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Muscle sarcoplasmic reticulum Ca²⁺ cycling adaptations during 16 h of heavy intermittent cycle exercise

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada

Submitted 22 December 2004 ; accepted in final form 20 April 2005

The repetition-dependent effects of a repetitive heavy exercise protocol previously shown to alter muscle mechanic behavior (Green HJ, Duhamel TA, Ferth S, Holloway GP, Thomas MM, Tupling AR, Rich SM, and Yau JE. J Appl Physiol 97: 2166–2175, 2004) on muscle sarcoplasmic reticulum (SR) Ca²⁺-transport properties, measured in vitro, were examined in 12 untrained volunteers [peak aerobic power (O_{2 peak}) = 44.3 ± 0.66 ml·kg^–1·min^–1]. The protocol involved 6 min of cycle exercise performed at 91% O_{2 peak} once per hour 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 (V_max) of 26 and 12% with exercise were only observed at R1 and R16, respectively. V_max remained depressed (P < 0.05) at R2 (B) but not at R9 (B) and R16 (B). No changes were observed in two 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% V_max). Changes in Ca²⁺ uptake (measured at 2,000 nM) with exercise and recovery generally paralleled V_max. The apparent coupling ratio, defined as the ratio between Ca²⁺ uptake and V_max, 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.

Ca²⁺ uptake; Ca²⁺ release; adaptation; repetitive activity; fatigue

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