Effects of elevated physiological temperatures on sarcoplasmic reticulum function in mechanically skinned muscle fibers of the rat

Properties of the sarcoplasmic reticulum (SR) with respect to Ca2+ loading and release were measured in mechanically skinned fiber preparations from isolated extensor digitorum longus (EDL) muscles of the rat that were either kept at room temperature (23 degrees C) or exposed to temperatures in the upper physiological range for mammalian skeletal muscle (30 min at 40 or 43 degrees C). The ability of the SR to accumulate Ca2+ was significantly reduced by a factor of 1.9 - 2.1 after the temperature treatments due to a marked increase in SR Ca2+ leak, which persisted for at least 3 h after treatment. Results with blockers of Ca2+ release channels (ruthenium red) and SR Ca2+ pumps [2,5-di(tert-butyl)-1,4-hydroquinone] indicate that the increased Ca2+ leak was not through the SR Ca2+ release channel or the SR Ca2+ pump, although it is possible that the leak pathway was via oligomerized Ca2+ pump molecules. No significant change in the maximum SR Ca2+-ATPase activity was observed after the temperature treatment, although there was a tendency for a decrease in the SR Ca2+-ATPase. The observed changes in SR properties were fully prevented by the superoxide (O-2(center dot-)) scavenger Tiron (20 mM), indicating that the production of O-2(center dot-) at elevated temperatures is responsible for the increase in SR Ca2+ leak. Results show that physiologically relevant elevated temperatures 1) induce lasting changes in SR properties with respect to Ca2+ handling that contribute to a marked increase in the SR Ca2+ leak and, consequently, to the reduction in the average coupling ratio between Ca2+ transport and SR Ca2+-ATPase and muscle performance, and 2) that these changes are mediated by temperature-induced O-2(center dot-) production.