Overdrive suppression of automaticity in cultured chick myocardial cells

Overdrive suppression in automatic cells and postdrive hyperpolarization in quiescent cells were examined in cultured embryonic chick heart cells. Cells were enzymatically separated from 3-day-old whole hearts and from 16-day-old atria and ventricles. The dispersed cells were allowed to reaggregate into small spheres (aneural) and maintained in culture for 1--4 wk. Quiescent reaggregates (all ventricular reaggregates and some of the atrial) demonstrated postdrive transient hyperpolarization of up to 5 mV. The spontaneously beating reaggregates (some of the atrial and all of the 3-day-old hearts) demonstrated overdrive suppression of automaticity. The faster the predrive spontaneous firing rate, the shorter was the overdrive suppression period. Prolongation of the drive period increased the duration of the overdrive suppression. The time courses of recovery from both overdrive suppression and postdrive hyperpolarization were similar. Atropine (10(-5) M) did not affect the phenomena. Ouabain (10(-6) M) reduced the postdrive hyperpolarization and shortened the duration of overdrive suppression. It was concluded that overdrive suppression and postdrive hyperpolarization a) occur in cultured chick ventricular and atrail cells; b) occur in early embryonic stages of development; c) occur independently of cholinergic receptors; d) are dependent on stimulation of an elecrogenic Na-K pump.