Increased vascular adrenergic vasoconstriction and decreased vasodilation in blacks - Additive mechanisms leading to enhanced vascular reactivity

Blood pressure reactivity is enhanced in young black subjects through mecha nisms that are poorly understood, We compared alpha -adrenergic-mediated va soconstrictor and beta -adrenergic vasodilator sensitivity and their relati on to sympathetic activity in blacks and whites. Ten healthy black (age, 29 .9+/-2.4 years) and 10 white (age, 28.3+/-1.9 years) men were studied. Fore arm blood flow was measured with strain-gauge plethysmography after the int rabrachial artery administration of phenylephrine (1.25 to 20 mug/min) and isoproterenol (60 and 400 ng/min) after application of lower-body negative pressure and after a cold presser test. Forearm and systemic norepinephrine spillover were measured with a radioisotope dilution technique, alpha -Adr energic, vasoconstriction was markedly increased (ANOVA P=0.008) and beta - adrenergic vasodilation decreased (ANOVA P=0.02) in blacks. Phenylephrine ( 10 mug/min) decreased forearm blood flow by 58.0+/-2.5% in blacks but only by 26.6+/-6.0% in whites (P<0.001). Vasoconstrictor response to endogenous norepinephrine, stimulated by a cold presser test, resulted in a higher for earm vascular resistance in blacks than in whites (107.3+/-13 versus 64.8+/ -13 mm Hg.mL(-1).100 mL(-1), P=0.03). There were no significant ethnic diff erences in basal or stimulated forearm or systemic norepinephrine spillover . Increased vasoconstrictor and decreased vasodilator responses in blacks w ere not correlated. Increased sympathetically mediated vascular tone caused by enhanced vasoconstriction and attenuated vasodilation, effects that wou ld be additive, and not increased sympathetic activity could enhance vascul ar reactivity and may play a role in the pathogenesis of hypertension in bl acks.