Selective activation of the adrenal medulla during acute bilateral carotid occlusion and its modulation by alpha-adrenergic receptors in the rat.

The sympathoadrenal activity was studied during baroreflex stimulation in chloralose anesthetized rats. Circulating norepinephrine (NE) and epinephrine (E) levels were used as indices of sympathetic fiber and adrenal medulla activities, respectively, under basal conditions and during a 1-min bilateral carotid occlusion (CO). In vagotomized rats, the CO induced a significant increase in mean arterial pressure (MAP) associated with an increase in circulating E levels, while this procedure did not alter blood pressure or circulating NE or E levels in intact animals. Following vagotomy, the baroreflex stimulation activated specifically the adrenal medulla, without alteration of the sympathetic fiber activity since the NE levels were not modified by the occlusion. Moreover, in support of that hypothesis, chemical sympathectomy did not decrease the pressure response to CO while bilateral adrenalectomy almost completely abolished this response. The elevation of circulating E induced by the CO was greatly potentiated by pretreatment with Yohimbine, a selective alpha 2-antagonist, and was completely abolished by administration of Clonidine, an alpha 2-agonist, while phenoxybenzamine, which is mainly an alpha 1-antagonist, did not potentiate significantly the E response to CO. These results therefore suggest that the baroreflex activation of the adrenal medulla induced by CO may be modulated in vivo via alpha 2-adrenergic receptors that could be localized on chromaffin cells.