Effects of a 21-Day Expedition to 6194 m on Human Skeletal Muscle SR Ca 2+ -ATPase

Abstract

We investigated the effects of a 21-day expedition to the summit of Mount Denali, Alaska (6194 m) on selected Ca²⁺ sequestration properties of sarcoplasmic reticulum (SR) calcium pump in vastus lateralis muscle. Muscle samples were obtained by biopsy from 5 male climbers (peak oxygen consumption, VO₂peak = 52.3 ± 2.1 mL⋅kg^-1⋅min^-1) approximately 7 days prior to (PRE) and 4 days following (POST) the expedition. A comparison of PRE versus POST measures of maximal Ca²⁺-ATPase activities (117 ± 8.5 vs. 97.6 ± 5.6 nmol⋅mg protein^-1⋅min^-1) and Ca²⁺-uptake (204 ± 15 vs. 161 ± 11 nmol⋅mg protein^-1⋅min^-1) measured in crude homogenates obtained from pre-exercised muscle, indicated only an effect (p < 0.05) of the expedition on Ca²⁺-uptake. The reduction in Ca²⁺-ATPase activity, representing 16.6%, was not significant (p = 0.089). The sarco endoplasmic reticulum calcium (SERCA)-ATPase isoforms, measured using Western blotting techniques, revealed a small reduction (p < 0.05) in SERCA 1 (-4.6 ± 1.9%), but not in SERCA 2a (+2.0 ± 1.4%). Prior to the expedition, both Ca²⁺-ATPase activity and Ca²⁺-uptake were reduced (p < 0.05) by approximately 34 and 18%, respectively, following 40 min of a two-step continuous cycling task (20 min at 59% VO₂peak and 20 min at 74% VO₂peak). The exercise-induced reduction in Ca²⁺-ATPase activity was independent of fiber type. Only in the case of Ca²⁺-uptake was a lower exercise response (p < 0.05) observed following the expedition, an effect that was due to the lower resting value. It is concluded that acclimatization as experienced during a mountaineering expedition induces changes in the properties of the SR Ca²⁺-pump, and particularly to Ca²⁺-sequestering function.