Adrenergic receptors in the forehead microcirculation.

The presence of alpha- and beta-adrenoceptors in the forehead microcirculation was investigated in 49 healthy subjects. Local vascular responses to noradrenaline, isoprenaline and adrenergic antagonists, administered transcutaneously by iontophoresis, were monitored via laser Doppler flowmetry. Iontophoresis of the alpha-adrenergic antagonist phentolamine induced a persistent increase in skin blood flow, whereas iontophoresis of saline induced a minor increase in skin blood flow which subsided rapidly. Skin blood flow increased moderately after the iontophoresis of the beta-adrenergic antagonist propranolol. Pretreatment of the experimental site with phentolamine blocked the normal vasoconstrictor response to noradrenaline, and unmasked a minor vasodilator component of response in some subjects. Iontophoresis of the beta-adrenergic agonist isoprenaline induced dose-dependent vasodilatation which was antagonised by propranolol. These findings indicate that alpha-adrenoceptors in the forehead microcirculation normally mediate a vasoconstrictor response to iontophoretically-applied noradrenaline. In addition, beta-adrenoceptors appear to mediate a minor vasodilator component of response.