Neonatal hyperthyroidism enhances growth of the heart but not of skeletal muscle

Abstract

Comparison of growth and biochemical development of the heart and skeletal muscle (gastrocnemius) in normal rats and those treated with triiodothyronine (T3) (0.1 μg/g bw, s.c.) daily, from birth until weaning (day 25), revealed that hyperthyroidism selectively enhanced cardiac growth as shown by marked increases in total content (per heart) of DNA (25%), RNA (22%), protein (31%), and myofibrillar protein (30%), as well as the activity of myosin ATPase (11–20%). On the contrary, the above parameters were significantly decreased in the skeletal muscle of hyperthyroid rats as was body weight. Termination of T3 treatment on day 25 led to resumption of rapid body growth which was not complete even by day 90. Cardiac growth decelerated in rehabilitating rats, however, resulting in final reduction of all growth parameters, except DNA content which was equal in 90-day control and rehabilitated animals. Muscle growth in terms of DNA and RNA content failed to respond to rehabilitation and continued to be lower than control animals while protein synthesis was relatively enhanced. It is concluded that neonatal hyperthyroidism markedly enhanced growth of the heart but not of skeletal muscle. This condition not only inhibited growth of the muscle during the T3 treatment period but also led to permanently reduced DNA content and other growth parameters even two months after discontinuation of treatment.