The influence of aging and growth on the postnatal development of cardiac muscle in rats.

Abstract

We studied the effects of different rates of postnatal body growth on the growth of rat myocytes. Growth rates were altered by adjusting the number of newborn animals per litter to 4 (fast-growing), 8 (normal), or 16 (slow-growing). On the 21st postnatal day, average left ventricular weights (mg SEM) were 80 5 in slowgrowing 131 8 in normal, and 183 5 in fast-growing rats. Average myocyte diameters, obtained from direct microscopic measurements in longitudinal sections prepared from eight animals in each group, were 8.6 0.3, 9.8 0.2, and 10.2 0.3 fim, respectively. The total number and total length of myocytes constituting the left ventricle were estimated, assuming a constant length-to-width ratio and cyclindrical model of myocytes. The estimated total number of myocytes constituting the respective left ventricles were 23.6 2.0, 24.5 2.2, and 31.1 2.1 millions. All differences between experimental groups were significant by analysis of variance. When the estimated total numbers of myocytes and their estimated total lengths were related to the left ventricular weights, a significant linear correlation was obtained, which indicated their dependence on the organ growth and their relative independence of age. These indications were strengthened further by analysis of covariance, which showed that the differences in all morphometric data among the three groups of hearts may be explained entirely by different growth rates of the left ventricle. These results also suggest that the accelerated growth of the heart during the early postnatal period is associated with increased proliferation of myocytes.