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The aim of this study was to assess the effects of acute or prolonged increases of ANG II on nitric oxide synthase (NOS) activities and protein expression in mesenteric resistance vessels, left ventricle, renal cortex, and renal medulla. The response of NOS activities to ANG II is compared with that induced by phenylephrine. ANG II or phenylephrine were infused over either 3 h or 3 days to conscious rats. NOS activity was examined by measuring the rate of conversion ofl-[¹⁴C]arginine tol-[¹⁴C]citrulline. Protein levels of endothelial (e) and neuronal (n) NOS were determined by Western blot analysis. Arterial pressure (AP) increased (P < 0.05) during acute and prolonged ANG II infusion. Ca²⁺-dependent NOS activity values (pmol of citrulline · min⁻¹ · g wet wt⁻¹) for control rats were 21 ± 9 in mesenteric arteries, 13 ± 7 in left ventricle, 14 ± 8 in renal cortex, and 411 ± 70 in renal medulla. Acute ANG II infusion increased …