Muscle Na-K-pump and fatigue responses to progressive exercise in normoxia and hypoxia.
To investigate the effects of hypoxia and incremental exercise on muscle contractility, membrane excitability, and maximal Na(+)-K(+)-ATPase activity, 10 untrained volunteers (age = 20 +/- 0.37 yr and weight = 80.0 +/- 3.54 kg; +/- SE) performed progressive cycle exercise to fatigue on two occasions: while breathing normal room air (Norm; Fi(O(2)) = 0.21) and while breathing a normobaric hypoxic gas mixture (Hypox; Fi(O(2)) = 0.14). Muscle samples extracted from the vastus lateralis before exercise and at fatigue were analyzed for maximal Na(+)-K(+)-ATPase (K(+)-stimulated 3-O-methylfluorescein phosphatase) activity in homogenates. A 32% reduction (P < 0.05) in Na(+)-K(+)-ATPase activity was observed (90.9 +/- 7.6 vs. 62.1 +/- 6.4 nmol.mg protein(-1).h(-1)) in Norm. At fatigue, the reductions in Hypox were not different (81 +/- 5.6 vs. 57.2 +/- 7.5 nmol.mg protein(-1).h(-1)) from Norm. Measurement of quadriceps neuromuscular function, assessed before and after exercise, indicated a generalized reduction (P < 0.05) in maximal voluntary contractile force (MVC) and in force elicited at all frequencies of stimulation (10, 20, 30, 50, and 100 Hz). In general, no differences were observed between Norm and Hypox. The properties of the compound action potential, amplitude, duration, and area, which represent the electromyographic response to a single, supramaximal stimulus, were not altered by exercise or oxygen condition when assessed both during and after the progressive cycle task. Progressive exercise, conducted in Hypox, results in an inhibition of Na(+)-K(+)-ATPase activity and reductions in MVC and force at different frequencies of stimulation; these results are not different from those observed with Norm. These changes occur in the absence of reductions in neuromuscular excitability.