Intracellular Ca2+ transients during fatigue in intact skeletal muscle: role of Ca2+ in low frequency fatigue

Abstract

We previously reported a surface fluorometry method for measurement of intracellular Ca2+ in intact skeletal muscles. Here we employ this method to examine mechanisms of muscle fatigue. Mouse epitrochlearis (EPI) and diaphragm (DIA) muscles were mounted in vertical chambers, stretched to LO, and loaded with Rhod‐2 AM. Force and intracellular Ca2+ were recorded at several stimulation frequencies, and muscles were then contracted with periodic increases in train frequency (0.16, 0.2, 0.25, 0.33, 0.5, 1 train/sec). Fatigue was defined by a 50% reduction in force. EPI and DIA showed markedly different times to fatigue, as expected (~7.0 min for EPI vs. ~10.8 min for DIA). Ca2+ transients (peak ΔF) showed no initial decline, but later decreased by ~22% for EPI and ~27% for DIA. Baseline Rhod‐2 fluorescence increased in both muscles during fatigue (EPI ~77% of peak ΔF, DIA ~58% of peak ΔF), indicating mitochondrial Ca2+ accumulation. Following 15 min of recovery, max tetanic force recovered fully while forces at lower frequencies were reduced from baseline (low‐frequency fatigue). In contrast, intracellular Ca2+ transients recovered fully at both high and low stimulation frequencies in this preparation suggesting that the primary mechanism for low frequency fatigue is related to inhibition of the contractile elements and not to processes involved in Ca+2 release. NHLBI 53333.