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1 School of Human Movement, Recreation and Performance; Centre for Ageing, Rehabilitation, Exercise and Sport, Victoria University of Technology, Melbourne, Victoria, Australia
2 Exercise, Muscle and Metabolism Unit, School of Health Sciences, Deakin University, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: michael.mcKenna{at}vu.edu.au.
We investigated whether depressed muscle Na+,K+-ATPase activity reflected a loss of Na+,K+-ATPase units, the time-course of its recovery post-exercise 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 post-exercise 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 site and Na+,K+-ATPase 
1, 2, 3, 
1, 2, and 3 isoform mRNA expression by Real-Time RT-PCR. Exercise (352±69 s; mean±SEM) did not affect [3H]ouabain binding sites, but decreased 3-O-MFPase activity by 10.7±2.3% (P<0.05), which had recovered by 3 h post-exercise, without further change at 24 h. Exercise elevated 1 mRNA by 1.5-fold at fatigue (P<0.05). This increase was inversely correlated with the percentage change in 3-O-MFPase activity from rest to fatigue (%
3-O-MFPaserest-fatigue) (r=-0.60, P<0.05). The average post-exercise (fatigue, 3, 24 h) 1 mRNA was increased by 1.4-fold (P<0.05) and approached a significant inverse correlation with %3-O-MFPaserest-fatigue (r=-0.56, P=0.08). Exercise elevated 2 mRNA at fatigue by 2.5-fold (P<0.05), which was inversely correlated with %3-O-MFPaserest-fatigue (r=-0.60, P=0.05). The average post-exercise 2 mRNA was increased 2.2-fold (P<0.05) and was inversely correlated with the %3-O-MFPaserest-fatigue (r=-0.68, P<0.05). Non-significant correlations were found between %3-O-MFPaserest-fatigue and other isoforms. Thus, acute exercise transiently decreased Na+,K+-ATPase activity, which was correlated to increased Na+,K+-ATPase gene expression. This suggests a possible signal transduction role for depressed muscle Na+,K+-ATPase activity with exercise.
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