Formation of binucleated cardiac myocytes in rat heart: I. Role of actin-myosin contractile ring

Cardiac myocytes in rat hearts lose their ability to undergo cytokinesis between day 3 and day 4, resulting in the formation of binucleated myocytes. Failure in the formation of the actin-myosin contractile ring could cause cardiac myocytes to be defective in cytokinesis. Enzymatically isolated cardiac myocytes from 2- and 4-day-old rats were employed to investigate the organisation and distribution of actin, myomesin, and myosin by rhodamine phalloidin, anti-myomesin, and isoform-specific anti-myosin antibodies, respectively. Interestingly, the actin-myosin contractile ring was formed in mitotic myocytes from both 2- and 4-day-old animals. The changes in organisation and distribution of actin, myosin and myomesin in mitotic myocytes from 4-day-old rats were similar to those from 2-day-old rats, except that there were longitudinal actin filaments in the cytoplasm of mitotic myocytes from 4-day-old rats. In mitotic myocytes from both 2- and 4-day-old rats, actin disassembled in prometaphase, concentrated in the equator of the mitotic spindle in late anaphase, and formed a circumferential intensely staining band in early telophase. Cytoplasmic myosin was evenly distributed in the cytoplasm as small spots, and appeared to associate with the cell membrane from interphase to early anaphase. It became progressively more concentrated in association with the cortical membrane in the equator region in late anaphase, formed a ring-like structure in early telophase, and remained associated with adjacent membrane at the cleavage furrow until late telophase. Sarcomeric myosin and myomesin were only partially disassembled in mitotic myocytes from both 2- and 4-day-old animals. The present study showed that the actin-myosin contractile ring was actually formed during the binucleation process of cardiac myocytes. Molecules involved in the latter stages of cytokinesis may be responsible for incomplete cytokinesis during the binucleation process.