Objective: Although it has been shown recently that acetylcholine (ACh)-induced vasodilation of forearm resistance vessels is predominantly mediated by nitric oxide, direct biochemical evidence for eNOS stimulation by bradykinin (BK) in the human arterial circulation is still lacking. Therefore, the present study was designed to test the hypothesis that in the human forearm vasculature eNOS stimulation significantly contributes to BK-induced vasodilation.
Methods: BK was infused in the presence and absence of the NOS inhibitor L-NMMA (8 micromol/min) into the brachial artery of 16 healthy volunteers and the effects compared to muscarinergic eNOS stimulation following acetylcholine infusion. Forearm blood flow (FBF) was measured by venous occlusion plethysmography, and plasma nitrite (NO(2)(-)), which represents a sensitive and specific marker of regional eNOS activity, was determined in the antecubital vein and brachial artery by flow injection analysis. Nitric oxide production was calculated as product of the veno-arterial difference of NO(2)(-) concentration times FBF.
Results: Kininergic (BK: 20, 60, 200 ng/min) as well as muscarinergic (ACh: 1, 3, 10 microg/min) stimulation resulted in a dose-dependent increase in FBF and NO(2)(-) in each individual. The relationship between FBF and NO production upon BK infusion was comparable to that obtained with ACh (r = 0.98; n = 64, p < 0.01). Moreover, NOS inhibition reduced both flow responses and NO production (BK: 54 and 75 %; ACh: 57 and 72 %) to a similar extent.
Conclusions: These data provide direct biochemical evidence for the involvement of eNOS in bradykinin-induced vasodilation of forearm resistance vessels in humans.