Local inhibition of nitric oxide generation in man reduces blood flow in finger pulp but not in hand dorsum skin.

1. Nitric oxide generation is important in the regulation of resistance vessel tone. Until now, however, there has been no evidence of such a role for basal generation of nitric oxide in the skin microcirculation of humans. 2. To investigate this, L-NG-monomethylarginine (L-NMMA), a competitive inhibitor of nitric oxide synthase, was administered at 1, 2 and 4 mumol min-1 (each for 10 min), via the brachial artery, in six healthy male subjects. 3. At each dose, using laser Doppler fluximetry, red blood cell flux was measured as an index of blood flow in the pulp of the thumb, an area rich in arteriovenous anastomoses, and on the dorsal surface of the hand, where arteriovenous anastomoses are rare. Finger nailfold capillary blood velocity was monitored at each dose using videomicroscopy. Forearm blood flow was measured by venous occlusion plethysmography, before, and 8 min after, completing infusion of L-NMMA. All data were obtained from both the infused and control arms. 4. L-NMMA reduced blood flow in the infused forearm by 37% (P = 0.005). In contrast, dorsum red cell flux, capillary blood velocity, and skin temperature were unchanged. There was, however, a significant reduction in thumb red cell flux (ANOVA, P = 0.0001), reaching a maximum reduction of 33% with 4 mumol min-1 L-NMMA. There were no effects apparent in the opposite arm. 5. These results suggest that endogenous nitric oxide production may be more important in regulating microvascular skin blood flow in regions rich in arteriovenous anastomoses than in areas containing mainly nutritive vessels.