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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Depressed Na⁺-K⁺-ATPase activity in skeletal muscle at fatigue is correlated with increased Na⁺-K⁺-ATPase mRNA expression following intense exercise

¹Muscle, Ions and Exercise Group, School of Human Movement, Recreation, and Performance, Centre for Ageing, Rehabilitation, Exercise, and Sport, Victoria University of Technology, and ²Exercise, Muscle, and Metabolism Unit, School of Health Sciences, Deakin University, Melbourne, Australia

Submitted 7 June 2004 ; accepted in final form 13 March 2005

We investigated whether depressed muscle Na⁺-K⁺-ATPase activity with exercise reflected a loss of Na⁺-K⁺-ATPase units, the time course of its recovery postexercise, and whether this depressed activity was related to increased Na⁺-K⁺-ATPase isoform gene expression. Fifteen subjects performed fatiguing, knee extensor exercise at 40% maximal work output per contraction. A vastus lateralis muscle biopsy was taken at rest, fatigue, 3 h, and 24 h postexercise and analyzed for maximal Na⁺-K⁺-ATPase activity via 3-O-methylfluorescein phosphatase (3-O-MFPase) activity, Na⁺-K⁺-ATPase content via [³H]ouabain binding sites, and Na⁺-K⁺-ATPase ₁-, ₂-, ₃-, ₁-, ₂- and ₃-isoform mRNA expression by real-time RT-PCR. Exercise [352 (SD 267) s] did not affect [³H]ouabain binding sites but decreased 3-O-MFPase activity by 10.7 (SD 8)% (P < 0.05), which had recovered by 3 h postexercise, without further change at 24 h. Exercise elevated ₁-isoform mRNA by 1.5-fold at fatigue (P < 0.05). This increase was inversely correlated with the percent change in 3-O-MFPase activity from rest to fatigue (%3-O-MFPase_rest-fatigue) (r = –0.60, P < 0.05). The average postexercise (fatigue, 3 h, 24 h) ₁-isoform mRNA was increased 1.4-fold (P < 0.05) and approached a significant inverse correlation with %3-O-MFPase_rest-fatigue (r = –0.56, P = 0.08). Exercise elevated ₂-isoform mRNA at fatigue 2.5-fold (P < 0.05), which was inversely correlated with %3-O-MFPase_rest-fatigue (r = –0.60, P = 0.05). The average postexercise ₂-isoform mRNA was increased 2.2-fold (P < 0.05) and was inversely correlated with the %3-O-MFPase_rest-fatigue (r = –0.68, P < 0.05). Nonsignificant correlations were found between %3-O-MFPase_rest-fatigue and other isoforms. Thus acute exercise transiently decreased Na⁺-K⁺-ATPase activity, which was correlated with increased Na⁺-K⁺-ATPase gene expression. This suggests a possible signal-transduction role for depressed muscle Na⁺-K⁺-ATPase activity with exercise.

Na⁺-K⁺ pump; signaling; muscle fatigue; ouabain binding

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