Endothelium-derived nitric oxide contributes to the regulation of venous tone in humans

Background-Although nitric oxide (NO) is known to play an important part in the regulation of arterial tone, little is known about its role in veins. The aim of this study was to investigate the role of basal and stimulated N O activity in the regulation of tone of the human venous capacitance bed. Methods and Results-We measured venous tone using radionuclide forearm veno us plethysmography in 24 healthy subjects with no cardiovascular risk facto rs. In 13 subjects, basal NO activity was assessed by measuring the effects on venous tone of an intra-arterial infusion of the NO synthase inhibitor N-monomethyl-L-arginine (L-NMMA). In the remaining 11 subjects, stimulated NO activity was evaluated by measuring the effects of an intra-arterial inf usion of incremental doses of carbachol, followed in a subgroup by coinfusi on with L-NMMA. Infusion of carbachol caused dose-dependent venodilation, w ith a maximal reduction in forearm venous tone of 40.1+/-12.5% (P<0.0001). Carbachol-induced venodilation was inhibited by L-NMMA (48.9+/-6.2% reversa l of maximal venodilation, P<0.01). Infusion of L-NMMA alone caused venocon striction (9.1+/-6.4% increase in venous tone, P=0.002). Conclusions-Human forearm capacitance veins exhibit: both stimulated and ba sal NO activity, which indicates that NO contributes not only to the regula tion of venous tone but also to resting venous tone in healthy human subjec ts.