Expression of myosin isoenzymes in cardiac-muscle cells in culture

Myosin isoenzyme profiles of rat and chicken embryonic cardiac myocytes were studied during differentiation and growth in vitro by native-gel electrophoresis and assay of Ca2+-activated ATPase. The electrophoretic pattern of myosin extracted from 18-day-embryonic-rat myocytes after 7 days in culture exhibits three isoenzyme bands, V1, V2 and V3, of which the slow-migrating V3 is predominant. This resembles the isoenzyme profiles from 18-20-day-embryonic ventricles in vivo. However, the isoenzyme profile of the 7-day-old culture differs from that of its counterpart in vivo, as well as from that of the young and adult rat ventricles, the last two containing the predominant fast-migrating component, V1. When embryonic cardiac myocytes were grown in vitro for 7 days in a medium containing a physiological concentration of L-thyroxine (T4), myosin isoenzyme profiles of these cells shifted to the adult form, with isoenzyme V1 predominating after day 4 of culture. The 7-day-old intact embryonic-chicken ventricles and isolated myocytes showed a single myosin isoenzyme band after 7 days of culture that resembles the pattern seen for the adult chicken. T4 had no effect on the electrophoretic mobility of this isoenzyme pattern. ATPase activity of isoenzyme V1 in cultured rat myocytes treated with T4 was comparable with that of V1 in the untreated adult heart. This study demonstrates that ATPase activity of the chicken myosin isoenzyme is significantly lower than that of isoenzyme V1, but is comparable with that of rat V3. This study shows that the expression of myosin isoenzyme profiles in cultured rat cardiac myocytes does not fully represent the situation in vivo. Physiological concentrations of T4 can modulate the predominant foetal-type isoenzyme V3 to the adult type V1 in cultured embryonic-rat cardiac myocytes within a brief period.