Role of nitric oxide in reactive hyperemia in human forearm vessels

The role of nitric oxide (NO) in reactive hyperemia (RH) is not well known. We investigated whether NO plays a role in RH in human forearm vessels by examining the effects of NG-monomethyl-L-arginine (L-NMMA), a blocker of NO synthesis, on reactive hyperemic flow. Forearm blood flow (FBF) was measured by strain-gauge plethysmography with a venous occlusion technique. The left brachial artery was cannulated for drug infusion and direct measurement of arterial pressure. To produce RH, blood flow to the forearm was prevented by inflation of a cuff on the upper arm to suprasystolic pressure for intervals of 3 and 10 minutes. After the release of arterial occlusion (AO), FBF was measured every 15 seconds for 3 minutes. Resting FBF was 4.3 +/- 0.3 mL.min-1.100 mL-1 before 3 minutes of AO and 4.1 +/- 0.6 mL.min-1.100 mL-1 before 10 minutes of AO. FBF increased to 32.3 +/- 1.9 and 38.2 +/- 3.1 mL.min-1.100 mL-1 immediately after 3 and 10 minutes of AO, respectively, and gradually decayed (n = 13). Intra-arterial infusion of L-NMMA (4 mumol/min for 5 minutes) decreased baseline FBF (P < .01) without changes in arterial pressure. L-NMMA did not affect the peak reactive hyperemic FBF after 3 and 10 minutes of AO. L-NMMA significantly decreased total reactive hyperemic flow (flow debt repayment) by 20% to 30% after 3 and 10 minutes of AO. Simultaneous infusion of L-arginine (a precursor of NO) with L-NMMA reversed the effects of L-NMMA. Our results suggest that NO plays a minimal role in vasodilation at peak RH but plays a modest yet significant role in maintaining vasodilation after peak vasodilation. Our results also suggest that reactive hyperemia in human forearms is caused largely by mechanisms other than NO.