inhibition with aminoguanidine on renal function and blood pressure was examined in rats. Intravenous aminoguanidine infusion (60 mg ⅐ kg Ϫ1 ⅐ hr Ϫ1) for 40 minutes to anesthetized Sprague-Dawley rats (nϭ7) resulted in no significant changes in mean arterial pressure or renal cortical blood flow, while medullary blood flow was slightly increased. Despite minimal effects on renal blood flow, urine flow was significantly decreased from 14.2Ϯ2.7 to 10.4Ϯ2.3 L ⅐ min Ϫ1 ⅐ g kidney wt Ϫ1 during aminoguanidine infusion. To examine the possible effects of inducible NOS on blood pressure, aminoguanidine (10 mg ⅐ kg Ϫ1 ⅐ h Ϫ1 IV) was infused chronically into uninephrectomized rats maintained on a high salt (4.0% NaCl) diet. Mean arterial pressure significantly increased from 104Ϯ2 to 118Ϯ3 mm Hg after 6 days of aminoguanidine infusion (nϭ7) and returned to levels not different from those in the control group after 2 days of postcontrol infusion. Calcium-independent NOS activity in the renal medulla, a tissue that expresses inducible NOS in normal rats, was significantly decreased by 49% in the aminoguanidine-infused group (nϭ6) compared with that activity in the vehicle-infused control animals (nϭ6). In contrast, calcium-dependent NOS activity in the renal medulla was not significantly altered by aminoguanidine infusion, indicating specificity of aminoguanidine for inducible NOS in these experiments. In a final group of rats (nϭ5), oral L-arginine administration in drinking water (2% wt/vol) increased plasma arginine levels from 118Ϯ5 to 232Ϯ16 mol/L and blocked the increase in arterial pressure after 6 days of aminoguanidine infusion. The present experiments provide evidence supporting a role for inducible NOS in the control of arterial pressure, possibly by renal tubular effects. 6-7 Because nNOS, iNOS, and eNOS are all present in the normal rat kidney, 8 -14 it is not clear which NOS isoform(s) is/are responsible for these functional effects. Of interest is the presence of iNOS in the normal rat kidney. In situ hybridization and reverse transcription-polymerase chain reaction of microdissected vascular and tubular segments have identified iNOS mRNA in arcuate and interlobular arteries, glomeruli, proximal tubules, thick ascending limbs, and collecting ducts. 8,11 The greatest amount of iNOS message was observed in the medullary thick ascending limbs and the inner medullary collecting ducts 11 ; this observation is consistent with protein blotting experiments that demonstrate greater amounts of iNOS immunoreactive protein in renal medullary than in renal cortical tissue homogenates.10 Despite the morphological localization of iNOS in the kidney, the role of this isoform in the control of normal renal function is unclear.Several indirect studies suggest that iNOS may participate in the regulation of renal function and arterial pressure. The genetic locus containing the gene for iNOS cosegregated with the high blood pressure phenotype...

Abstract-We hypothesized that the relatively high doses of angiotensin (Ang) II required to produce hypertension in rats were related to stimulation of renal medullary nitric oxide production, which in turn blunted reductions in medullary blood flow and the development of hypertension. Ang II was infused (5 days at 3 ng ⅐ kg Ϫ1 ⅐ min Ϫ1 IV) to uninephrectomized Sprague-Dawley rats in the presence and absence of a continuous medullary interstitial N G -nitro-L-arginine methyl ester (L-NAME) infusion. Renal cortical and medullary blood flows were determined with the use of implanted optical fibers and laser-Doppler flowmetry. Ang II in the absence of medullary nitric oxide synthase inhibition did not change cortical or medullary blood flow or mean arterial pressure. A threshold dose of L-NAME was determined (75 g ⅐ kg Ϫ1 ⅐ h Ϫ1 ) that did not produce significant short-or long-term changes in medullary blood flow and mean arterial pressure. In rats with blunted medullary nitric oxide synthase activity, Ang II infused intravenously resulted in a 30% reduction in medullary blood flow (from 1.3 to 0.9Ϯ0.2V) and Ϸ20 mm Hg increase in mean arterial pressure with Ang II infusion over 5 days. During 70 minutes after the start of intravenous Ang II, there was an immediate reduction in medullary blood flow, with no changes in cortical blood flow or mean arterial pressure. We conclude that the relative insensitivity of rats to long-term elevations of circulating Ang II is due to the potent counterregulatory actions of the nitric oxide system, specifically within the renal medulla. The results provide novel insights of how the organism attempts to protect itself from the hypertensive effects of Ang II. Key Words: hypertension, renal Ⅲ kidney Ⅲ angiotensin II Ⅲ renal blood flow Ⅲ nitric oxide Ⅲ nitric oxide synthase I t is well recognized that elevations of circulating angiotensin (Ang) II cause resetting of the pressure-natriuresis relationship to elevated levels of arterial blood pressure through the renal retention of sodium and volume expansion. 1 Yet considerable species differences have been found in the concentrations of Ang II required to produce chronic hypertension. We have found that Ang II infused long term at concentrations of 3 to 5 ng ⅐ kg Ϫ1 ⅐ min Ϫ1 produce Ϸ30 mm Hg increase in mean arterial pressure (MAP) over a period of 1 week in mongrel dogs maintained on a normal daily NaCl diet. 2 Humans exhibit even greater pressor sensitivity to prolonged infusions of Ang II. 3 In contrast, prolonged infusion of Ang II at 3 to 5 ng ⅐ kg Ϫ1 ⅐ min Ϫ1in Sprague-Dawley rats does not lead to a significant increase in MAP, 4 and investigators generally administer Ang II at rates of 30 to 60 ng ⅐ kg Ϫ1 ⅐ min Ϫ1 to induce prolonged hypertension in rats. 5 A number of studies have demonstrated that the medullary vasculature of Sprague-Dawley rats is relatively refractive to the vasoconstrictor effects of Ang II compared with the cortical circulation. 6,7 Subpressor doses of Ang II administered intravenously that can significan...

Abstract-In the present study, we assessed whether activation of the nitric oxide (NO) system within the renal medulla could serve as a buffer against the chronic hypertensive effects of arginine vasopressin (AVP). NO concentration in the renal medulla of Sprague-Dawley rats was measured with in vivo microdialysis/oxyhemoglobin NO trapping. The results showed that medullary interstitial [NO] was increased after 2 hours of AVP infusion and remained elevated even after 10 days (by 62Ϯ8% and 42Ϯ13%, respectively). Western blot analysis showed that 2 days of AVP infusion was insufficient to increase protein expression of any of the NO synthase (NOS) isoforms, but after 10 days of AVP infusion, endothelial NOS expression was significantly increased in the inner medulla with no significant changes in noninducible NOS and inducible NOS levels. When renal medullary NOS enzyme activity was blunted with a nonpressor dose of) that was chronically infused locally into the renal medulla, intravenous AVP infusion (which was shown earlier to be subpressor in chronic studies) produced a sustained elevation in arterial pressure (from 107Ϯ2 to 121Ϯ2 mm Hg). These data indicate that chronic elevations in plasma AVP enhance renal medullary endothelial NOS protein expression, which enables sustained elevations of NO concentrations in this region of the kidney to buffer the hypertensive effects of AVP. Key Words: arterial pressure Ⅲ kidney Ⅲ vasopressins Ⅲ nitric oxide Ⅲ endothelium Ⅲ nitric oxide synthase Ⅲ arginine A rginine vasopressin (AVP) is one of the most potent circulating vasoconstrictor hormones, 1-6 and it has long been recognized as the antidiuretic hormone. 2 Its combined vasoconstrictor and volume-retaining actions suggest that chronic elevations in circulating AVP should result in hypertension. Hypertension would also be predicted on the basis of observations that small elevations in AVP can significantly reduce medullary blood flow (MBF) and blunt pressure natriuresis, 4,6 both of which generally lead to hypertension. 3 However, AVP does not produce sustained hypertension in normal rats, dogs, or humans 3,7,8 maintained on a normal sodium intake.Recent studies in conscious rats in which changes in regional renal blood flow were determined with laser-Doppler flowmetry provide, in part, an answer to this puzzle. It was found that although AVP produced immediate reductions in MBF, the reductions were not sustained during a chronic 5-day medullary interstitial infusion of AVP, nor was hypertension observed. 8 In contrast, a medullary infusion of a vasopressin V 1 receptor (V 1 R) agonist produced a sustained reduction in MBF with a corresponding chronically sustained increase in mean arterial pressure (MAP). The results of these studies suggested that a parallel stimulation of both V 1 R and vasopressin V 2 receptors (V 2 Rs) with endogenous AVP resulted in an increase in a V 2 R-induced counterregulatory system. This was shown to be a V 2 R-mediated increase in medullary nitric oxide (NO) production. 9 These studies al...