FATIGUE-INDUCED ALTERATIONS IN CA2+ AND CAFFEINE SENSITIVITIES OF SKINNED MUSCLE-FIBERS

The purpose of this investigation was to examine th Ca2+ and caffeine sensitivities of skinned skeletal muscle fibers after fatigue. Single frog semitendinosus fibers were chemically skinned in either a rested state or after tetanic contractions (80 Hz, 100 ms) evoked at 2 s-1 for 5 min. This protocol reduced tetanic force to 1.8 +/- 0.2% of control. Maximal Ca2+-activated force (F0, 20-degrees-C) was not significantly different between rested and fatigued fibers. However, the concentration of Ca2+ required to evoke 50% of F0 was significantly lower in the fatigued fibers (1.80 +/- 0.18 vs. 1.33 +/- 0.16 muM; P < 0.05), an effect that persisted as the skinned fiber was allowed to incubate in the relaxing solution for >90 min. The addition of caffeine (25 mM) after Ca2+ loading of the sarcoplasmic reticulum (SR) for periods of 5-30 s (0.25 muM free [Ca2+]) evoked smaller contractures in fatigued fibers than in rested fibers. However, when the loading period was prolonged (60-240 s), force developed after caffeine application was not significantly different between conditions. This suggests that the rate, but not the maximal capacity of Ca2+ loading by the SR, is reduced by fatigue. After Ca2+ loading (120 s), the minimal caffeine concentration required to evoke a contracture in fatigued fibers (5.7 +/- 0.3 mM) was significantly greater than that of control fibers (3.1 +/- 0.4 mM), an effect that persisted with prolonged incubation of the skinned fibers. In addition, the rate of force increase in response to 8 mM caffeine was reduced in fatigued fibers by 41%. Taken together, these latter findings suggest that the caffeine sensitivity of skinned fibers is reduced after fatigue. It appears that fatigue exerts direct effects on both the contractile apparatus and the SR of skeletal muscle.