Sympathetic regulation of cutaneous circulation in the rat

The purpose of the present study was to characterize, in vivo, the function and origin of peripheral alpha-adrenergic mechanisms in acral regions of the cutaneous microvasculature. Laser-Doppler flowmetry was used to continuously monitor changes in local cutaneous microvascular perfusion (CP) measured at the plantar surface of the terminal phalange in the rat. In ketamine-anesthetized rats, the intravenous administration of phentolamine (a nonselective alpha-adrenoceptor antagonist) elicited a dose-dependent (0.01-1.0 mg/kg, i.v.) increase in CP, a decrease in mean arterial pressure (MAP) and a marked reduction in cutaneous vascular resistance (CVR). These results demonstrate a high degree of endogenous alpha-adrenergic tone in the cutaneous microvasculature of the rat. In pithed rats, cutaneous vasoconstriction could be evoked by ganglionic stimulation with DMPP or by electrical stimulation of the peripheral cut end of the sciatic nerve trunk. DMPP and sciatic nerve responses were not altered by propranolol or atropine, but were inhibited by phentolamine. In addition, the effects of DMPP on CP were abolished by bilateral adrenal demedullation, but were unaltered by hindlimb denervation. These results suggest that the predominant vasoconstrictor tone in the cutaneous vasculature is mediated by a humoral action of circulating catecholamines at postjunctional alpha-adrenoceptors. The adrenal medulla appears to be the origin of this humoral tone. Postganglionic sympathetic nerves play an insignificant role in mediating cutaneous vasoconstriction elicited by ganglionic stimulation.