Rapid, simple and sensitive microassay for skeletal muscle homogenates in the functional assessment of the Ca-release channel of sarcoplasmic reticulum: Application to diagnosis of susceptibility to malignant hyperthermia

A microassay is demonstrated for functional characterization of the Ca2+-release channel (CRC) of sarcoplasmic reticulum (SR) of skeletal muscle using swine with susceptibility to malignant hyperthermia (MH). Diluted muscle homogenates, indo-1 and ratiometric dual-emission spectrofluorometry are used to monitor Ca2+-lowering activity in real-time in the presence and absence of ryanodine at exposures that open and close the CRC. Reactions are initiated with 50 mu M CaCl2 to raise ionized Ca2+ concentration near 1 mu M and MgATP to activate the Ca2+-ATPase pump. Oxalate is included to precipitate Ca2+ within the SR. The assay requires less than 30 mg muscle, which may be cryopreserved, and is completed within 20 min of thawing the tissue. Maximum SR Ca2+ ATPase pumping and CRC activities, degree of CRC activation, and Ca2+-buffering capacity can be determined. Using this assay we studied muscle from MH-susceptible swine and demonstrated that whereas maximal Ca2+-ATPase pumping and CRC activities are normal, the CRC activity after addition of a bolus of Ca2+ is 50% greater in heterozygotes and 100% greater in homozygotes for the MH mutation. Hypersensitivity to CRC agonists, such as caffeine, and an associated hyposensitivity to CRC antagonists such as Mg2+ is also demonstrated. Genotypes for the MH mutation site can be discriminated from each other by determining Ca2+-lowering activities and the effect of ryanodine on them.