A damped oscillation in the intramembranous charge movement and calcium release flux of frog skeletal muscle fibers

Asymmetric membrane currents and calcium transients were recorded simultaneously from cut segments of frog skeletal muscle fibers voltage clamped in a double Vaseline-gap chamber in the presence of high concentration of EGTA intracellularly. An inward phase of asymmetric currents following the hump component was observed in all fibers during the depolarization pulse to selected voltages (≃ -45 mV). The average value of the peak inward current was 0.1 A/F (SEM = 0.01, n = 18), and the time at which it occurred was 34 ms (SEM = 1.8, n = 18). A second delayed outward phase of asymmetric current was observed after the inward phase, in those experiments in which hump component and inward phase were large. It peaked at more variable time (between 60 and 130 ms) with amplitude 0.02 A/F (SEM = 0.003, n = 11). The transmembrane voltage during a pulse, measured with a glass microelectrode, reached its steady value in less than 10 ms and showed no oscillations. The potential was steady at the time when the delayed component of asymmetric current occurred. ON and OFF charge transfers were equal for all pulse durations. The inward phase moved 1.4 nC/μF charge (SEM = 0.8, n = 6), or about one third of the final value of charge mobilized by these small pulses, and the second outward phase moved 0.7 nC/μF (SEM = 0.8, n = 6), bringing back about half of the charge moved during the inward phase. When repolarization intersected the peak of the inward phase, the OFF charge transfer was independent of the repolarization voltage in the range -60 to - 90 mV. When both pre- and post-pulse voltages were changed between -120 mV and -60 mV, the equality of ON and OFF transfers of charge persisted, although they changed from 113 to 81% of their value at -90 mV. The three delayed phases in asymmetric current were also observed in experiments in which the extracellular solution contained Cd²⁺, La³⁺ and no Ca²⁺. Large increases in intracellular [Cl^-] were imposed, and had no major effect on the delayed components of the asymmetric current. The Ca²⁺ transients measured optically and the calculated Ca²⁺ release fluxes had three phases whenever a visible outward phase followed the inward phase in the asymmetric current. Several interventions intended to interfere with Ca release, reduced or eliminated the three delayed phases of the asymmetric current. We conclude that these phases are capacitive, with no significant ionic component. All results could be well described by a model of EC coupling in which the oscillations of the asymmetric current are intramembranous charge movements driven by changes in local voltage. In the model, the oscillations are the consequence of two feedback processes. One is positive: Ca²⁺, released from the SR binds to a hypothetical site near the voltage sensors, increasing the local voltage. The other is the spontaneous inactivation of release, presumably also induced by Ca²⁺, which reduces the local Ca²⁺ concentration and the transmembrane voltage.