title: Thermal instability of rat muscle sarcoplasmic reticulum Ca(2+)-ATPase function.
abstract: To examine the thermal instability and the role of sulfhydryl (SH) oxidation on sarcoplasmic reticulum (SR) Ca(2+)-ATPase function, crude homogenates were prepared from the white portion of the gastrocnemius (WG) adult rat muscles (n = 9) and incubated in vitro for < or =60 min either at a normal resting body temperature (37 degrees C) or at a temperature indicative of exercise-induced hyperthermia (41 degrees C) with DTT and without DTT (CON). In general, treatment with DTT resulted in higher Ca(2+)-ATPase and Ca(2+) uptake values (nmol. mg protein(-1). min(-1), P < 0.05), an effect that was not specific to time of incubation. Incubations at 41 degrees C resulted in lower (P < 0.05) Ca(2+) uptake rates (156 +/- 18 and 35.9 +/- 3.3) compared with 37 degrees C (570 +/- 54 and 364 +/- 26) at 30 and 60 min, respectively. At 37 degrees C, ryanodine (300 microM), which was used to block Ca(2+) release from the calcium release channel, prevented the time-dependent decrease in Ca(2+) uptake. A general inactivation (P < 0.05) of maximal Ca(2+)-ATPase activity (V(max)) in CON was observed with incubation time (0 > 30 > 60 min), with the effect being more pronounced (P < 0.05) at 41 degrees C compared with 37 degrees C. The Hill slope, a measure of co-operativity, and the pCa(50), the cytosolic Ca(2+) concentration required for half-maximal activation of Ca(2+)-ATPase activity, decreased (P < 0.05) at 41 degrees C only. Treatment with DTT attenuated the alterations in enzyme kinetics. The increase in V(max) with the Ca(2+) ionophore A-23187 was less pronounced at 41 degrees C compared with 37 degrees 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(2+) uptake and occurs as a result of increases in membrane permeability to Ca(2+). Moreover, the decreases in Ca(2+)-ATPase kinetics in WG with sustained heat stress result from SH oxidation.
authors.affiliation: Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
authors.forename: J D
authors.initials: JD
authors.lastname: Schertzer
authors.forename: H J
authors.initials: HJ
authors.lastname: Green
authors.forename: A R
authors.initials: AR
authors.lastname: Tupling
mesh_terms: D000818:Animals; D001831:Body Temperature / Q000502:physiology; D001833:Body Temperature Regulation / Q000187:drug effects / Q000502:physiology*; D002118:Calcium / Q000493:pharmacokinetics; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004229:Dithiothreitol / Q000494:pharmacology; D005260:Female; D018882:Heat Stress Disorders / Q000378:metabolism; D018482:Muscle, Skeletal / Q000201:enzymology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases
chemical_list: D012433:Ryanodine; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D002118:Calcium; D004229:Dithiothreitol