Blacks have a greater sensitivity to α1-adrenoceptor-mediated venoconstriction compared with whites

Abstract

Blacks have increased hemodynamic responses to both physiological and pharmacological adrenergic stimulation compared with whites, and this may contribute to the greater prevalence of hypertension in this ethnic group. A small study suggested enhanced α1-adrenoreceptor-mediated arterial vasoconstriction in the forearm vasculature of blacks compared with whites, but it is unknown whether this reflects a generalized vascular phenomenon. The objective of this study was to examine the hypothesis that there are ethnic differences in venous α1-adrenoreceptor responsiveness. Using a linear variable differential transformer, we measured local dorsal hand vein responses to increasing doses of the selective α1-adrenoreceptor agonist, phenylephrine, in 106 subjects (64 whites and 42 blacks). There was wide interindividual variability in responses to phenylephrine. The dose that produced 50% of maximal constriction (ED50) ranged from 11 to 5442 ng/min, and maximal venoconstriction (Emax) ranged from 13.5% to 100%. Blacks (geometric mean ED50 =172 ng/min; 95% confidence interval, 115-256 ng/min) were more sensitive to phenylephrine than whites (310 ng/min; 95% confidence interval, 222-434 ng/min; unadjusted P=0.026; adjusted P=0.003). Median Emax was slightly higher in blacks (89%; interquartile range, 82% to 98%) compared with whites (85%; interquartile range, 75% to 95%; P=0.07). Taken together with previous findings in arterial vessels, our results suggest a generalized increased sensitivity to α1-adrenoreceptor-mediated vasoconstriction in blacks. Increased vascular α-adrenoreceptor sensitivity could predispose to hypertension, and future studies addressing the contribution of this mechanism to ethnic differences in the prevalence of hypertension will be of interest.

Figure 1

Dose-response curves in Caucasians (blue, n=64) and African Americans (red, n=42) based on averaged hand vein responses to phenylephrine. African Americans have a leftward shift in the dose-response curve indicating increased sensitivity with a significantly lower ED₅₀ and a trend to increased efficacy (increased E_max). The dotted lines represent the 95% CI.

Figure 2

Vascular sensitivity to phenylephrine (expressed as Log ED₅₀) in Caucasians and African Americans. The horizontal lines represent the mean, and the whiskers represent the standard error of the mean. African Americans were more sensitive to phenylephrine than Caucasians (unadjusted p=0.026; adjusted p=0.003).

Figure 3

Efficacy of phenylephrine (expressed as E_max) in Caucasians and African Americans. The horizontal lines represent the median, and the whiskers represent the interquartile range.