DNA synthesis in myocytes from different myocardial compartments of young rats in norm, after experimental infarction and in vitro.

Following 30 repeated 3H-thymidine (3HTdr) injections to rats beginning from days 13 and 21 after birth ca. 20 and 10% of labeled nuclei were observed in the left and right atria, respectively, while in both ventricles cumulative labeling of myocytes was nearly ten times lower. In rats of the same age with experimental myocardial infarction of the left ventricle the number of myonuclei labeled after 30-fold 3HTdr injections increased in atria up to 40-50%, in the perinecrotic area of the left ventricle up to 8-11% and in myofibers of the left and right ventricles located far from the necrotic focus up to 3-4 and 2-3%, respectively. In some of the rats subendocardial and/or subepicardial layers of the surviving left ventricular tissue contained up to 15-35% of labeled myonuclei. Thus, the reproduction capacity of ventricular myocytes in neonates is significantly higher than in adults. Reactive proliferation of cardiomyocytes in the atria of young rats, as opposed to adults, starts on day 3 rather than day 5 after infarction, however the DNA synthesis reactivation is of the same order as found in similar experiments on adult rats. The cultivation in vitro does not eliminate the specificity of the atrial myocytes behavior, their proliferation intensity being 5-7 times higher than that of ventricular ones. Active DNA synthesis and subsequent acytokinetic mitoses of atrial myocytes lead to a significant increase in the number of binucleated cells both after infarction and in vitro.