title: Prolonged exercise to fatigue in humans impairs skeletal muscle Na+-K+-ATPase activity, sarcoplasmic reticulum Ca2+ release, and Ca2+ uptake.
abstract: Prolonged exhaustive submaximal exercise in humans induces marked metabolic changes, but little is known about effects on muscle Na+-K+-ATPase activity and sarcoplasmic reticulum Ca2+ regulation. We therefore investigated whether these processes were impaired during cycling exercise at 74.3 +/- 1.2% maximal O2 uptake (mean +/- SE) continued until fatigue in eight healthy subjects (maximal O2 uptake of 3.93 +/- 0.69 l/min). A vastus lateralis muscle biopsy was taken at rest, at 10 and 45 min of exercise, and at fatigue. Muscle was analyzed for in vitro Na+-K+-ATPase activity [maximal K+-stimulated 3-O-methylfluorescein phosphatase (3-O-MFPase) activity], Na+-K+-ATPase content ([3H]ouabain binding sites), sarcoplasmic reticulum Ca2+ release rate induced by 4 chloro-m-cresol, and Ca2+ uptake rate. Cycling time to fatigue was 72.18 +/- 6.46 min. Muscle 3-O-MFPase activity (nmol.min(-1).g protein(-1)) fell from rest by 6.6 +/- 2.1% at 10 min (P <0.05), by 10.7 +/- 2.3% at 45 min (P <0.01), and by 12.6 +/- 1.6% at fatigue (P <0.01), whereas 3[H]ouabain binding site content was unchanged. Ca2+ release (mmol.min(-1).g protein(-1)) declined from rest by 10.0 +/- 3.8% at 45 min (P <0.05) and by 17.9 +/- 4.1% at fatigue (P < 0.01), whereas Ca2+ uptake rate fell from rest by 23.8 +/- 12.2% at fatigue (P=0.05). However, the decline in muscle 3-O-MFPase activity, Ca2+ uptake, and Ca2+ release were variable and not significantly correlated with time to fatigue. Thus prolonged exhaustive exercise impaired each of the maximal in vitro Na+-K+-ATPase activity, Ca2+ release, and Ca2+ uptake rates. This suggests that acutely downregulated muscle Na+, K+, and Ca2+ transport processes may be important factors in fatigue during prolonged exercise in humans.
authors.affiliation: Muscle, Ions, and Exercise Group, School of Human Movement, Recreation and Performance, (FO22 Victoria Univ. of Technology, PO Box 14428, MCMC, Melbourne, Victoria 8001, Australia.
authors.forename: James A
authors.initials: JA
authors.lastname: Leppik
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Aughey
authors.forename: Ivan
authors.initials: I
authors.lastname: Medved
authors.forename: Ian
authors.initials: I
authors.lastname: Fairweather
authors.forename: Michael F
authors.initials: MF
authors.lastname: Carey
authors.forename: Michael J
authors.initials: MJ
authors.lastname: McKenna
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D002118:Calcium / Q000378:metabolism*; D004789:Enzyme Activation; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D005260:Female; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010807:Physical Endurance / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013848:Thigh / Q000502:physiology
chemical_list: D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium