Characteristics and functions of two types of efferent fibers in the adrenal nerve.

Two types of adrenal nerve fibers, AD and NA types, were identified by the presence or absence of inhibition of efferent activity after an intracarotid injection of isoproterenol (a beta-adrenergic stimulant). Efferent activity of both types was suppressed by adrenaline or noradrenaline injection. In AD-type fibers, suppression due to adrenaline or isoproterenol injection was blocked by a beta-adrenergic blocking agent (propranolol). In NA type fibers, the suppressive effect of adrenaline or noradrenaline was blocked by an alpha-adrenergic blocking agent (phenoxybenzamine). Dose-response characteristics showed that in AD-type fibers suppressive effect of adrenaline was always larger than that of noradrenaline, whilst in NA type fibers the two catecholamines caused the same degree of suppression of nerve activity. Two negative feedback loops for catecholamine secretion are suggested; one is between adrenaline level in the circulating blood and AD type nerve activity mediated be beta-adrenergic receptors, and the other is between noradrenaline level and NA type nerve activity, mediated by alpha-adrenergic receptors. Considering that AD-type fibers terminate on adrenaline-storing cells and NA type fibers terminate on noradrenaline-storing cells, it is suggested that beta-receptor--AD-type fibers--adrenaline-storing cell system controls adrenaline level and alpha-receptor--NA type fibers--noradrenaline-storing cell system controls noradrenaline level.