Abnormal membrane properties of the sarcoplasmic reticulum of pigs susceptible to malignant hyperthermia: Modes of action of halothane, caffeine, dantrolene, and two other drugs

Abstract

The role of sarcoplasmic reticulum (SR) in malignant hyperthermia (MH) was studied using the heavy microsomal fraction prepared from semitendinosus muscles of both normal and genetically MH-susceptible pigs. In the presence of ATP, SR was loaded with 70 nmol Ca 2+/mg SR protein. Under these conditions, MH-SR demonstrated Ca 2+-induced Ca 2+ release (Ca-ICaR) and halothane-induced Ca 2+ release (halothane-ICaR; halothane concentrations as low as 10 μ m). Normal SR did not demonstrate these release phenomena. Dantrolene inhibited the halothane-ICaR, but did not inhibit the Ca-ICaR. Ruthenium red and tetracaine inhibited both types of Ca 2+ release. From the measurement of passive Ca 2+ efflux, it was shown that dantrolene did not affect the Ca 2+ permeability of the SR itself, but suppressed only the halothane-induced increment of the permeability. The membrane order parameter of the SR, as measured by the spin-probe EPR technique, indicated that halothane disordered the lipid bilayer of MH-SR to a greater extent than it did of normal SR. This halothane disordering effect on MH-SR was antagonized by dantrolene. Ruthenium red and tetracaine did not antagonize the halothane disordering effect. These results raise the possibility that halothane could disturb the structure of the lipoprotein complex in MH-SR in such a way that it could open the Ca 2+-release channels. The Ca 2+ thus released further opens the channel through the Ca-ICaR mechanism in a positive feedback fashion, thus triggering the MH syndrome. The efficacy of dantrolene in ameliorating the MH syndrome might be related to the inhibition of this halothane effect.