Voltage dependence of membrane charge movement and calcium release in frog skeletal muscle fibres

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Abstract
Voltage dependent membrane charge movement (gating current) and the release of Ca2+ from intracellular stores have been measured simultaneously in intact frog skeletal muscle fibres. Charge movement was measured using the three microelectrode voltage clamp technique. Ca2+ release was measured using the metallochromic indicator dye arsenazo III. Fibres were bathed in 2.3×hypertonic solutions to prevent contraction. Rb+, tetraethylammonium and tetrodotoxin (TTX) were used to eliminate voltage-dependent ionic currents. The maximum rate of Ca2+ release from the sarcoplasmic reticulum in response to voltage-clamp step depolarizations to 0 mV was calculated using the dye-related parameters of model 2 of Bayloret al. (1983) and a method described in the Appendix for calculating a scaling factor (1+p) that accounts for the additional Ca2+ buffering power of the indicator dye. The estimates of the maximum rate of Ca2+ release at 5–6° C ranged from 3 to 19 µM ms−1 in the 17 fibres examined. The mean value was 8.9±1.1 µM ms−1 (S.E.M.) The maximum rate of Ca2+ release was linearly related to the magnitude of the nonlinear membrane change moved during suprathreshold depolarizing steps. The voltage dependence of charge movement and the maximum rate of Ca2+ releases were nearly identical at 6° C. The voltage-dependence of the delay between the test step and the onset of Ca2+ release could be adequately described by an equation having the same functional form as the voltage dependence of nonlinear charge movement. The relationship between the test pulse voltage and the delay was shifted to more negative voltages and to shorter delays as the temperature was raised from 6° C to 15° C. The inactivation of Ca2+ release was found to occur at more negative holding voltages and to be more steeply voltage dependent than the immobilization of nonlinear membrane charge movement. The above data are discussed using the ‘hypothetical coupler’ model of excitation-contraction coupling (Milediet al., 1983b) applied to the specific case in which each mobile charge group controls the gating of one Ca2+ release site in the sarcoplasmic reticulum.