Impaired sarcoplasmic reticulum Ca2+ release rate after fatiguing stimulation in rat skeletal muscle

The purpose of the study was to characterize the sarcoplasmic reticulum (SR ) function and contractile properties before and during recovery from fatig ue in the rat extensor digitorum longus muscle. Fatiguing contractions (60 Hz, 150 ms/s for 4 min) induced a reduction of the SR Ca2+ release rate to 66% that persisted for 1 h, followed by a gradual recovery to 87% of prefat igue release rate at 3 h recovery. Tetanic force and rate of force developm ent (+dF/dt) and relaxation (-dF/dt) were depressed by similar to 80% after stimulation. Recovery occurred in two phases: an initial phase, in which d uring the first 0.5-1 h the metabolic state recovered to resting levels, an d a slow phase from 1-3 h characterized by a rather slow recovery of the me chanical properties. The recovery of SR Ca2+ release rate was closely corre lated to +dF/dt during the slow phase of recovery (r(2) = 0.51; P < 0.05). Despite a slowing of the relaxation rate, we did not find any significant a lterations in the SR Ca2+ uptake function. These data demonstrate that the Ca2+ release mechanism of SR is sensitive to repetitive in vitro muscle con traction. Moreover, the results indicate that +dF/dt to some extent depends on the rate of Ca2+ release during the slow phase of recovery.