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1 University of Copenhagen, Institute of Exercise and Sport Sciences, Copenhagen, Denmark
2 Rigshospitalet, Section 7652, Copenhagen Muscle Research Centre, Copenhagen, Denmark
3 August Krogh Institute, Copenhagen Muscle Research Centre, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: jbangsbo{at}aki.ku.dk.
The present study tested the hypothesis that exercise with a large compared with a small active muscle mass results in a higher contraction induced increase in Na+, K+ -ATPase mRNA expression, due to greater hormonal responses. Furthermore, the relative abundance of Na+, K+ -ATPase subunits 
1, 2, 3, 4, 
1, 2 and 3 mRNA in human skeletal muscle was investigated. On two occasions, eight subjects performed one-legged knee-extension exercise (L) or combined one-legged knee-extension and bi-lateral arm cranking (AL) for 5.00, 4.25, 3.50, 2.75 and 2.00 min separated by 3 min of rest. Leg exercise power output was the same in AL and L (57±5 vs. 59±5 W), but heart rate (HR) at the end of each exercise interval was higher in AL compared with L (91±3% vs. 61±12% of maximal HR, P<0.001). One minute after exercise, arm venous blood lactate was higher (P<0.001) in AL than i L (11.4±4.2 vs. 4.2±2.2 mM). A higher (P<0.05) level of blood epinephrine (141%) and nor-epinephrine (380%) was evident three minutes after exercise in AL compared with L. Nevertheless, none of the exercise induced increases (P<0.001) in 1, 2, 1 and 3 mRNA expression levels were higher in AL compared with L. The most abundant Na+, K+ -ATPase subunit at the mRNA level was 1, which was 3.4 times more expressed than 2 (P<0.001). Expression of 1, 2 and 3 was less than 5% of the 2 expression, and no reliable detection of 3 and 4 was possible. In conclusion, activation of additional muscle mass does not result in a higher exercise induced increase in Na+, K+ -ATPase subunit specific mRNA.
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