Malleability of human skeletal muscle sarcoplasmic reticulum to short-term training

This study investigated the hypothesis that adaptations would occur in the sarcoplasmic reticulum in vastus lateralis soon after the onset of aerobic-based training consistent with reduced Ca2+-cycling potential. Tissue samples were extracted prior to (0 days) and following 3 and 6 days of cycling performed for 2 h at 60%-65% of peak aerobic power (VO2peak) in untrained males (VO2peak = 47 +/- 2.3 mL . kg(-1) . min(-1); mean +/- SE, n = 6) and assessed for changes (nmol . mg protein(-1) . min(-1)) in maximal Ca2+-ATPase activity (V-max), Ca2+-uptake, and Ca2+-release (phase 1 and phase 2) as well as the sarcoplasmic (endoplasmic) reticulum Ca2+-ATPase (SERCA) isoforms. Training resulted in reductions (p < 0.05) in SERCA1a at 6 days (-14%) but not at 3 days. For SERCA2a, reductions (p < 0.05) were also noted only at 6 days (-7%). For V-max, depressions (p < 0.05) were found at 6 days (172 +/- 11) but not at 3 days (176 +/- 13; p < 0.10) compared with 0 days (192 +/- 11). These changes were accompanied by a lower (p < 0.05) Ca2+-uptake at both 3 days (-39%) and 6 days (-48%). A similar pattern was found for phase 1 Ca2+-release with reductions (p < 0.05) of 37% observed at 6 days and 23% (p = 0.21) at 3 days of training, respectively. In a related study using the same training protocol and participant characteristics, microphotometric determinations of V-max indicated reductions (p < 0.05) in type I at 3 days (-27%) and at 6 days (-34%) and in type IIA fibres at 6 days (-17%). It is concluded that in response to aerobic-based training, sarcoplasmic reticulum Ca2+-cycling potential is reduced by adaptations that occur soon after training onset.