Tetrodotoxin‐resistant electric activity in chick skeletal muscle cells differentiated in vitro

The embryonic chick skeletal muscle cells differentiated in cell culture from trypsin‐dissociated myoblasts produce a spike response which is tetrodotoxin‐sensitive. It has been found that many cells also produce a plateau response which is resistant to tetrodotoxin. The plateau response frequently occurs even in the muscle cells which do not normally exhibit the spike response. During the plateau response membrane resistance is greatly reduced below its resting value. The current‐voltage relation in muscle cells with the plateau response is always S‐shaped. It is suggested that the plateau arises from a voltage‐dependent increase in permeability to external cations whose influx produce the maintained depolarization, and from low level of repolarizing potassium outflux. The plateau response is sensitive to manganese ions. This finding, together with resistibility to tetrodotoxin, suggests that calcium ions are the dominant carriers for the depolarizing current.