Sarcolemmal properties implicated in the skeletal muscle disorder, malignant hyperthermia (MH), were examined using sarcolemma-membrane vesicles isolated from normal and MH-susceptible (MHS) porcine skeletal muscle. MHS and normal sarcolemma did not differ in the distribution of the major proteins, cholesterol or phospholipid content, vesicle size and sidedness, (Na+ + K+)-ATPase activity, ouabain binding, or adenylate cyclase activity (total and isoproterenol sensitivity). The regulation of the initial rates of MHS and normal sarcolemmal ATP-dependent calcium transport (calcium uptake after 1 min) by Ca2+ (K 1 2 = 0.64-0.81 μM), calmodulin, and cAMP-dependent protein kinase were similar. However, when sarcolemmal calcium content was measured at either 2 or 20 min after the initiation of active calcium transport, a significant difference between MHS and normal sarcolemmal calcium uptake became apparent, with MHS sarcolemma accumulating approximately 25% less calcium than normal sarcolemma. Calcium transport by MHS and normal sarcolemma, at 2 or 20 min, had a similar calmodulin dependence (C 1 2 = 150 nM), and was stimulated to a similar extent by cAMP-dependent protein kinase or calmodulin. Halothane inhibited MHS and normal sarcolemmal active calcium uptake in a similar fashion (half-maximal inhibition at 10 mM halothane), while dantrolene (30 μM) and nitrendipine (1 μM) had little effect on either MHS or normal sarcolemmal calcium transport. After 20 min of ATP-supported calcium uptake, 2 mM EGTA plus 10 μM sodium orthovanadate were added to initiate sarcolemmal calcium efflux. Following an initial rapid phase of calcium release, an extended slow phase of calcium efflux (k = 0.012 min-1) was similar for both MHS and normal sarcolemma vesicles. We conclude that although a number of sarcolemmal properties, including passive calcium permeability, are normal in MH, a small but significant defect in MHS sarcolemmal ATP-dependent calcium transport may contribute to the abnormal calcium homeostasis and altered contractile properties of MHS skeletal muscle.
Bibliographical noteFunding Information:
The authors wish to express their appreciation to Dr. William Rempel for the supply of experimental animals and Dr. Tim Walseth for assistance in preparing 32p-labelled nucleotides. Thanks also to the Veterinary Biology secretarial staff for excellent typing services. Portions of this research were conducted during the tenure of a Postdoctoral Fellowship from the Muscular Dystrophy Association (J.R.M.). This project was supported by grants from the Muscular Dystrophy Association and NIH GM-31382.
- Calcium ion transport
- Malignant hyperthermia
- Membrane vesicle
- Skeletal muscle