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

Abstract

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 NaCI. 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.

Reperfusion paradox: A novel mode of glial cell injury

“…Sodium entry must also be through calcium channels, which are known to lose their specificity under low calcium conditions (Tunstall et al, 1986) since cadmium and nickel, two well-described calcium channel blockers (Frank et al, 1982;Rich and Langer, 1982), prevent the paradoxical increase as well. The depolarization caused by low external calcium (Almers et al, 1984;Minota and Koketsu, 1983) and intracellular sodium loading by either means are, however, apparently able to shift the exchanger into its reverse phase when it is reactivated by external calcium during reperfusion (Caputo et al, 1989;DiPolo and Beauge, 1988).…”