title: Effects of prolonged low frequency stimulation on skeletal muscle sarcoplasmic reticulum.
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 is 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 mumol.mg-1 protein.min-1 in 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+ bd 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.
authors.affiliation: Department of Kinesiology, University of Waterloo, ON, Canada.
authors.forename: E R
authors.initials: ER
authors.lastname: Chin
authors.forename: H J
authors.initials: HJ
authors.lastname: Green
authors.forename: F
authors.initials: F
authors.lastname: Grange
authors.forename: J
authors.initials: J
authors.lastname: Dossett-Mercer
authors.forename: P J
authors.initials: PJ
authors.lastname: O'Brien
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation; D005260:Female; D066298:In Vitro Techniques; D018763:Muscle Fatigue / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D013997:Time Factors
chemical_list: D002118:Calcium