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Am J Physiol Endocrinol Metab 283: E722-E728, 2002; doi:10.1152/ajpendo.00204.2002
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Vol. 283, Issue 4, E722-E728, October 2002

Thermal instability of rat muscle sarcoplasmic reticulum Ca2+-ATPase function

J. D. Schertzer, H. J. Green, and A. R. Tupling

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

To examine the thermal instability and the role of sulfhydryl (SH) oxidation on sarcoplasmic reticulum (SR) Ca2+-ATPase function, crude homogenates were prepared from the white portion of the gastrocnemius (WG) adult rat muscles (n = 9) and incubated in vitro for <= 60 min either at a normal resting body temperature (37°C) or at a temperature indicative of exercise-induced hyperthermia (41°C) with DTT and without DTT (CON). In general, treatment with DTT resulted in higher Ca2+-ATPase and Ca2+ uptake values (nmol · mg protein-1 · min-1, P < 0.05), an effect that was not specific to time of incubation. Incubations at 41°C resulted in lower (P < 0.05) Ca2+ uptake rates (156 ± 18 and 35.9 ± 3.3) compared with 37°C (570 ± 54 and 364 ± 26) at 30 and 60 min, respectively. At 37°C, ryanodine (300 µM), which was used to block Ca2+ release from the calcium release channel, prevented the time-dependent decrease in Ca2+ uptake. A general inactivation (P < 0.05) of maximal Ca2+-ATPase activity (Vmax) in CON was observed with incubation time (0 > 30 > 60 min), with the effect being more pronounced (P < 0.05) at 41°C compared with 37°C. The Hill slope, a measure of co-operativity, and the pCa50, the cytosolic Ca2+ concentration required for half-maximal activation of Ca2+-ATPase activity, decreased (P < 0.05) at 41°C only. Treatment with DTT attenuated the alterations in enzyme kinetics. The increase in Vmax with the Ca2+ ionophore A-23187 was less pronounced at 41°C compared with 37°C. It is concluded that exposure of homogenates to a temperature typically experienced in exercise results in a reduction in the coupling ratio, which is mediated primarily by lower Ca2+ uptake and occurs as a result of increases in membrane permeability to Ca2+. Moreover, the decreases in Ca2+-ATPase kinetics in WG with sustained heat stress result from SH oxidation.

muscle; Ca2+ regulation; temperature; Ca2+ uptake; Ca2+-ATPase


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