The role of excitation-contraction coupling in skeletal muscle fatigue

The effects of fatigue on excitation-contraction coupling (ECC) in frog skeletal muscle was examined using conventional and ion-selective microelectrodes, and the vaseline gap method of voltage clamping. As a result of stimulation (100 ms trains at 150 Hz, 1/sec for 5 min), the resting membrane potential (RMP) depolarized 9 mV, the action potential (AP) overshoot (OS) fell 12 mV, the duration of the AP was prolonged 2 fold, the intracellular Na$\sp+$ concentration rose from 16 to 50 mM while intracellular K$\sp+$ fell from 141 to 100 mM. The RMP and OS recovered by 2 min and the intracellular ions by 5 min, at which time the duration was still prolonged. In addition, as fatigue developed, the membrane failed to respond to each stimulus with an AP. In the worse case, the muscle responded to 15 stimuli with only 3 APs. Since a 9 mV depolarization of the RMP would not inactivate a significant percentage of the voltage sensors, and the amount of transverse-tubular (t-tubular) charge moved is maximal at potential achieved by the OS, it was concluded that the changes in the AP did not cause fatigue. However, alterations in the t-tubule AP may have been more extreme. The decrease in the ability of the sarcolemma to respond to each stimulus with an AP was also discounted, since fatigued muscles stimulated at 30 Hz were able to generate tension equal to that produced by 150 Hz stimulation. Alterations in the intracellular concentrations of Na$\sp+$ and K$\sp+$ were discounted as causing fatigue, since the time course of their recovery did not correspond to the recovery of force. The affects of depolarization, elevated extracellular Ca$\sp{2+},$ and low intracellular pH on the t-tubular voltage sensor were examined. Depolarization of the holding potential from $-$90 to $-$75 or $-$60 mV significantly decreased the maximal amount of charge moved in response to test pulses. Raising extracellular Ca$\sp+$ from 2 to 10 mM prevented the decrease in charge movement upon depolarization to $-$75 but not $-$60 mV. Decreasing intracellular pH from 7.0 to 6.2 had no effect on the total or the $\gamma$ component of the charge movement. The affects of fatigue on sarcoplasmic reticulum Ca$\sp{2+}$ release has not yet been determined