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

J. D. Schertzer, H. J. Green, A. R. Tupling
2002 American Journal of Physiology. Endocrinology and Metabolism  
Thermal instability of rat muscle sarcoplasmic reticulum Ca 2ϩ -ATPase function. Am J Physiol Endocrinol Metab 283: E722-E728, 2002; 10.1152 10. /ajpendo.00204.2002ine the thermal instability and the role of sulfhydryl (SH) oxidation on sarcoplasmic reticulum (SR) Ca 2ϩ -ATPase function, crude homogenates were prepared from the white portion of the gastrocnemius (WG) adult rat muscles (n ϭ 9) and incubated in vitro for Յ60 min either at a normal resting body temperature (37°C) or at a
more » ... ) 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 Ca 2ϩ -ATPase and Ca 2ϩ uptake values (nmol ⅐ mg protein Ϫ1 ⅐ min Ϫ1 , P Ͻ 0.05), an effect that was not specific to time of incubation. Incubations at 41°C resulted in lower (P Ͻ 0.05) Ca 2ϩ uptake rates (156 Ϯ 18 and 35.9 Ϯ 3.3) compared with 37°C (570 Ϯ 54 and 364 Ϯ 26) at 30 and 60 min, respectively. At 37°C, ryanodine (300 M), which was used to block Ca 2ϩ release from the calcium release channel, prevented the time-dependent decrease in Ca 2ϩ uptake. A general inactivation (P Ͻ 0.05) of maximal Ca 2ϩ -ATPase activity (Vmax) in CON was observed with incubation time (0 Ͼ 30 Ͼ 60 min), with the effect being more pronounced (P Ͻ 0.05) at 41°C compared with 37°C. The Hill slope, a measure of co-operativity, and the pCa50, the cytosolic Ca 2ϩ concentration required for half-maximal activation of Ca 2ϩ -ATPase activity, decreased (P Ͻ 0.05) at 41°C only. Treatment with DTT attenuated the alterations in enzyme kinetics. The increase in Vmax with the Ca 2ϩ 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 Ca 2ϩ uptake and occurs as a result of increases in membrane permeability to Ca 2ϩ . Moreover, the decreases in Ca 2ϩ -AT-Pase kinetics in WG with sustained heat stress result from SH oxidation. muscle; Ca 2ϩ regulation; temperature; Ca 2ϩ uptake; Ca 2ϩ -ATPase SKELETAL MUSCLE CONTRACTION is regulated by the concentration of free cytosolic Ca 2ϩ ([Ca 2ϩ ] f ), which is primarily controlled by the sarcoplasmic reticulum (SR) (23). The SR is an elaborate muscle membrane system residing at the junction between the transverse tubule, the sarcolemmal membrane, and the contractile apparatus (4, 23). Sarcolemmal and T-tubule depo-
doi:10.1152/ajpendo.00204.2002 pmid:12217889 fatcat:2bdbyexjrrc5xbqb25zjqtk3su