EFFECTS OF PROLONGED LOW-FREQUENCY STIMULATION ON SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM

The role of prolonged electrical stimulation on sarcoplasmic reticulum (SR) Ca2+ sequestration measured in vitro and muscle energy status in fast white and red skeletal muscle was investigated. Fatigue was indu ced by 90 min intermittent 10-Hz stimulation of rat gastrocnemius musc le, which led to reductions (p < 0.05) in ATP, creatine phosphate, and glycogen of 16, 55, and 49%, respectively, compared with non-stimulat ed muscle. Stimulation also resulted in increases (p < 0.05) in muscle lactate, creatine, Pi, total ADP, total AMP, IMP, and inosine. Calcul ated free ADP (ADP(f)) and free AMP (AMP(f)) were elevated 3- and 15-f old, respectively. No differences were found in the metabolic response between tissues obtained from the white (WG) and red (RG) regions of the gastrocnemius. No significant reductions in SR Ca2+ ATPase activit y were observed in homogenate (HOM) or a crude SR fraction (CM) from W G or RG muscle following exercise. Maximum Ca2+ uptake in HOM and CM p reparations was similar in control (C) and stimulated (St) muscles. Ho wever, Ca2+ uptake at 400 nM free Ca2+ was significantly reduced in CM from RG (0.108 +/- 0.04 to 0.076 +/- 0.02 mu mol . mg(-1) protein . m in(-1) in RG-C and RG-St, respectively). Collectively, these data sugg est that reductions in muscle energy status are dissociated from chang es in SR Ca2+ ATPase activity in vitro but are related to Ca2+ uptake at physiological free [Ca2+] in fractionated SR from highly oxidative muscle. Dissociation of SR Ca2+ ATPase activity from Ca2+ uptake may r eflect differences in the mechanisms evaluated by these techniques.