Co-existence of nonmuscle-specific and cardiac muscle-specific myosin in myofibrils of cultured adult cardiac muscle cells.

Immunofluorescence microscopy revealed that nonmuscle myosin and cardiac muscle-specific myosin were present in the myofibrils of cultured adult cardiac muscle cells at different stages of redifferentiation including fully redifferentiated cells. Nonmuscle myosin and cardiac muscle-specific myosin were observed in amorphous or fibroamorphous form predominantly in unspread and partially spread cardiac myocytes. Nonmuscle myosin and cardiac muscle-specific myosin were present in the same striated myofibrils of the same fully redifferentiated cells. Nonmuscle myosin was localized in the Z-lines of sarcomeres of myofibrils. In double stained cells, alpha-actinin was colocalized with nonmuscle myosin in the Z-lines. The in vivo adult cardiac muscle cells contained nonmuscle myosin and cardiac muscle-specific myosin in the same myofibrils of the same cells with the localization of nonmuscle myosin in Z-lines and intercalated discs. It is evident that the nonmuscle myosin is an integral part of the sarcomeric structure of myofibrils in cardiac muscle cells.