Pacemaker currents in chick embryonic heart cells change with development.

The initial heartbeat of the chick embryo occurs shortly after the first day of incubation. The pacemaker of this beat originates in the region of the primitive heart destined to become sinoatrial tissue in the adult. Individual cells isolated from the atrial and ventricular portions of the embryonic heart are also capable of beating spontaneously at this stage. However, the intrinsic activity of these cells gradually diminishes from about day 7 until day 21 when the chick hatches. We have investigated these changes in automaticity by measuring membrane pacemaker currents with the voltage-clamp technique from aggregates of cells prepared from 7-, 12- and 17-day-old chick embryo cardiac ventricle. We report that there is a voltage and time dependent conductance at 7 days which is markedly reduced by 17 days. The reduction of this pacemaker current parallels the decrease of spontaneous activity in these preparations.