Changes in thiol reactivity and extractability of myofibril bound cardiac troponin C in porcine malignant hyperthermia

We recently showed that a shift (∼44%) in cardiac myosin isozyme from V3 to V1 occurs in the hearts of malignant hyperthermia (MH)-susceptible pigs and may be important in the disease. To follow up this finding, we investigated whether this myosin isozyme shift results in conformational changes in cardiac troponin C (cTnC). The two cysteine residues (Cys-35 and Cys-84) in the regulatory domain of cTnC make it possible to attach conformational probes to this region. Incorporation of a fluorescent probe, 7-diethylamino-3-(4′maleimidylphenyl)-4-methylcoumarin (CPM), into myofibril-bound cTnC was measured by alkaline urea gel electrophoresis, followed by quantification of the protein and the fluorescent label on the gels. The structural stability of cTnC incorporated into cardiac myofibrils was compared between normal and MH-susceptible pigs by selective cTnC extraction and re-incorporation. Changes were detected in both the reactivity of cTnC with CPM in rigor myofibrils and cTnC incorporation into myofibrils from the hearts of MH-susceptible pigs. These changes are very likely to be a consequence of the cardiac myosin isozyme shift in the hearts of MH-susceptible pigs, which may contribute to the changes in the myofilament response to Ca²⁺ binding and to the modulation of cardiac contractility seen in this disease.