Action of insulin on thyroid protein synthesis in dogs

The study was performed on dog thyroid slices after in vivo administration of monocomponent insulin (carotid perfusion of a lobe: 25 mIU/kg/min for 15 min, and 2nd lobe perfused as control: saline) and after in vitro hormone addition (100 mIU/ml of glucose-free incubation medium). The 3H-leucine incorporation in thyroid proteins (dpm/mg) was determined following incubation of slices for 1 h and 2 h. Insulin given in situ significantly increased the mean incorporation into homogenate and cytosol proteins: about 130% after 1 h of incubation, for an identical hypoglycaemia in both lobes. When insulin was added to the medium, a similar response was observed after 1 h of incubation, the effect being less pronounced after 2 h but still significant. The sucrose gradient analysis of the soluble fraction showed that the stimulation affected all thyroid proteins. Polyribosomes isolated from in vivo insulin-perfused thyroids exhibited an increased activity when tested in cell-free system. These data suggest that insulin stimulated the thyroid protein synthesis in dogs through a direct action on the thyroid cell.