Interactions of K, Na, and vagal stimulation in the S-A node of the rabbit

Abstract

Right atrial preparations with functional vagi were isolated from rabbits and maintained in modified Ringer solution at 30[degree] C. The effects of changes in [Na]0, [Ca]0, and [Mg]0 were observed on automaticity and on electrical properties of pacemaker cells in the S-A [Sino-atrial] node, with and without the intervention of cholinergic nerve stimulation. The spontaneous cycle duration was related inversely to [Ca]0; the slope of the curve was shallow at normal [Na]o but steep at 30% of normal [Na]0. In Na-deficient solution the time to onset of asystole was related inversely to the [Ca]0. The magnitude of the negative chronotropic response and of membranal hyperpolarization following vagal stimulation was related directly to the [Ca]0. These effects were potentiated by reduction of [Na]0. Mg increased spontaneous cycle duration, but the negative chronotropic response to vagal stimulation was decreased by Mg. An increase in the [Ca]0 antagonized the vagaldepressant action of [Mg]0 without modifying its negative chronotropic action. It was concluded that Ca can modify vagal influence by affecting the release of cholinergic transmitter, and that it can also modify spontaneous excitability and vagal influence by competing with Na at the cell membrane.