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1 School of Human Movement, Recreation and Performance, and 2 School of Life Science, Centre for Rehabilitation, Exercise and Sports Science, Victoria University of Technology, Melbourne, Victoria, 8001, Australia; and 3 Department of Medicine B, The Heart Centre, Rigshospitalet, Copenhagen, DK-2100 Denmark
This study investigated whether
fatiguing dynamic exercise depresses maximal in vitro
Na+-K+-ATPase activity and whether any
depression is attenuated with chronic training. Eight untrained (UT),
eight resistance-trained (RT), and eight endurance-trained (ET)
subjects performed a quadriceps fatigue test, comprising 50 maximal
isokinetic contractions (180°/s, 0.5 Hz). Muscle biopsies (vastus
lateralis) were taken before and immediately after exercise and were
analyzed for maximal in vitro
Na+-K+-ATPase (K+-stimulated
3-O-methylfluoroscein phosphatase) activity. Resting samples were analyzed for [3H]ouabain binding
site content, which was 16.6 and 18.3% higher (P < 0.05) in ET than RT and UT, respectively (UT 311 ± 41, RT 302 ± 52, ET 357 ± 29 pmol/g wet wt).
3-O-methylfluoroscein phosphatase activity was depressed at
fatigue by
13.8 ± 4.1% (P < 0.05), with no
differences between groups (UT
13 ± 4, RT
9 ± 6, ET
22 ± 6%). During incremental exercise, ET had a lower ratio of rise in plasma K+ concentration to work than UT
(P < 0.05) and tended (P = 0.09) to be
lower than RT (UT 18.5 ± 2.3, RT 16.2 ± 2.2, ET 11.8 ± 0.4 nmol · l
1 · J
1).
In conclusion, maximal in vitro Na+-K+-ATPase
activity was depressed with fatigue, regardless of training state,
suggesting that this may be an important determinant of fatigue.
3-O-methylfluoroscein phosphatase; exercise; Na+-K+ pump; potassium
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