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1 Muscle, Ions and Exercise Group, School of Human Movement, Recreation and Performance, Centre for Ageing, Rehabilitation and Sport Science (CARES), Victoria University of Technology, Melbourne, Victoria, Australia; Institute of Physiology and Biophysics, University of Aarhus, Aarhus, Denmark
2 Institute of Physiology and Biophysics, University of Aarhus, Aarhus, Denmark
3 Muscle, Ions and Exercise Group, School of Human Movement, Recreation and Performance, Centre for Ageing, Rehabilitation and Sport Science (CARES), Victoria University of Technology, Melbourne, Victoria, Australia
* To whom correspondence should be addressed. E-mail: km{at}fi.au.dk.
This study investigated the effects of electrical stimulation on Na+,K+-ATPase isoform mRNA, with the aim to identify factors modulating Na+,K+-ATPase mRNA in isolated rat EDL muscle. Interventions designed to mimic exercise-induced increases in intracellular Na+ and Ca2+ contents, and membrane depolarization, were examined. Muscles were mounted on force transducers and stimulated with 60 Hz 10 s pulse trains producing tetanic contractions three times at 10 min intervals. Ouabain (1.0 mM, 120 min), veratridine (0.1 mM, 30 min) and monensin (0.1 mM, 30 min) were used to increase intracellular Na+ content. High extracellular K+ ([K+]o) (13 mM, 60 min) and the Ca2+ ionophore A23187 (0.02 mM, 30 min) were used to induce membrane depolarization and elevated intracellular Ca2+ content, respectively. Muscles were analyzed for Na+,K+-ATPase 
1-3 and 
1-3 mRNA (Real-Time RT-PCR). Electrical stimulation had no immediate effect on Na+,K+-ATPase mRNA, but at 3 h post-stimulation increased 1, 2 and 3 mRNA by 223, 621 and 892%, respectively (P=0.010), without changing mRNA. Ouabain, veratridine and monensin increased intracellular Na+ content by 769, 724 and 598%, respectively (P=0.001), but did not increase mRNA of any isoform. High [K+]o elevated 1 mRNA by 160% (P=0.021), whereas A23187 elevated 3 mRNA by 123% (P=0.035) but reduced 1 mRNA by 76% (P=0.001). In conclusion, electrical stimulation induced subunit-specific increases in Na+,K+-ATPase mRNA in isolated rat EDL muscle. Furthermore, Na+,K+-ATPase mRNA appears to be regulated by different stimuli, including cellular changes associated with membrane depolarization and increased intracellular Ca2+ content, but not increased intracellular Na+ content.
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