Effects of prolonged low frequency stimulation on skeletal muscle sarcoplasmic reticulum

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Abstract
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 induced by 90 min intermittent 10-Hz stimulation of rat gastrocnemius muscle, which led to reductions (p < 0.05) in ATP, creatine phosphate, and glycogen of 16, 55, and 49%, respectively, compared with non-stimulated muscle. Stimulation also resulted in increases (p < 0.05) in muscle lactate, creatine, Pi, total ADP, total AMP, IMP, and inosine. Calculated free ADP (ADPf) and free AMP (AMPf) were elevated 3- and 15-fold, 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 activity were observed in homogenate (HOM) or a crude SR fraction (CM) from WG or RG muscle following exercise. Maximum Ca2+uptake in HOM and CM preparations was similar in control (C) and stimulated (St) muscles. However, Ca2+uptake at 400 nM free Ca2+was significantly reduced in CM from RG (0.108 ± 0.04 to 0.076 ± 0.02 μmol∙mg−1protein∙min−1in RG–C and RG–St, respectively). Collectively, these data suggest that reductions in muscle energy status are dissociated from changes 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 reflect differences in the mechanisms evaluated by these techniques.Key words: sarcoplasmic reticulum, contractile activity, Ca2+sequestration, energy status, red and white gastrocnemius.

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