The Ca-Releasing Action of Halothane on Fragmented Sarcoplasmic Reticulum1

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Abstract
Ca-releasing action of halothane on fragmented sarcoplasmic reticulum (FSR) from bullfrog and rabbit skeletal muscle was examined to understand the mechanism of Ca release in reference to the etiology of malignant hyperthermia. Halothane has dual action on FSR: the Ca release and the inhibition of Ca uptake. On addition of halothane to loaded FSR, a rapid Ca release was followed by a sluggish Ca leakage which was probably due to a decreased capacity for Ca uptake. The properties of the rapid Ca release by halothane are similar to those of caffeine. It was inhibited by procaine or high concentrations of Mg 2+ . It was stimulated by a high concentration of ATP. A low temperature was stimulatory for Ca-releasing action on frog FSR while it was inhibitory on rabbit FSR. The result that caffeine shifted the dose response curve for Ca release by halothane to a steeper relation to a range of much lower concentrations suggests that the action of halothane may not be identical with that of caffeine in spite of many similarities.