Impaired sarcoplasmic reticulum Ca release rate after fatiguing stimulation in rat skeletal muscle

Ørtenblad, Niels, Gisela Sjøgaard, and Klavs Madsen. Impaired sarcoplasmic reticulum Ca release rate after fatiguing stimulation in rat skeletal muscle. J Appl Physiol 89: 210–217, 2000.—The purpose of the study was to characterize the sarcoplasmic reticulum (SR) function and contractile properties before and during recovery from fatigue in the rat extensor digitorum longus muscle. Fatiguing contractions (60 Hz, 150 ms/s for 4 min) induced a reduction of the SR Ca release rate to 66% that persisted for 1 h, followed by a gradual recovery to 87% of prefatigue release rate at 3 h recovery. Tetanic force and rate of force development (1dF/dt) and relaxation (2dF/dt) were depressed by ;80% after stimulation. Recovery occurred in two phases: an initial phase, in which during the first 0.5–1 h the metabolic state recovered to resting levels, and a slow phase from 1–3 h characterized by a rather slow recovery of the mechanical properties. The recovery of SR Ca release rate was closely correlated to 1dF/dt during the slow phase of recovery (r 5 0.51; P , 0.05). Despite a slowing of the relaxation rate, we did not find any significant alterations in the SR Ca uptake function. These data demonstrate that the Ca release mechanism of SR is sensitive to repetitive in vitro muscle contraction. Moreover, the results indicate that 1dF/dt to some extent depends on the rate of Ca release during the slow phase of recovery.