Microassay for malignant hyperthermia susceptibility: hypersensitive ligand-gating of the Ca channel in muscle sarcoplasmic reticulum causes increased amounts and rates of Ca-release.

A crude preparation of heavy sarcoplasmic reticulum (HSR) was isolated using 1 gram of muscle obtained from swine susceptible to malignant hyperthermia (MH) and from control swine. The caffeine and ATP concentration-dependency of Ca-release was determined using suction filtration with radioisotopic 45Ca as a tracer. Rates of release were determined using a rapid filtration system. Caffeine and ATP-induced Ca-release from MH-susceptible (MHS) HSR occurred at one-tenth the concentration of agonist that was required for control muscle HSR. No differences in rates and amounts of release were observed when agonist concentrations were used that caused maximum release for controls. However, at the threshold concentration of caffeine causing release for control HSR, the MHS HSR released 4-times as much Ca and at 3-times the rate of controls. These findings indicate that increased rates and amounts of Ca-release are due to the hypersensitivity of the Ca-release channel of HSR and that this abnormality can be detected using 1 gram of muscle.