Cultivation, morphology, and electrophysiology of contractile rat myoballs.

Myoballs were cultured from neonatal rat skeletal muscle without the use of antimitotic drugs. Electron microscopic investigation showed that 7-day-old myoballs are multinucleated syncytia in a state of differentiation where filaments are abundant and already in hexagonal arrays. The resting potential of 142 myoballs kept at 20 degrees C was not correlated with the cell size. Its mean value was -64 mV. Cells with a high resting potential were capable of generating action potentials with a threshold of -51 mV, an overshoot of +31 mV, and a rate of rise of 100 V/s. The steady-state current-voltage relation showed inward rectification on hyperpolarization and outward rectification on depolarization. The dynamic sodium and potassium currents were investigated at 37 degrees C with the whole-cell-recording technique. The sodium current had its maximum at -20 mV. The potassium current showed delayed activation and a very slow and incomplete inactivation. The electrophysiological results from these cultured cells are very similar to those obtained from adult cells.