Superoxide Inhibits Neuronal Nitric Oxide Synthase Influences on Afferent Arterioles in Spontaneously Hypertensive Rats

Abstract

Abstract-This study was designed to determine the influence of increased superoxide anion in neuronal nitric oxide synthase (nNOS)-dependent regulation of afferent arterioles in spontaneously hypertensive rats (SHR). Afferent arteriolar diameters of male Wistar-Kyoto rats (WKY) and SHR were assessed in vitro with the blood-perfused juxtamedullary nephron technique and averaged 21.6Ϯ1.6 (nϭ6) and 18.8Ϯ1.2 (nϭ7) m, respectively. The superoxide dismutase mimetic Tempol (1, 10, and 100 mol/L) did not influence afferent arterioles of WKY but significantly increased afferent arteriolar diameters of SHR by 20.6Ϯ5.5%, 25.2Ϯ5.4%, and 23.3Ϯ4.9%, respectively. In WKY (nϭ6), the nNOS inhibitor S-methyl-L-thiocitrulline (L-SMTC; 10 mol/L) and the NOS inhibitor N -nitro-L-arginine (L-NNA; 100 mol/L) significantly decreased afferent arteriolar diameters (19.6Ϯ1.6 m) by 11.9Ϯ3.1% and 21.0Ϯ3.9%, respectively. In SHR (nϭ7), L-SMTC did not influence afferent arteriolar diameters (21.0Ϯ1.5 m), but L-NNA exerted an afferent arteriolar constriction (14.8Ϯ3.2%) that was similar to the response observed in WKY. Experiments were also performed in the presence of 100 mol/L Tempol. In afferent arterioles of WKY (nϭ6), Tempol treatment did not modulate the basal diameters (21.5Ϯ1.2 m) or the constrictor response to L-SMTC (10.6Ϯ2.1%) or L-NNA (19.3Ϯ3.3%). In SHR (nϭ8), Tempol significantly increased afferent arteriolar diameters by 22.5Ϯ4.3% and enhanced afferent arteriolar constrictor responses to L-SMTC (18.4Ϯ2.7%) and L-NNA (31.9Ϯ2.6%). However, the nitric oxide donor S-nitroso-N-acetylpenicillamine (10 mol/L), which similarly increased afferent arteriolar diameters (17.2Ϯ2.3%, nϭ6), did not affect afferent arteriolar responses to L-SMTC (1.5Ϯ2.7%) or L-NNA (18.6Ϯ2.3%). These suggest that superoxide anion inhibits the control of afferent arteriolar diameters by nNOS in SHR. Key Words: Tempol Ⅲ nitric oxide synthase Ⅲ arterioles Ⅲ rats, spontaneously hypertensive Ⅲ kidney N itric oxide (NO) is recognized as a major paracrine regulator of renal microvascular tone. 1 Inhibition of NO synthase (NOS) causes renal microvascular constriction with consequent decreases in renal blood flow and glomerular filtration rate. Therefore, renal NO deficiency may theoretically lead to the development of hypertension. However, constitutive NOS activity has been reported to be maintained in the kidneys of various models of hypertension. [2][3][4][5] In kidneys of spontaneously hypertensive rats (SHR), constitutive NOS activity is also maintained or elevated, 6 -9 suggesting that renal NO production is not reduced. However, the endothelium-dependent relaxation of renal vasculatures has been reported to be impaired in this model of hypertension. 10,11 This finding can be explained by the reduced production of NO-independent vasodilators, such as endothelium-derived hyperpolarizing factor, or a decrease in NO bioavailability. Recent studies have demonstrated that superoxide anion (O 2 Ϫ ) production is increased in the aorta of stroke-prone SHR 12,13 and in...

Role of oxidative stress in angiotensin-induced hypertension

“…This investigator found that in the SHR, a decrease in the synthesis of NO by neuronal NO synthase is responsible for increasing the TGF response and that this alteration is corrected by the microperfusion of tempol into the juxtaglomerular apparatus (178). This observation was confirmed later on by Ichihara et al (66). An additional observation that confirmed the participation of NO in the TGF response in SHR is that the microperfusion of NO donors (such as nitroprusside) into the macula densa of SHR nephrons produces a dose-dependent blunting of the TGF response, which is enhanced by microperfusion of tempol (178).…”

Renal Renin-Angiotensin System

“…Thus, superoxide may mediate the decreased ability of nNOS-derived NO to counteract the TGFmediated afferent arteriolar constriction in Ang II-induced hypertension. This concept is supported by recent evidence that the superoxide dismutase mimetic, tempol, restores the reduced bioavailability of nNOS-derived NO in spontaneously hypertensive rats [38]. In addition, the expression of the nNOS gene and protein in the macula densa are upregulated in angiotensinogen-gene-knockout mice [39] and AT1 receptor-deficient mice [40].…”

NO-independent mechanism mediates tempol-induced renal vasodilation in SHR

“…This is supposedly due to the reaction between O 2 Ϫ and nitric oxide (NO) to form peroxinitrite (ONOO Ϫ ) (3). This, in turn, lowers the levels of NO, thereby causing vasoconstriction and hypertension (20,23,31,42,51). It has been proposed that such a mechanism may play a role in determining renal blood flow.…”

“…In that study, the investigators found that ANG II induced an NADPH oxidase-dependent production of O 2 Ϫ , which reduced the bioavailability for NO and in turn contributed to renal vasoconstriction. Direct measurements of rat afferent arteriolar diameter utilizing the blood perfused juxtamedullary nephron preparation show that tempol causes vasodilation of this vessel in SHR but is without effect in WKY (20). It was suggested that O 2 Ϫ selectively inhibited the action of neuronal nitric acid synthase (nNOS), which in the kidney emanates from the macula densa cells.…”