Voltage sensors of the frog skeletal muscle membrane require calcium to function in excitation‐contraction coupling.

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Abstract
1. Intramembrane charge movements and changes in intracellular Ca2+ concentration (Ca2+ transients) elicited by pulse depolarization were measured in frog fast twitch cut muscle fibres under voltage clamp. 2. Extracellular solutions with very low [Ca2+] and 2 mM-Mg2+, shown in the previous paper to reduce Ca2+ release from the sarcoplasmic reticulum (SR), were found to cause two changes in charge movement: (a) a decrease (-12 nC/.mu.F) in the charge that moves during depolarizing pulses from -90 to O mV, termed here ''charge 1''; (b) an increase (+7 nC/.mu.F) in the charge moved by hyperpolarizing pulses from -90 to -180 mV, termed ''charge 2''. 3. The increase in charge moved by hyperpolarizing pulses was correlated (r = 0.64) with the decrease in charge moved by depolarizing pulses and both were correlated with the inhibition of Ca2+ release recorded in the same fibres. 4. The low Ca2+ solutions caused a shift to more negative voltages of the dependence relating charge movement and holding potential (VH). This shift is of similar magnitude (about 22 mV) and direction as the shift in the curve relating Ca2+ release flux to VH (previous paper). 5. In solutions with normal [Ca2+] a conditioning depolarization to 0 mV, of 2 s duration, placed 100 ms before a test pulse from -70 to 0 mV, reduced by 30% the amount of charge displaced by the test pulse. Conditioning pulses of 1 s or less caused potentiation of charge movement by up to 30%. 6. In low Ca2+ solutions, reduction of charge was observed at all durations of the conditioning pulse. The duration for half-inhibition was near 200 ms. 7. An extracellular solution with no metal cations caused a more radical inhibition than the low Ca2+ solutions that contained Mg2+. The inhibition of Ca2+ release was essentially complete (90-100%). The charge moved by a pulse to 0 mV was reduced by 20 nC/.mu.F and the charge moved by a pulse to -170 mV increased 8 nC/.mu.F. This shows that Mg2+ supports excitation-contraction (E-C) coupling to some extent. 8. A state model of the voltage sensor of E-C coupling explains qualitatively the observations in both papers. The voltage sensor has at least four states: a resting state, an active state, which signals the SR to release Ca2+, and two inactivated states. Depolarization drives the resting .fwdarw. active transition and moves charge 1. When the depolarization is maintained the system goes to inactivated states; changes in potential drive transitions between these and cause a charge movement with very different properties (charge 2). 9. In this model Ca2+ has high affinity for the resting and active states and low affinity for the inactivated states. Low [Ca2+]o, as well as prolonged depolarization, favour the inactivated states, diminish charge 1 and Ca2+ release, and increase charge 2. In the total absence of Ca2+ and Mg2+ the system is inactivated even at normal resting potentials. 10. The observations support the concept that a large portion of intramembrane charge movement originates at the voltage sensor of excitation-contraction coupling.