ROLE OF SARCOLEMMA ACTION-POTENTIALS AND EXCITABILITY IN MUSCLE FATIGUE

The purposes of this study were to characterize the alterations in the sarcolemma action potential (AP) waveform and sarcolemma excitability as a result of fatiguing stimulation of the frog semitendinosus muscl e and to relate these changes to the decrease in the force-generating ability of the muscle. Trains of APs were recorded before and after st imulation (100-ms trains, 150 Hz, 1/s for 5 min). The resting membrane potential (RMP), AP overshoot (OS), and duration at 50% of peak magni tude (DUR) were - 84.3 +/- 2.0 mV, 19.5 +/- 1.9 mV, and 1.3 +/- 0.1 ms , respectively, before stimulation. The stimulation protocol caused RM P to depolarize to - 75.1 +/- 2.0 mV, OS to fall to 7.3 +/- 1.9 mV, an d DUR to increase to 2.5 +/- 0.4 ms. RMP and OS recovered fully in 5 m in after the cessation of stimulation, whereas DUR was still prolonged . Before the stimulation protocol, AP frequency matched the stimulatio n frequency at all stimulation rates less than or equal to 150 Hz. At 200-Hz stimulation, AP frequency was 192 +/- 6 Hz. After 5 min of stim ulation, AP frequency matched the stimulation frequency only at less t han or equal to 60 Hz. At 100-, 150-, and 200-Hz stimulation, AP frequ encies were 89 +/- 8, 84 +/- 17, and 79 +/- 15 Hz, respectively. Becau se of a decreased fusion frequency at fatigue, the fall in the sarcole mma AP frequency did not contribute to the decreased force. The stimul ation-induced alterations in the AP waveform were moderate and unlikel y to have caused fatigue. However, the alterations in AP may have been more extreme in the depths of the transverse tubules.