Development of electrical coupling and action potential synchrony between paired aggregates of embryonic heart cells

Pairs of spheroidal aggregates of embryonic chick heart cells, held in suction pipettes were brought into contact and allowed to synchronize their spontaneous action potentials. Contractions were suppressed with cytochalasin B. Both intracellular and extracellular electrodes were used to analyze the development of synchrony. Electric coupling occurred in three phases. During phase I electrical interactions were absent despite close physical contact. Phase II was characterized by partial synchrony. Action potentials in the faster aggregate (F) induced small depolarizations in the other member of the pair (S). These depolarizations sometimes triggered action potentials in S depending on when during the diastolic depolarization in S they occurred. In these cases both the latency between the action potentials (L) and the fluctuations in latency (VL) were large. At the end of phase II the aggregates often passed through a brief period when functuation in interbeat interval in both increased noticeably. In phrase III, beginning about 8 min after initial contact, action potentials were completely entrained at a certain L. During the subsequent 20--40 min L fell along an approximately exponential time course from about 130 to less than 1 msec, while VL declined in parallel. When well-coupled aggregates were pulled apart and immediately pressed back together, they re-established synchronization according to the usual three-phase time course. Synchronized aggregates could be partially decoupled by separating them just far enough to reduce the area of mutual contact. Pairs joined only by cellular strands maintained entrained action potentials with long latencies for many minutes. These results indicate that electronic junctions form between the paired heart cell aggregates causing the gradual development of action potential synchrony.