Effects of inhibition of the L-arginine/nitric oxide pathway on vasodilation caused by beta-adrenergic agonists in human forearm.

Background: We examined whether vasodilator responses to beta-agonists in human forearm vasculature are mediated in part through the nitric oxide pathway.

Methods And Results: We measured forearm blood flow responses to brachial artery infusions of beta-adrenergic agonists in healthy men. Salbutamol was more than 100 times as potent as dobutamine. Cumulative doses of salbutamol (0.3 to 3.5 nmol.min-1) did not cause tachyphylaxis to an identical repeated infusion after a 24-minute recovery period. Vasodilators were infused with this sequence during coinfusion of saline and NG-monomethyl-L-arginine (L-NMMA, 4 mumol.min-1), an inhibitor of nitric oxide synthase. L-NMMA coinfusion inhibited responses (area under the dose-response curve) to isoproterenol (0.01 to 0.1 nmol.min-1) by 59 +/- 7% (n = 5) and inhibited those to salbutamol (0.3 to 3.5 nmol.min-1) by 52 +/- 6% (n = 8). L-NMMA had no significant effect on vasodilator responses to nitroprusside (2.7 to 11.0 nmol.min-1, n = 8), verapamil (20 to 80 nmol.min-1, n = 8), or prostacyclin (0.08 to 0.24 nmol.min-1, n = 8).

Conclusions: These results suggest that beta-adrenergic vasodilator responses in human forearm vasculature are mediated predominantly through beta 2-adrenergic receptors and are dependent on nitric oxide synthesis.