Dexamethasone-induced differentiation of atrial myocytes in culture

Atrial and ventricular myocytes from fetal and newborn rats were cultured in medium supplemented with fetal or newborn calf serum with and without glucocorticoid. Myocyte morphology was examined by light and electron microscopy, and the amount of stored and secreted atrial natriuretic peptide (ANP) was measured. Without dexamethasone, neonatal atrial myocytes cultured for 7 days contained myofibrils organized into sarcomeres and numerous endocrine granules containing immunostainable ANP. Secretion of immunoreactive ANP reached a peak between days 7 and 9 of culture. Myocytes from fetal rats secreted ANP but contained few endocrine granules, and myofilaments were poorly organized. By contrast, the addition of dexamethasone (1 nM-1 microM) to the culture medium of newborn myocytes promoted development of numerous endocrine storage granules, mitochondria, and myofibrils with prominent Z-bands. Dexamethasone also increased the cellular content of ANP and ANP-specific mRNA in both atrial and ventricular myocytes. In the presence of dexamethasone myocytes maintained their structural integrity for periods of at least 45 days.