Blunted blood pressure response to central sympathoinhibition in normotensive blacks: increased importance of nonsympathetic factors in blood pressure maintenance in blacks

Enhanced sympathetic reactivity may predispose blacks to the development of hypertension and may occur because of increased sympathetic stimulation and/or attenuated sympathoinhibition. A potential site for such attenuated sympathetic inhibition may be at the level of central a₂-adrenergic receptors, which play an important role in the feedback inhibition of norepinephrine release. We used cumulative doses (1, 2, and 3 µg/kg IV) of the centrally acting a₂-adrenergic agonist clonidine to measure the sensitivity of a₂-adrenoceptor–mediated sympathoinhibition and the resultant hypotensive response in 8 normotensive blacks and 10 normotensive whites. Sympathetic activity was determined by radioisotope dilution methodology. Basal norepinephrine spillover was similar in blacks (0.80±0.14 µg/min) and whites (0.73±0.19 µg/min, P=NS) and after clonidine decreased significantly in both blacks (0.21±0.07 µg/min, P<.0001) and whites (0.24±0.06 µg/min, P<.0001), with no difference between the groups (P=NS). Despite this similar degree of sympathoinhibition, the hypotensive response to clonidine was markedly blunted in blacks, such that mean arterial pressure decreased by only 10% in blacks but by 21% in whites (P<.0001). The smaller blood pressure decrement after clonidine in normotensive blacks, in the face of an equal degree of sympathoinhibition, suggests that even when sympathetic tone is decreased to the same level in blacks and whites, normotensive blacks have less reduction in blood pressure than whites, implying that nonadrenergic mechanisms contribute more to blood pressure maintenance in blacks than whites. Whether a similar  interethnic difference in response to sympathoinhibition occurs in hypertensive patients is as yet unknown.