Electrotonic interactions between aggregates of chick embryo cardiac pacemaker cells.

Synchronization of spontaneously active heart cell aggregates occurs shortly after they are brought into contact. The synchronous rate is determined by pacemaker phase resetting and passive subthreshold electrotonic interactions. To further study the effects of passive electrical interactions, we have used 150-microns diameter aggregates prepared from cells of 4d (4-day ventricle + 1 day in vitro), 7d, and 14d embryonic chick ventricle as models of primary, latent, and nonpacemaker tissues, respectively. Coupling of 4d and 7d aggregates (4d/7d pairs) leads to intermediate synchronous rates. We show here that elevating external K+ from 1.3 to 2.8 mM, which has no effect on 4d/4d pairs but selectively reduces the beat rate of 7d/7d pairs by 42%, slows the synchronous beat rate of 4d/7d pairs by 23%. Increases in electrical coupling in newly joined 4d/14d pairs cause the 4d rate to slow to a minimum value (16 +/- 13 beats/min, n = 16) just prior to the onset of synchronous activity. The rate slowly recovers to a final value of 40 +/- 12 beat/min. We conclude that the spontaneous beat rate of a primary pacemaker is modulated by both active and passive interactions with latent or nonpacemaker tissues.