Regional Blood Flow Responses to Acute ANG II Infusion: Effects of Nitric Oxide Synthase Inhibition

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Abstract
We hypothesized that nitric oxide (NO) opposes regional vasoconstriction caused by acute angiotensin II (ANG II) infusion in conscious rats. Mean arterial pressure (MAP), blood flow, and vascular conductance (regional blood flow/MAP; ml/min/100 g/mm Hg) were measured and/or calculated before and at 2 min of ANG II infusion (0.05 or 1 μg/kg/min, i.a.) in the absence and presence of NO synthase (NOS) inhibition [N G-nitro-L-arginine methyl ester (L-NAME), 0.25 or 1 mg/kg, i.a.]. ANG II reduced stomach and hindlimb conductance only after NOS inhibition. For example, whereas 0.05 μg/kg/min ANG II did not attenuate conductance in the stomach (i.e., 1.04 ± 0.08 to 0.93 ± 0.12 ml/min/100 g/mm Hg), this variable was reduced (i.e., 0.57 ± 0.14 to 0.34 ± 0.05 ml/min/100 g/mm Hg; p < 0.05) when ANG II was infused after 0.25 mg/kg L-NAME. In addition, whereas hindlimb conductance was similar before and after administering 1 μg/kg/min ANG II (i.e., 0.13 ± 0.01 and 0.09 ± 0.02, respectively), this variable was reduced (i.e., 0.07 ± 0.01 and 0.02 ± 0.00, respectively; p < 0.05) when ANG II was infused after 1 mg/kg L-NAME. These findings indicate that NO opposes ANG II-induced vasoconstriction in the stomach and hindlimb. In contrast, whereas both doses of ANG II decreased (p < 0.05) vascular conductance in the kidneys and small and large intestine regardless of whether NOS inhibition was present, absolute vascular conductance was lower (p < 0.05) after L-NAME. For example, 1 μg/kg ANG II reduced renal conductance from 3.34 ± 0.31 to 1.22 ± 0.14 (p < 0.05). After 1 mg/kg L-NAME, renal conductance decreased from 1.39 ± 0.18 to 0.72 ± 0.16 (p < 0.05) during ANG II administration. Therefore the constrictor effects of NOS inhibition and ANG II are additive in these circulations. Taken together, our results indicate that the ability of NO to oppose ANG II-induced constriction is not homogeneous among regional circulations.