Attenuation of cyclic nucleotide-mediated smooth muscle relaxation in blacks as a cause of racial differences in vasodilator function.

Vasodilator reactivity is attenuated in normotensive blacks, and this may contribute to their enhanced susceptibility to hypertension and its complications. However, the mechanisms responsible for this phenomenon are unknown. We therefore studied nitric oxide (NO)-dependent and -independent vasorelaxation in healthy blacks and whites to investigate the nature of racial differences in vasodilator function. Forearm flow responses to intra-arterial infusion of increasing doses of acetylcholine (a vasodilator that stimulates endothelial release of NO), sodium nitroprusside (an exogenous NO donor), and isoproterenol (a beta-adrenergic agonist whose vasodilator effect stems from the combination of direct smooth muscle stimulation and endothelial NO release) were studied in 18 normotensive whites and 18 blacks by use of strain-gauge plethysmography. A blunted vasodilator response to acetylcholine (7.2+/-1.1 versus 14.4+/-1.8 mL. min-1. dL-1; P<0.001) and sodium nitroprusside (8.2+/-1.1 versus 12.1+/-1.3 mL. min-1. dL-1; P<0.001) was observed in blacks compared with whites, suggesting decreased cGMP-mediated smooth muscle relaxation. The vasodilator effect of isoproterenol was lower in blacks than in whites both before (10.9+/-1.7 versus 14.9+/-1.5 mL. min-1. dL-1; P=0.006) and after NG-monomethyl-L-arginine (6.1+/-1.2 versus 10. 1+/-0.8 mL. min-1. dL-1; P<0.001), implying that cAMP-dependent vasodilator response to isoproterenol is diminished in blacks. No significant difference was observed in the hyperemic response to forearm ischemia. Compared with whites, healthy blacks have reduced vasodilation in response to NO-dependent and -independent stimuli. This difference seems to be related to an attenuation in cyclic nucleotide-mediated vascular smooth muscle relaxation and may play a role in the increased prevalence of hypertension and its complications in blacks.