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 (T3) and thyroxine (T4) increase approximately six- and fourfold respectively between the end of gestation and weaning in the rat. This early postnatal surge of thyroid hormones was experimentally modified to produce either a state of hypo- or hyperthyroidism. The growth and rates of protein turnover 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 lactation. Hyperthyroidism was achieved by giving the suckling pups daily injections of T4 from day 3 postpartum onwards. Between 12 and 20 days the weight and protein mass of the combined ventricles of the euthyroid animals approximately doubled, along with substantial increases (50%) in the RNA and DNA contents. Over this same 8 days, growth in the combined atria was much slower. During the same period, hypothyroidism significantly retarded the growth of these immature rats and their atria and ventricles. Both the rates of protein synthesis and protein degradation were decreased in the atria and ventricles. In contrast, hyperthyroidism significantly increased growth in both types of cardiac tissue, this being more pronounced in the atria than in the ventricles between 12 and 20 days. The rates of protein synthesis were increased accordingly, principally by increases in the ribosomal activities. In conclusion, thyroid hormones clearly influence the early postnatal growth of the atria and ventricles of the heart in the rat.