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Human muscle sarcoplasmic reticulum function during submaximal exercise in normoxia and hypoxia

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Submitted 4 September 2003 ; accepted in final form 14 March 2004

In this study, the response of the sarcoplasmic reticulum (SR) to prolonged exercise, performed in normoxia (inspired O₂ fraction = 0.21) and hypoxia (inspired O₂ fraction = 0.14) was studied in homogenates prepared from the vastus lateralis muscle in 10 untrained men (peak O₂ consumption = 3.09 ± 0.25 l/min). In normoxia, performed at 48 ± 2.2% peak O₂ consumption, maximal Ca²⁺-dependent ATPase activity was reduced by 25% at 30 min of exercise compared with rest (168 ± 10 vs. 126 ± 8 µmol·g protein^–1·min^–1), with no further reductions observed at 90 min (129 ± 6 µmol·g protein^–1·min^–1). No changes were observed in the Hill coefficient or in the Ca²⁺ concentration at half-maximal activity. The reduction in maximal Ca²⁺-dependent ATPase activity at 30 min of exercise was accompanied by oxalate-dependent reductions (P < 0.05) in Ca²⁺ uptake by 20% (370 ± 22 vs. 298 ± 25 µmol·g protein^–1·min^–1). Ca²⁺ release, induced by 4-chloro-m-cresol and assessed into fast and slow phases, was decreased (P < 0.05) by 16 and 32%, respectively, by 90 min of exercise. No differences were found between normoxia and hypoxia for any of the SR properties examined. It is concluded that the disturbances induced in SR Ca²⁺ cycling with prolonged moderate-intensity exercise in human muscle during normoxia are not modified when the exercise is performed in hypoxia.

calcium, cycling; vastus lateralis; prolonged activity

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