Chronic intermittent hypoxia and incremental cycling exercise independently depress muscle in vitro maximal Na+-K+-ATPase activity in well-trained athletes

Athletes commonly attempt to enhance performance by training in normoxia but sleeping in hypoxia [ live high and train low (LHTL)]. However, chronic hypoxia reduces muscle Na+- K+-ATPase content, whereas fatiguing contractions reduce Na+-K+-ATPase activity, which each may impair performance. We examined whether LHTL and intense exercise would decrease muscle Na+-K+-ATPase activity and whether these effects would be additive and sufficient to impair performance or plasma K+ regulation. Thirteen subjects were randomly assigned to two fitness-matched groups, LHTL ( n = 6) or control ( Con, n = 7). LHTL slept at simulated moderate altitude ( 3,000 m, inspired O-2 fraction = 15.48%) for 23 nights and lived and trained by day under normoxic conditions in Canberra ( altitude similar to 600 m). Con lived, trained, and slept in normoxia. A standardized incremental exercise test was conducted before and after LHTL. A vastus lateralis muscle biopsy was taken at rest and after exercise, before and after LHTL or Con, and analyzed for maximal Na+-K+-ATPase activity [K+-stimulated 3-O-methylfluorescein phosphatase (3-O-MFPase)] and Na+-K+-ATPase content ([H-3] ouabain binding sites). 3-O-MFPase activity was decreased by - 2.9 +/- 2.6% in LHTL ( P < 0.05) and was depressed immediately after exercise ( P < 0.05) similarly in Con and LHTL ( - 13.0 +/- 3.2 and - 11.8 +/- 1.5%, respectively). Plasma K+ concentration during exercise was unchanged by LHTL; [H-3] ouabain binding was unchanged with LHTL or exercise. Peak oxygen consumption was reduced in LHTL ( P < 0.05) but not in Con, whereas exercise work was unchanged in either group. Thus LHTL had a minor effect on, and incremental exercise reduced, Na+-K+-ATPase activity. However, the small LHTL-induced depression of 3-O-MFPase activity was insufficient to adversely affect either K+ regulation or total work performed.