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1 Muscle, Ions & Exercise Group, Centre for Ageing, Rehabilitation, Exercise and Sport, School of Human Movement, Recreation and Performance, Victoria University, Melbourne, Australia
2 Exercise Metabolism Group, School of Medical Sciences, Faculty of Life Sciences, RMIT University, Melbourne, Australia
3 Exercise, Muscle and Metabolism Unit, School of Exercise and Nutrition Sciences, Deakin University, Melbourne, Australia
* To whom correspondence should be addressed. E-mail: michael.mckenna{at}vu.edu.au.
The Na+,K+ATPase enzyme is vital in skeletal muscle function. We investigated the effects of acute high-intensity interval exercise, before and following high intensity training (HIT), on muscle Na+,K+ATPase maximal activity, content, and isoform mRNA expression and protein abundance. Twelve endurance-trained athletes were tested at Baseline, Pre-Train and after 3-wks HIT (Post-Train), which comprised seven sessions of 8x5 min interval cycling at 80% Peak Power Output. Vastus lateralis muscle was biopsied at rest (Baseline) and both at rest and immediately post-exercise during the first (Pre-Train) and seventh (Post-Train) training sessions. Muscle was analysed for Na+,K+ATPase maximal activity (3-O-MFPase), content ([3H]-ouabain binding), isoform mRNA expression (RT-PCR) and protein abundance (Western-blotting). All Baseline-to-Pre-Train measures were stable. Pre-Train, acute exercise decreased 3-O-MFPase activity (12.7±5.1%, P<0.05), increased 
1, 2 and 3 mRNA expression (1.4-, 2.8- and 3.4-fold, respectively, P<0.05), with unchanged 
-isoform mRNA, or protein abundance of any isoform. In resting muscle, HIT increased (P<0.05) 3-O-MFPase activity by 5.5±2.9%, and 3 and 3 mRNA expression by 3.0- and 0.5-fold, respectively, with unchanged Na+,K+ATPase content or isoform protein abundance. Post-Train, the acute exercise-induced decline in 3-O-MFPase activity, and increased 1 and 3 mRNA each persisted (P<0.05); the post-exercise 3-O-MFPase activity was also higher after HIT (P<0.05). Thus, HIT augmented Na+,K+ATPase maximal activity despite unchanged total content or isoform protein abundance. Elevated Na+,K+ATPase activity post-exercise may contribute to reduced fatigue after training. The Na+,K+ATPase mRNA response to interval exercise of increased -, but not -mRNA was largely preserved Post-Train, suggesting a functional role of mRNA upregulation.
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