Electrophysiological properties of automatic fibers in rabbit atrioventricular valves

Subsidiary atrial automaticity generated by cardiac fibers of the rabbit tricuspid valve was studied in vitro with the use of standard microelectrode techniques. The time course of diastolic depolarization of leading pacemaker fibers was characteristically biphasic over a potential range of -80 to -55 mV, with a brief primary phase of relatively steep slope and a longer secondary phase of lesser slope. The regenerative upstroke of pacemaker potentials was suppressed by cobalt ions and varied with changes in extracellular Ca²⁺ concentration in a manner suggesting a slow channel process. The time course of each diastolic phase, estimated by measuring slope, varied in a reproducible manner with changes in membrane potential elicited with long duration current pulses. Cesium ions reduced the slope of both diastolic phases at all potentials with greater effects on the primary diastolic phase, but spontaneous activity in the absence of current pulses was not significantly altered. Furthermore, pacemaker activity varied with changes in extracellular K⁺ concentration and in response to barium ions in manners suggesting the presence of a decaying K⁺ conductance. These experiments demonstrate the utility of valve tissue as a model of subsidiary atrial pacemaker activity and suggest the presence of current systems qualitatively similar to those observed in the sinoatrial node.