VASODILATORY EFFECT OF ARGININE-VASOPRESSIN IS MEDIATED BY NITRIC-OXIDE IN HUMAN FOREARM VESSELS

Arginine vasopressin (AVP) causes biphasic changes in vascular resistance in human forearms; vasoconstriction at lower doses and vasodilation at higher doses. Vasoconstriction is mediated by the V1 receptor. However, the mechanism of AVP-induced vasodilation is not known. We investigated whether AVP-induced vasodilation is mediated by nitric oxide (NO) in human forearms by examining the effects Of L-arginine (a precursor of NO) and N(G)-monomethyl-L-arginine (L-NMMA, a blocker of NO synthase) on AVP-induced vasodilation. AVP was infused intraarterially at doses of 0.05,0.1, 0.2,0.5, and 1.0 ng/kg per min (n = 8). The lower doses of AVP (less-than-or-equal-to 0.1 ng/ kg per min) increased, whereas the higher doses of AVP (greater-than-or-equal-to 0.5 ng/kg per min) decreased forearm vascular resistance (FVR) (P < 0.01). Intraarterially infUSed L-arginine at 10 mg/min did not alter arterial pressure, baseline FVR, or heart rate. L-arginine did not alter the magnitude of AVP-induced vasoconstriction at the lower doses, but L-arginine augmented the magnitude of AVP-induced vasodilation at doses of 0.2 (P < 0.05), 0.5 (P < 0.01), and 1.0 (P < 0.05) ng/kg per min. In another group (n = 6), intraarterially infused L-NM M A (4 mumol / min for 5 min) increased baseline FVR without systemic effects, and inhibited acetylcholine-induced vasodilation (P < 0.01). L-NMMA at this dose inhibited AVP-induced vasodilation (P < 0.01 ) but did not affect vasoconstriction. L-arginine reversed the inhibitory effect of L-NMMA. Our results suggest that the vasodilatory effect of AVP may be mediated by NO in human forearms.