Role of intracellular calcium in fatigue in single skeletal muscle fibers isolated from the rat

The aim of the present study was to demonstrate the role of intracellular calcium ([Ca2+]i) in the performance of fatigued muscle fibers isolated from the skeletal muscle of the rat. We measured developed tension of a single myocyte during short tetanic electrical stimulation of various intensities along with [Ca2+]i dynamics by fura-2. The performance of individual muscle fiber was assessed by developed tension during 100 Hz tetanic stimulation (‘100 Hz force’). We regarded the muscle fiber fatigued when, after repeated tetanic stimulations, the developed tension declined to 50% of the initial level. When fatigue was induced by maximal stimulation (100 Hz tetani), the 100 Hz force measured immediately following completion of fatigue was considerably decreased (48% of control). This change in the muscle performance was associated with significant increase in the resting [Ca2+]i (280% of control) and decrease in Ca2+ transient (54% of control). The 50% relaxation time after cessation of tetanic stimulation (RT50) was also prolonged. In contrast, when fatigue was induced by low frequency electrical stimulation (30 Hz tetani), neither the 100 Hz force, RT50, nor Ca2+ transient in fatigue were significantly different from the controls, while the resting [Ca2+]i increased only slightly. These findings suggest a tight relationship between [Ca2+]i and the performance of fatigued single isolated skeletal muscles. Also, the results show that performance of the fatigued muscle fiber may in part depend on the protocol used to produce muscle fatigue.