Prolonged action potential of frog skeletal muscle membrane in Ca-free EGTA solution

The membrane of isolated frog skeletal muscle fibers with or without T-system is depolarized to about -30 mV in a Ca-free solution containing 2 mM EGTA. Under such a condition, the action potential can be produced by a cathodal pulse when the membrane is previously hyperpolarized to -70- -100 mV by a conditioning anodal current. The action potential consists of two different potential components, namely a spike potential and a following slow depolarizing response forming a plateau phase (plateau potential). The membrane conductance during the plateau potential is increased. Both spike and plateau potentials are abolished in the absence of NaCl. TTX blocks a spike potential without affecting a plateau potential, whereas Mn and D-600 act contrarily. The i-v relation shows an anomalous rectification in Na-free solution, suggesting that the K conductance during the plateau phase is decreased. It is suggested that spike and plateau potentials are produced by a movement of sodium ions through sodium and calcium channels of the membrane, respectively.