Inhibition of NAD(P)H oxidase potentiates AT2 receptor agonist-induced natriuresis in Sprague-Dawley rats

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Abstract
A positive association between renin-angiotensin system, especially AT1 receptor, and oxidative stress in the pathogenesis of hypertension and cardiovascular/renal diseases has been suggested. However, the role of oxidative stress, especially superoxide radicals in renal sodium handling in response to AT1 and AT2 receptors, is not known. Therefore, the present study was designed to investigate the role of NAD(P)H oxidase (NOX), a major superoxide radical producing enzyme, in AT1 and AT2 receptor function on natriuresis/diuresis in Sprague-Dawley rats. The rats under anesthesia were intravenously infused with NOX inhibitor apocynin (3.5 μg·kg−1·min−1), the AT1 receptor antagonist candesartan (100 μg/kg; bolus), and the AT2 receptor agonist CGP-42112A (1 μg·kg−1·min−1) alone and in combinations. Candesartan alone significantly increased urinary flow (UF; μl/30 min) by 53 and urinary Na excretion (UNaV; μmol/min) by 0.4 over basal. Preinfusion of apocynin had no effect on the net increase in UF or UNaV in response to candesartan. On the other hand, apocynin preinfusion caused profound increases in CGP-42112A-induced UF by 72, UNaV by 1.14, and fractional excretion of Na by 7.8. Apocynin and CGP-42112A alone did not cause significant increase in UF or UNaV over the basal. CGP-42112A infusion in the presence of apocynin increased urinary nitrite/nitrates and cGMP over basal. The infusion of candesartan, apocynin, and CGP-42112A alone or in combinations had no effect on the blood pressure or the glomerular filtration rate, suggesting tubular effects on natriuresis/diuresis. The data suggest that NOX may have an antagonistic role in AT2 receptor-mediated natriuresis/diuresis possibly via neutralizing nitric oxide and thereby influence fluid-Na homeostasis.