Why does halothane relax cardiac muscle but contract malignant hyperthermic skeletal muscle?

We have studied the question of the possible role of sarcoplasmic reticulum (SR) in the interaction of volatile anesthetics (such as halothane, enflurane and isoflurane) with muscle. We used two cardiac muscle models, i.e., isolated rat myocytes and Langendorff perfused rat hearts. We compared the results with those for skeletal muscle SR from rabbits, rats and pigs susceptible to malignant hyperthermia (MH). In both skeletal and cardiac muscle SR, volatile anesthetics enhanced the calcium release from the SR. In cardiac muscle, these agents are known to decrease contractility (negative inotropism). We found that caffeine, a well-known agent which releases calcium from the SR, also had a negative inotropic effect in cardiac muscle, raising the possibility of an unexpected link between the potentiation of calcium release and mechanism underlying the observed negative inotropism. Current understanding of anesthetic mechanisms does not include this possibility. We further found that both volatile anesthetics and caffeine decrease the content of calcium in the SR, suggesting that the increase of calcium permeability results in the decrease of calcium ions in the SR which are available for excitation-contraction (E-C) coupling. In MH-susceptible skeletal muscle, a similar increase in calcium permeability does not cause a decrease of contractility, but rather may contribute to a fatal syndrome of temperature increase provoked by abnormal contracture. This difference may be because in skeletal myoplasm calcium ions recycle internally, while in the cardiac muscle cell they are in dynamic equilibrium with extracellular calcium ions.