Attenuation of isoproterenol-mediated vasodilatation in blacks

To the Editor: The study by Lang and colleagues (July 20 issue)¹ is provocative but lacks two important control observations. First, as compared with the white subjects, the black subjects had markedly attenuated forearm blood-flow responses to isoproterenol yet similar increases in regional norepinephrine spillover. Hyperresponsiveness to the α-adrenergic effects of endogenous norepinephrine is therefore at least as likely an explanation of the findings as hyporesponsiveness to β-adrenergic stimulation by isoproterenol. No mention is made of this possibility; a direct infusion of norepinephrine or phenylephrine would easily settle the issue. Second, since regional norepinephrine spillover is heavily dependent on blood.