Calcium currents in rat myoballs and their inhibition by insulin.

Two Ca2+ currents were found in myoballs prepared from primary culture of hindlimb muscles from rat embryos. One whole cell current, not described previously, was early, fast, and transient. It depended on the presence of Na+ in the bathing solution and was blocked by tetrodotoxin. Despite this behavior suggesting that it might be via a Na+ channel, its reversal potential exceeded 80 mV compared to 46 mV for the Na+ equilibrium potential. It was increased by increased Ca2+ concentration in the bathing solution and eliminated by Co2+ and by diltiazem. The other Ca2+ current resembled the slow inward Ca2+ current ICa(si), described in other cell types. Insulin decreased both Ca2+ currents; even at 64 pM insulin reduced ICa(si). When outward K+ currents were prevented, the myoball action potential was altered greatly, owing to the unopposed effect of ICa(si). Its duration was on the order of seconds. Insulin, in a concentration-dependent manner, beginning at 60 pM, reduced the duration of this action potential more effectively than nimodipine. This is the most sensitive response to insulin observed in skeletal muscle.