In vivo specific uptake of labeled insulin by turtle (Chrysemys dorbigni) thyroid gland

Insulin labeled with 125I was injected into turtles to study its specific uptake by the thyroid gland. The gel filtration behavior of labeled material in blood and thyroid gland was determined in order to ascertain if the uptake is specific. Most animals were pretreated with KI to saturate the gland with iodide. Maximum specific uptake of radioactivity by the thyroid gland was only detected in animals pretreated with KI. A significant dose-related reduction (ED50 = 0.5 micrograms/kg) was observed when unlabeled insulin was administered simultaneously with 125I-insulin. Prolactin, glucagon and growth hormone (2 mg/kg) did not affect 125I-insulin uptake. Most of the radioactive material extracted from the turtle thyroid 15 min after 125I-insulin injection coeluted with 125I-insulin on Sephadex G-50. This peak decreased as a function of time after 125I-insulin administration. Similar elution patterns were found for thyroid extracts from turtles previously treated with KI. The labeled hormone in the gland was rapidly degraded or processed to both higher and lower molecular weight compounds. Prior administration of KI suppressed the former, whereas when unlabeled insulin was injected simultaneously with 125I-insulin the amount of degradation products was reduced. The demonstration of radioactive degradation products is consistent with the intracellular receptor-mediated degradation hypothesis. These findings indicate the presence of specific insulin-binding sites in the thyroid gland.