In vivo specific uptake of labeled insulin by liver,adipose tissue,pituitary, and adrenals in the turtle Chrysemys dorbigni

Insulin labeled with ¹²⁵I was injected into turtles (Chrysemys dorbigni) to study its specific uptake by tissues. The maximum specific uptake of radioactivity by turtle tissues was obtained 1 hr after administration of ¹²⁵I]iodoinsulin. Besides liver and adipose tissue, specific uptake of labeled insulin was detected in some endocrine glands, such as pituitary and adrenals. Both glands were as active in concentrating labeled insulin as liver and adipose tissue. A significant reduction of the uptake was observed when unlabeled insulin was injected together with the labeled hormone. This reduction was dose dependent, and the concentration of unlabeled insulin that prevented 50% of the tissue uptake of ¹²⁵I]iodoinsulin was of 1 to 10 μg/kg body weight. These doses were able to induce blood glucose decrease in the turtle. Prolactin, growth hormone, or glucagon were unable to displace labeled insulin uptake. The major proportion of the radioactive material extracted from liver and pituitary 1 hr after ¹²⁵I]iodoinsulin injection into turtle coeluted with ¹²⁵I]iodoinsulin in Sephadex G-50 column. The presence of radioactive degradation products are consistent with the intracellular receptor mediated degradation hypothesis. These findings suggest the presence of specific insulin binding sites in liver, adipose tissue, pituitary, and adrenal glands from turtles.