Mechanical stimuli exciting type A atrial vagal receptors in the cat.

The activity of type A right atrial vagal receptors was recorded from the right cervical vagus in cats anesthetized with sodium pentobarbital, immobilized with gallamine, and with their chests open. Nerve impulses initiated by receptor activation were recorded simultaneously with instantaneous right atrial pressure and dimensional changes under various hemodynamic conditions. Atrial volume changes induced by infusion of saline, bleeding, and occlusion of the inferior vena cava did not alter consistently the systolic activity of the receptors. Electrical stimulation of the right stellate ganglion significantly increased the frequency of discharge during systole, whereas electrical stimulation of the left thoracic vagus significantly reduced the frequency of discharge. These inotropic interventions produced similar effects when the heart was paced at a fixed rate. Pacing the right atrial appendage increased the systolic discharge of the receptors only when at high rates the atrium contracted against closed atrioventricular valves. To investigate the influence of tonic efferent sympathetic activity on spontaneous receptor discharge, three receptors were studied before and after bilateral surgical stellectomy, and in cats with their chest closed three receptors were studied before and after infusion of propranolol. Both of these interventions markedly reduced the systolic activity. In addition to having effects on systolic activity, injection of saline, vagal stimulation, and sympathetic "denervation" always activated the receptors during filling. Our results indicate that: (1) the systolic discharge of type A receptors is a function of the active tension developed by atrial muscle during contraction; and (2) the pattern of discharge of the receptors during the atrial cycle depends on both the degree of atrial distention and the state and extent of contraction.