Electrical activity in embryonic heart cell aggregates. Pacemaker oscillations.

Aggregates were formed from dissociated heart cells of 7-day chick embryos. When spontaneous action potentials were blocked with 10-8 to 10-7 g/ml tetrodotoxin (TTX) oscillatory pacemaker potentials were sometimes seen. The emergence of these pacemaker potentials was critically dependent on the external potassium concentration. In 1.3 mM potassium medium TTX suppression of action potential generation always led to a stable resting potential close to the threshold level (-55 to -50 mV). In 4.3 mM potassium TTX suppression was followed by a train of pacemaker potentials which usually gave way to a stable resting potential of about -70 mV. Raising the calcium concentration from 1.8 to 5 mM often induced long lasting (3 hrs) pacemaker oscillations of 20 to 30 mV peak to peak amplitude. These were abolished by raising the potassium concentration to 8.3 mM or upon the addition of 1.5 mM Mn2 plus. The responses of TTX-treated aggregates are discussed in terms of Noble and Tsien's pacemaker theory for Purkinje fibers. The results are well described by assuming the existence of an ik2-like potassium current in embryonic heart cells. The role of calcium is unclear but it may help provide the inward current against which the outward potassium current can function.