The effects of volatile anesthetics on calcium regulation by malignant hyperthermia-susceptible sarcoplasmic reticulum

To clarify the mechanism by which volatile anesthetics initiate malignant hyperthermia (MH), we examined the effect of halothane, isoflurane, and enflurane on Ca2+ uptake and release by sarcoplasmic reticulum vesicles isolated from MH-susceptible (MHS) and normal pig muscle. Clinical concentrations of these anesthetics (0.1-0.5 mM) stimulated sarcoplasmic reticulum ATP-dependent Ca2+ uptake (maximal at approximately 4 mM), whereas 10-20 times the clinical anesthetic concentration inhibited Ca2+ uptake. There was no significant difference between MHS and normal sarcoplasmic reticulum in any aspect of Ca2+ uptake. Ca2+ release from 45Ca(2+)-filled sarcoplasmic reticulum vesicles in a 10(-8) M Ca(2+)-containing medium (pH 7.0) was significantly stimulated at clinical concentrations of all three volatile anesthetics (anesthetic concentration for the 50% stimulation of Ca2+ release = 0.096-0.22 mM); however, the rate constant for Ca2+ release from MHS sarcoplasmic reticulum was in all cases significantly greater than that from normal sarcoplasmic reticulum. Furthermore, 0.5 mM halothane had no effect on Ca2+ release from normal sarcoplasmic reticulum at pH values less than 6.8, although it could still significantly stimulate Ca2+ release from MHS sarcoplasmic reticulum even at pH 6.4; similar results were obtained for isoflurane and enflurane. These studies thus demonstrate that the interaction of volatile anesthetics with the sarcoplasmic reticulum Ca(2+)-release channel is altered in MHS porcine muscle such that the channel may be activated even at a Ca2+ concentration or pH that would be expected to maintain the channel in the closed state.