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1 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
* To whom correspondence should be addressed. E-mail: green{at}healthy.uwaterloo.ca.
The repetition-dependent effects of a repetitive heavy exercise protocol previously shown to alter muscle mechanic behaviour (Green et al 2004) on muscle sarcoplasmic reticulum (SR) Ca2+- transport properties, measured "in vitro", were examined in 12 untrained volunteers [peak aerobic power (VO2peak) = 44.3±0.66 ml/kg/min]. The protocol involved 6 min of cycle exercise performed at ~91% VO2peak once per h for 16 h. Tissue samples were obtained from the vastus lateralis before (B) and after (A) exercise at repetitions 1 (R1), 2 (R2), 9 (R9) and 16 (R16). Reductions (P<0.05) in maximal Ca2+-ATPase activity (Vmax) of 26% and 12% with exercise were only observed at R1 and R16, respectively. Vmax remained depressed (P<0.05) at R2 (B) but not at R9 (B) and R16 (B). No changes were observed in 2 other kinetic properties of the enzyme, namely the Hill coefficient (defined as the slope of the relationship between Ca2+- ATPase activity and free Ca2+-concentration) and the Ca50 (defined as the free Ca2+- concentration needed to elicit 50% Vmax). Changes in Ca2+-uptake (measured at 2000 nM) with exercise and recovery generally paralleled Vmax. The apparent coupling ratio, defined as the ratio between Ca2+-uptake and Vmax, was unaffected by the intermittent protocol. Reductions (P<0.05) in Phase 1 Ca2+-release (32%) were only observed at R1. No differences were observed between B and A for R2, R9 and R16 or between B and B for R1, R2, R9 and R16. The changes in Phase 2 Ca2+-release were as observed for Phase 1 Ca2+-release. It is concluded that the SR Ca2+- handling properties, in general, display rapid adaptations to repetitive exercise.
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