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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Contraction-induced increases in Na⁺-K⁺-ATPase mRNA levels in human skeletal muscle are not amplified by activation of additional muscle mass

¹Institute of Exercise and Sport Sciences, University of Copenhagen, ²Rigshospitalet, Sect. 7652, and ³August Krogh Institute, Copenhagen, Denmark

Submitted 11 November 2004 ; accepted in final form 8 February 2005

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 subunit 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 bilateral 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, but heart rate at the end of each exercise interval was higher in AL compared with L. One minute after exercise, arm venous blood lactate was higher in AL than in L. A higher level of blood epinephrine and norepinephrine was evident 3 min after exercise in AL compared with L. Nevertheless, none of the exercise-induced increases 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 expressed 3.4 times than ₂. 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.

exercise; gene expression; hormonal response; lactate

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