Intra-arterial vasopressin in the human forearm: pharmacodynamics and the role of nitric oxide.

BACKGROUND AND OBJECTIVES: Diverse vascular effects have been ascribed to vasopressin, including the potential to cause vasodilation, vasoconstriction, and nitric oxide release. The objective of this study was to establish the pharmacodynamics, reproducibility, and nitric oxide dependence of the vasomotor actions of vasopressin in the forearm resistance vessels. METHODS: Blood flow in both forearms of 12 healthy men was measured with venous occlusion plethysmography. Continuous and discontinuous doses of 1 to 300 pmol/min vasopressin were administered by the intrabrachial route. For assessment of the contribution of nitric oxide, vasopressin was coadministered with a "nitric oxide clamp," a balanced coinfusion of 4 micromol/min L-N(G)-monomethylarginine (a nitric oxide synthase inhibitor) and 0.3 to 0.8 nmol/min sodium nitroprusside (an exogenous nitric oxide donor) to block endogenous nitric oxide production and restore normal basal blood flow, respectively. RESULTS: Vasopressin produced a dose-dependent biphasic change in blood flow with a maximum reduction in percentage change in blood flow ratio of infused and control arms of 22% +/- 5% at 3 pmol/min (P