Excitation contraction uncoupling by high intracellular [Ca²⁺] in frog skeletal muscle: a voltage clamp study

Raising the intracellular [Ca²⁺] ([Ca²⁺]_i) was previously found to produce uncoupling between the electrical depolarization of the transverse tubules and contraction in skinned muscle fibers. Here we study the effect of elevated [Ca²⁺]_i in voltage clamped cut fibers of frog skeletal muscle to establish how the charge movement, a measure of the activation of the dihydropyridine receptors (DHPR)-voltage sensors, and Ca²⁺ release, a consequence of the opening of the ryanodine receptor (RyR)-release channels, were affected. [Ca²⁺]_i was raised by various procedures (pharmacological release from the sarcoplasmic reticulum, application of high [Ca²⁺]_i intracellular solution, permeabilization of the plasma membrane by a Ca²⁺ ionophore) all of which produced impairment of excitation-contraction coupling. The charge movement was reduced from 20.2 ± 1.24 to 9.9 ± 0.94 nC/μF meanwhile the Ca²⁺ release flux was reduced from 13.5 + 0.7 to 2.2 ± 0.3 μM/ms (n = 33). This suggests that a significant fraction of the DHPRs that remained functional, could not activate RyRs, and were therefore presumably disconnected. These results are broadly consistent with the original reports in skinned fibers. Uncoupling was prevented by the addition to the intracellular solution of the protease inhibitor leupeptin. In approximately 40 % of the uncoupled cells we observed that the [Ca²⁺]_i transient continued to rise after the voltage clamp pulse was turned off. This loss of control by membrane voltage suggests that the uncoupled release channels might have another mechanism of activation, likely by Ca²⁺.