Malignant hyperthermia susceptibility: biochemical basis for pathogenesis and diagnosis.

Malignant hyperthermia (MH) is a hypermetabolic and hypercontractile syndrome triggered by anesthesia or various stressors that cause a sustained increase in sarcoplasmic ionized Ca. Susceptibility is apparently inherited in an autosomal dominant pattern. The primary molecular defect results in hypersensitive ligand-gating of the Ca-release channel of sarcoplasmic reticulum (SR) in skeletal muscle: channel opening is stimulated by abnormally low concentrations of agonist. We attribute MH to a mutation in the gene for the fast twitch muscle isoform of the Ca-channel, resulting in the expression of a cardiac-like isoform in fast muscle. Syndromes with some resemblance to MH can occur due to other genetic or acquired imbalances in Ca-flux across SR that favor net release of Ca. Either defective uptake or release can be detected as increased sensitivity of muscle to the contracture-producing effects of caffeine and halothane. Thus, caffeine and/or halothane contracture tests for MH-susceptibility may give false positives when there is decreased Ca-uptake, such as in muscular dystrophies. Ca-channel hypersensitivity and decreased Ca-uptake activity can be detected by assays using isolated SR. Functional assays using lymphocytes are being assessed as potential replacements for muscle contracture tests. Polymorphism analysis of proteins or nucleic acids for the MH or closely-linked genes has been used to trace the inheritance of MH-susceptibility.