Effects of progressive exercise and hypoxia on human muscle sarcoplasmic reticulum function

This study examined the effects of progressive exercise to fatigue in normoxia (N) on muscle sarcoplasmic reticulum (SR) Ca2+ cycling and whether alterations in SR Ca2+ cycling are related to the blunted peak mechanical power output (POpeak) and peak oxygen consumption (Vo(2 peak)) observed during progressive exercise in hypoxia (H). Nine untrained men (20.7 +/- 0.42 yr) performed progressive cycle exercise to fatigue on two occasions, namely during N (inspired oxygen fraction = 0.21) and during H (inspired oxygen fraction = 0.14). Tissue extracted from the vastus lateralis before exercise and at power output corresponding to 50 and 70% of Vo(2 peak) (as determined during N) and at fatigue was used to investigate changes in homogenate SR Ca2+-cycling properties. Exercise in H compared with N resulted in a 19 and 21% lower (P < 0.05) POpeak and Vo(2 peak), respectively. During progressive exercise in N, Ca2+-ATPase kinetics, as determined by maximal activity, the Hill coefficient, and the Ca2+ concentration at one-half maximal activity were not altered. However, reductions with exercise in N were noted in Ca2+ uptake (before exercise = 357 +/- 29 mumol(.)min(-1.)g protein(-1); at fatigue = 306 +/- 26 mumol(.)min(-1.)g protein(-1); P < 0.05) when measured at free Ca2+ concentration of 2 muM and in phase 2 Ca2+ release (before exercise = 716 +/- 33 mumol(.)min(-1.)g protein(-1); at fatigue = 500 +/- 53 mumol(.)min(-1.)g protein(-1); P < 0.05) when measured in vitro in whole muscle homogenates. No differences were noted between N and H conditions at comparable power output or at fatigue. It is concluded that, although structural changes in SR Ca2+-cycling proteins may explain fatigue during progressive exercise in N, they cannot explain the lower POpeak and Vo(2) (peak) observed during H.