No abstract

Administration of growth hormone to animals has been found to reduce fat stores (1, 2), to increase liver fat (3,4) and to cause ketosis (5) and a depression of respiratory quotient (6, 7). The increase in mobilization and metabolism of fat implied by these findings has been thought to contribute either in an auxiliary or in an essential way to the anabolic stimulus induced by growth hormone.

The effect of thyroid state on the activity of myosin adenosinetriphosphatase (ATPase) was examined in the rat and the rabbit. Cardiac myosin from thyroxinetreated rabbits showed enzymatic properties characterized by high Ca 2+ -activated ATPase activity, low activating energy, lower rate of inactivation at alkaline pH, and no activation by Af-ethylmaleimide compared with the same properties in the normal rabbit; thyroidectomy did not affect the enzymatic properties of rabbit cardiac myosin. These findings suggest a difference in the myosin molecule at or near the active site, involving some sulfhydryl groups, between hyperthyroid and euthyroid rabbits. However, rat cardiac myosin showed a pattern of activity in the euthyroid state similar to that of the hyperthyroid rabbit and changed to the euthyroid type after thyroidectomy. These changes were specific for cardiac myosin, since no change was observed in skeletal myosin. It is unlikely that there are major differences in the myosin molecule associated with the two types of activity, since similar proportion and amino acid composition of the subunits of cardiac myosin were observed in the different thyroid states. Thus, we concluded that the administration of thyroxine to the rabbit stimulates the synthesis of new cardiac myosin with altered enzymatic properties and that synthesis of this type of cardiac myosin is maintained by the normal level of thyroid hormone in the rat. KEY 208myosins from different species of animals has been demonstrated (8), and the present study shows that the different properties of cardiac myosin can be converted by thyroid hormone (9). The experiments in the present study attempted to demonstrate the effect of thyroid hormone on the enzymatic properties of different types of cardiac myosin and to determine the mechanism of thyroxine-induced activation of myosin ATPase. Methods MODIFICATION OF THYROID ACTIVITY IN ANIMALSMale rats (200-250g) and rabbits (2.0-2.5 kg) were studied. The hyperthyroid animals were given daily injections of /-thyroxine (300 /xg/kg, sc) for 21 days. Normal control animals (euthyroid) were injected daily for 21 days with an equal volume of 0.9% saline solution. Weight loss was observed in all animals given thyroxine. After 21 days of thyroxine treatment, the body weights of the treated rats and rabbits decreased 35% and 42%, respectively, in contrast to a weight gain in normal rats and rabbits of 40% and 29%, respectively. About 25% of the thyroxine-treated animals died within 3 weeks. Hypothyroidism was produced by total resection of the thyroid glands. The animals were killed 21 days after surgery. The increase in body weight was less than 10% in the hypothyroid rats and rabbits. Serum thyroxine and cholesterol concentrations in normal control rabbits were approximately 1-2 \i% and 40-

No abstract