THE INFLUENCE OF THYROID-HORMONES ON THE GROWTH OF THE ATRIA AND VENTRICLES OF THE HEART IN IMMATURE RATS

The normal plasma concentrations of tri-iodothyronine (T-3) and thyrox ine (T-4) increase approximately six- and fourfold respectively betwee n the end of gestation and weaning in the rat. This early postnatal su rge of thyroid hormones was experimentally modified to produce either a state of hypo- or hyperthyroidism. The growth and rates of protein t urnover in the atria and ventricles of the heart were studied, 12 and 20 days postpartum, both as a function of age and of changing thyroid status. Neonatal hypothyroidism was induced by adding propylthiouracil to the mothers' drinking water late in gestation and throughout lacta tion. Hyperthyroidism was achieved by giving the suckling pups daily i njections of T-4 from day 3 postpartum onwards. Between 12 and 20 days the weight and protein mass of the combined ventricles of the euthyro id animals approximately doubled, along with substantial increases (50 %) in the RNA and DNA contents. Over this same 8 days, growth in the c ombined atria was much slower. During the same period, hypothyroidism significantly retarded the growth of these immature rats and their atr ia and ventricles. Both the rates of protein synthesis and protein deg radation were decreased in the atria and ventricles. In contrast, hype rthyroidism significantly increased growth in both types of cardiac ti ssue, this being more pronounced in the atria than in the ventricles b etween 12 and 20 days. The rates of protein synthesis were increased a ccordingly, principally by increases in the ribosomal activities. In c onclusion, thyroid hormones clearly influence the early postnatal grow th of the atria and ventricles of the heart in the rat.