ATTENUATION OF ISOPROTERENOL-MEDIATED VASODILATATION IN BLACKS

Background. The mechanism of enhanced vascular reactivity in young bla cks, which may play a part in the development of hypertension, has not been defined. To determine the contribution of blunted vasodilatation mediated by beta(2)-adrenergic receptors to this phenomenon, we compa red forearm blood-flow responses to isoproterenol in young black and w hite normotensive men. Methods. We used venous-occlusion plethysmograp hy to measure the responses of blood flow in the forearm to the intraa rterial administration of isoproterenol (10 to 400 ng per minute) in 9 normotensive black men (mean [+/-SD] age, 31.3+/-8.0 years) and 13 no rmotensive white men (mean age, 32.91+/-5.6 years). Sympathetic activi ty in the forearm was measured simultaneously by isotope-dilution tech niques. Results. Base-line blood flow in the forearm was similar in bl acks and whites, but the degree of vasodilatation in response to isopr oterenol was markedly lower in blacks. Isoproterenol at an infusion ra te of 400 ng per minute produced a 9-fold increase in blood flow in wh ites but only a 3.5-fold increase in blacks (P<0.001). The base-line r ate of norepinephrine spillover in the forearm was higher in blacks (2 .0+/-1.3 ng per minute [11.8+/-7.7 nmol per minute]) than in whites (0 .6+/-0.5 ng per minute [3.5+/-3.0 nmol per minute], P=0.002), but ther e was no difference between the groups after isoproterenol stimulation . Conclusions. Forearm blood-flow responses to isoproterenol were mark edly attenuated in normotensive blacks, indicating a blunting of vasod ilatation mediated by beta(2)-adrenergic receptors. Our findings sugge st that the mechanisms responsible for blunted vasodilatation in respo nse to the administration of isoproterenol may contribute to enhanced vascular reactivity in blacks and may play a part in the pathogenesis of hypertension in blacks.