Inhibition of crossbridge cycling improves cytosolic Ca 2+ handling during high‐frequency stimulation of isolated skeletal myocytes.

This investigation examined Ca 2+ release and re‐uptake by the sarcoplasmic reticulum (SR) during fatiguing contractions following inhibition of crossbridge cycling in isolated Xenopus single skeletal muscle fibers. Cytosolic [Ca 2+ ] ([Ca 2+ ] c ) and tension were measured in fibers (n=9) during two sequential tetanic contraction trials, separated by 45 min rest. In each trial, contraction frequency was increased every two min until impairment of SR Ca 2+ release had occurred (peak [Ca 2+ ] c = 75% of initial). Following the initial control trial, fibers were incubated with 12.5 μM N‐benzyl‐p‐toluene sulfonamide (BTS), an inhibitor of crossbridge cycling. Developed tension was significantly (p<0.05) decreased (12±2%) following exposure to BTS, while initial relative peak [Ca 2+ ] c release remained unchanged. However, time to impairment of SR Ca 2+ release was significantly increased (p<0.05) following exposure to BTS (259±29 sec) compared to control fibers (113±14 sec). In addition, relative baseline [Ca 2+ ] c significantly (p<0.05) increased (15.7±4%) in control fibers at the SR Ca 2+ release impairment time point, whereas fibers exposed to BTS had a significantly lower (p<0.05) increase in baseline [Ca 2+ ] c (9+3%). These data demonstrate an improvement in SR Ca 2+ release and re‐uptake following inhibition of crossbridge cycling, suggesting that factors originating from the contractile sites contribute to inhibition of SR Ca 2+ release and re‐uptake during fatigue. Supported by NIH AR40155