[Proliferative processes in the myocardium in different parts of the heart during the creation of experimental myocardial infarct of the left ventricle in 2- and 3-week-old rats].

As a result of 30 times repeated injections of 3H-thymidine (3HTdr) to neonate rats, beginning from days 13 or 21 post partum, ca. 20 and 10% of myonuclei in the left and right atria were labeled, respectively, while in both ventricles cumulative labeling of myocytes was nearly ten times lower. In rats of the same age with experimental infarction of the left ventricular myocardium the number of myonuclei labeled after 30-fold 3HTdr injections increased in atria up to 40-50%, in perinecrotic myofibers of the left ventricles up to 8-11%, and in myofibers of the left and right ventricle located far from the necrotic foci up to 3-4 and 2-3%, respectively. In some of rats subendocardial and/or subepicardial layers of the surviving left ventricular myocardium contained up to 15-35% of labeled myonuclei. Thus, in neonatal rats the extent of DNA synthesis reactivation in the nuclei of cardiomyocytes, the majority of which have recently completed normal ontogenetic proliferation, is, on the whole, of the same order as found in similar experiments on adult rats (Rumiantsev, Kassem, 1976; Oberpriller et al., 1984). However, still immature ventricular myocytes of neonatal rats resume mitotic cycle easier than those of adult animals which is evidenced not only by higher numbers of 3HTdr labeled myonuclei in subepicardial and subendocardial ventricular myocardia of some rats, but even more by reactivation of DNA synthesis in a limited fraction (2-3%) of the whole population of non-perinecrotic myocytes in both ventricles. Besides, reactive proliferation of cardiomyocytes in the atria of neonate rats, unlike in adults, starts on day 3 rather than on day 5 after infarction is induced. In the atria of neonatal rats polyploidization of myonuclei at later postinfarction stages is less pronounced than in adult rats which may be accounted for by formation of individual daughter nuclei during acytokinetic mitoses or, more seldom, by completion of cytotomy.