FUNCTIONAL-ASPECTS OF SKELETAL-MUSCLE CONTRACTILE APPARATUS AND SARCOPLASMIC-RETICULUM AFTER FATIGUE

This study examined the effects of fatigue on the functional aspects o f the contractile apparatus and sarcoplasmic reticulum (SR). Frog semi tendinosus muscles were stimulated to fatigue, and skinned fibers or a homogenate fraction was prepared from both fatigued and rested contra lateral muscles. In fatigued fibers, maximal Ca2+-activated force of t he contractile apparatus was unaltered, whereas maximal actomyosin-ATP ase activity was depressed by 20%. The Ca2+ sensitivity of force was i ncreased, whereas that of actomyosin-ATPase was not altered. Also, the rate constant for tension redevelopment was decreased at submaximal C a2+ concentration. These latter findings suggest that fatigue slows th e dissociation of force-generating myosin cross bridges. Ca2+ uptake a nd Ca2+-ATPase activity of the SR were depressed by 46 and 21%, respec tively, in the fatigued muscles. Fatigue also reduced the rates of SR Ca2+ release evoked by AgNO3 and 4-chloro-m-cresol by 38 and 45%, resp ectively. During fatigue, the contractile apparatus and SR undergo int rinsic functional alterations. These changes likely result in altered force production and energy consumption by the intact muscle.