Abstract
Under a high-Ca condition (>5mM), myotubes of a mouse myogenic cell line MC3T3-A1/M13 generated a long-lasting Ca action potential and a prolonged afterhyperpolarization (a. h. p.) during their in vitro development. The action potential was sensitive to Co or verapamil. Under a voltage-clamp condition, membrane depolarization more positive than -20mV evoked a Ca-dependent inward current, which was apparently prolonged and responsible for the generation of the long-lasting action potential. The appearance of the Ca action potential preceded that of a Na spike by about 24h, and it developed so that the maximum rate of rise became 26±4V/s by day 7. Then this Ca-dependent potential faded as the myotubes matured, until the action potentials became solely Na-dependent. The a. h. p. was evoked accompanying the Ca action potential and was inhibited by quinine or quinidine, showing that it is operated by Ca-activated K channels. This channel developed together with the Ca channel and continued to exist during the myotube maturation process. These results indicate that the MC3T3-A1/M13 myotube at the initial stage of development has a highly developed Ca spike system that is due mostly to a high-threshold-type Ca channel.
