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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Glucose supplements increase human muscle in vitro Na⁺-K⁺-ATPase activity during prolonged exercise

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada

Submitted 3 October 2006 ; accepted in final form 2 April 2007

Regulation of maximal Na⁺-K⁺-ATPase activity in vastus lateralis muscle was investigated in response to prolonged exercise with (G) and without (NG) oral glucose supplements. Fifteen untrained volunteers (14 males and 1 female) with a peak aerobic power (O_2peak) of 44.8 ± 1.9 ml·kg^–1·min^–1; mean ± SE cycled at 57% O_2peak to fatigue during both NG (artificial sweeteners) and G (6.13 ± 0.09% glucose) in randomized order. Consumption of beverage began at 30 min and continued every 15 min until fatigue. Time to fatigue was increased (P < 0.05) in G compared with NG (137 ± 7 vs. 115 ± 6 min). Maximal Na⁺-K⁺-ATPase activity (V_max) as measured by the 3-O-methylfluorescein phosphatase assay (nmol·mg^–1·h^–1) was not different between conditions prior to exercise (85.2 ± 3.3 or 86.0 ± 3.9), at 30 min (91.4 ± 4.7 vs. 91.9 ± 4.1) and at fatigue (92.8 ± 4.3 vs. 100 ± 5.0) but was higher (P < 0.05) in G at 90 min (86.7 ± 4.2 vs. 109 ± 4.1). Na⁺-K⁺-ATPase content (_max) measured by the vanadate facilitated [³H]ouabain-binding technique (pmol/g wet wt) although elevated (P < 0.05) by exercise (0<30, 90, and fatigue) was not different between NG and G. At 60 and 90 min of exercise, blood glucose was higher (P < 0.05) in G compared with NG. The G condition also resulted in higher (P < 0.05) serum insulin at similar time points to glucose and lower (P < 0.05) plasma epinephrine and norepinephrine at 90 min of exercise and at fatigue. These results suggest that G results in an increase in V_max by mechanisms that are unclear.

contractile activity; Na⁺-K⁺-pump; glucose; regulation

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