Reexpression of α-smooth muscle actin isoform in cultured adult rat cardiomyocytes

Expression of α-smooth muscle (sm) actin in regenerating adult cardiomyocytes in culture was investigated. No α-sm-actin could be detected in adult ventricular tissue or in newly dissociated rod-shaped cells, whereas a fraction of the polymorphic flattened out adult cardiac cells in culture did express the protein. Immunofluorescence studies revealed a characteristic staining pattern, suggesting the preferential presence of α-sm-actin in stress fiber-like structures, while newly formed myofibrils contained only little α-sm-actin isoprotein. Cell-cell contacts were resumed, but formation of new gap junctions, as revealed by microinjecting Lucifer yellow, was not dependent on α-sm-actin expression. The behavior corresponds to fetal cardiomyocytes either in tissue or as single cells in culture where expression of α-sm-actin can be observed. Such immunofluorescence staining patterns with corresponding immunoblot data can be expected when a return to a less differentiated, more fetal state of the adult cardiomyocyte in culture is assumed. The possible role of the α-sm-actin and α-sarcomeric actin isoforms during reformation of myofibrillar sarcomeres is discussed.