Key result
Incubation of rat muscle homogenates at 41°C resulted in significantly lower Ca2+ uptake rates compared with 37°C (P<0.05), an effect that was attenuated by DTT treatment.
p-value: p=<0.05
Exposure of rat muscle homogenates to exercise-induced hyperthermia reduces Ca(2+) uptake and Ca(2+)-ATPase kinetics, which is mediated by sulfhydryl oxidation.
Animal data link hyperthermia to impaired muscle Ca2+ handling via oxidation; leaves open applicability to human exercise physiology.
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.
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Schertzer et al. (2002) studied Thermal instability of sarcoplasmic reticulum Ca2+-ATPase function (n=9). Incubation at 41 degrees C (exercise-induced hyperthermia) with or without DTT vs. Incubation at 37 degrees C (normal resting body temperature) was evaluated on Ca(2+) uptake rates (p=<0.05). Incubation of rat muscle homogenates at 41°C resulted in significantly lower Ca2+ uptake rates compared with 37°C (P<0.05), an effect that was attenuated by DTT treatment.
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