THYROXINE AND INSULIN INTERACTION
Rate of body weight gain of both the young chick and the young rat was markedly inhibited following ingestion of amounts of thyroxine which were greatly in excess of physiological requirements. The rate of body weight gain of the young rat was also markedly inhibited in alloxan diabetes. The administration of insulin partly or largely prevented the observed effect on body weight in both conditions. The administration of thyroxine caused a reduction of rat skeletal muscle mass which was partially prevented when insulin was given concurrently with thyroxine.Reduced body weight gain and muscle mass in the rat were accompanied by marked increases in the muscle DNA and nuclei concentration. Nuclear diameter and the amount of DNA per nucleus remained unchanged. RNA concentration of the muscle was not reduced. The beneficial effect of insulin administration on body weight and muscle mass was accompanied by a preventive effect against changes in muscle DNA and nuclei concentration. However, since the DNA content of muscle bundles remained relatively unchanged and since muscle bundle weights changed proportionately with body weight it would appear that DNA content does not reflect change in muscle mass.The effectiveness of insulin in the thyroxine-treated animal appears to be due to an improved utilization of energy. A hypoglycemic action of thyroxine implies an antagonism between the two hormones in their mechanism of action. The relationship between the two hormones is not direct since the administration of insulin does not affect the increased plasma protein-bound iodine levels resulting from thyroxine treatment.