CONTRACTILE APPARATUS AND SARCOPLASMIC-RETICULUM FUNCTION - EFFECTS OF FATIGUE, RECOVERY, AND ELEVATED CA2+

This investigation tested the notion that fatiguing stimulation induce s intrinsic changes in the contractile apparatus and sarcoplasmic reti culum (SR) and that these changes are initiated by elevated intracellu lar Ca2+ concentration ([Ca2+](i)). Immediately after stimulation of f rog semitendinosus muscle, contractile apparatus and SR function were measured. Despite a large decline in tetanic force (P-o), maximal Ca2-activated force (F-max) of the contractile apparatus was not signific antly altered. However, Ca2+ sensitivity was increased. In conjunction , the rate constant of Ca2+ uptake by the SR was diminished, and the c affeine sensitivity of Ca2+ release was decreased. During recovery, P- o, contractile apparatus, and SR function each returned to near-initia l levels. Exposure of skinned fibers to 0.5 mu M free Ca2+ for 5 min d epressed both F-max and Ca2+ sensitivity of the contractile apparatus. In addition, caffeine sensitivity of Ca2+ release was diminished. Res ults suggest that fatigue induces intrinsic alterations in contractile apparatus and SR function. Changes in contractile apparatus function do not appear to be mediated by increased [Ca2+](i). However, a portio n of the change in SR Ca2+ release seems to be due to elevated [Ca2+]( i).