Effects of Barium on Membrane Potentials of the Sino-Atrial Node in Response to Sympathetic Nerve Stimulation

Electrophysiological studies prove that consistent alterations induced by Ba++ in the transmembrane potential from cardiac, skeletal and smooth muscles are represented by a reduction in the resting potential and a prolongation of the action potential duration (1–3), which would be considered to correlate intimately to a production of automaticity in quiescent muscles unless driven by other cells or by artificial stimuli. Pacemaker activity induced by Ba++ in the atrial and ventricular myocardium mimics the normal impulse generation with respect to the responses of the spontaneous rate and of the membrane potential to exogenous adrenaline (2) and to electrical rapid stimulation of intracardiac sympathetic nerve fibers (4). It is generally accepted that adrenaline and noradrenaline increase spontaneity of cardiac muscles in association with alterations in the permeability of membrane to cations (5). However, an earlier report (4) shows that cardiac noradrenaline does not seem to participate in automaticity induced by Ba++.