Two components of calcium channel current in embryonic chick skeletal muscle cells developing in culture

The properties of the Ca channel currents in chick skeletal muscle cells (myoballs) in culture were studied using a suction pipette technique which allows internal perfusion and voltage clamp. The Ca channel currents as carried by Ba ions were recorded, after suppression of currents through ordinary Na, K and Cl channels by absence of Na, K and Cl ions, by external TEA, by internal EGTA and by observing the Ba currents instead of the Ca currents. Two components of Ba current could be distinguished. One was present only if the myoballs were held at relatively negative holding potentials below -50 mV. This component first became detectable at clamp potentials of about -50 mV and reached a maximum between -10 and -20 mV. During long clamp steps, it became inactivated completely. The inactivation process of this component at a clamp potential of -30 mV was well fitted to a single exponential with a time constant of about -20 ms. Half-maximal steady-state inactivation was observed at -63 mV. The other component persisted even at relatively positive holding potentials above -40 mV, was observed during clamp pulses to -20 mV and above, and reached a maximum between +10 and +20 mV. This component inactivated very little; a substantial fraction of this component remained at the end of clamp pulses lasting 1 s. The inactivation process of this component at a clamp potential of -10 mV apparently followed a single exponential with a time constant of about 1 s. Half-maximal steady-state inactivation was attained at -33 mV. Both components of Ba current were blocked by Co ions, but organic Ca channel blocker D600 preferentially blocked the high-threshold, slowly inactivating component. The relationship between the current amplitude and the concentration of the external Ba ions was different between the two components. Furthermore, the two components of Ba current also differed in their developmental profile. These findings demonstrate the existence of two distinct types of Ca channels in the early stages of chick muscle cell development.