Apoptosis, rather than oncosis, is the predominant mode of spontaneous death of isolated adult rat cardiac myocytes in culture

A number of isolated adult cardiomyocytes dies within a few days of culture and the mode of death has recently been suggested to be apoptosis, based o n its association with the appearance of DNA fragmentation. However, morpho logical evidence is still lacking and precise analysis, including quantific ation, has not been performed. Adult rat ventricular cardiomyocytes isolate d by enzymatic dissociation were incubated for 7 days in a serum-free mediu m (the rapid attachment model) and after various incubation periods, both a ttached and floating cells were counted and classified based on combined cr iteria of morphology and membrane permeability (dye exclusion): type 1, rod cells with intact membranes; type 2, non-rod cells with intact membranes; and type 3, non-rod cells with ruptured membranes. The number of both rod-s haped and dye-excluding cells decreased with the incubation period. After 7 days culture, the number of residual cells decreased to 12% of the initial value. Electron microscopy identified type I cells as viable, type 2 cells as viable or apoptotic, and type 3 as undergoing oncosis (primary necrosis ) or secondary post-apoptotic necrosis. Ultrastructural morphometry reveale d that oncotic cell death occurred predominantly during the early phase of culture whereas the more abundant apoptotic cell death occurred throughout the culture period. In conclusion, although both apoptotic and oncotic deat h occur in the natural course of adult rat cardiomyocytes in short-term cul ture, apoptosis is more predominant. Because of the high incidence of spont aneous cell death predominantly via apoptosis, this information is importan t for the interpretation of studies using this cell type in culture.