L-NMMA INCREASES ARTERIOLAR BUT NOT VENOUS TONE IN HUMANS - THE MECHANISM APPEARS TO BE INDEPENDENT OF SYMPATHETIC TONE

L-NMMA increases arteriolar but not venous tone in humans but the mechanism underlying this arteriovenous difference is unclear. One possibility is that the basal sympathetic tone in arterioles stimulates basal nitric oxide (NO) release, or that NO interferes with sympathetic neurotransmission. In this study, we have investigated the effects of local infusions of N(G)-monomethyl-L-arginine (L-NMMA) on the norepinephrine dose-response and the response to sympathetically mediated venoconstriction in human hand veins in vivo. The results show that L-NMMA has no effect on sympathetic venoconstriction or the constriction produced by infusion of norepinephrine. The lack of effect of L-NMMA on resting venous tone cannot be explained by the absence of basal norepinephrine release from sympathetic nerves. We can find no evidence for interaction between NO and sympathetic nerves in these vessels.