CHARACTERIZATION OF INSULIN-RECEPTORS IN THE BOVINE ADRENAL-CORTEX AND MEDULLA

In order to identify insulin receptors in the bovine adrenal cortex and medulla, we have studied I-125-porcine insulin binding to the membrane preparations from the bovine adrenal cortex and medulla. I-125-porcine insulin bound not only to the bovine adrenal cortex but to the medulla in time-, temperature-, and pH-dependent manners. The maximum levels of I-125-porcine insulin binding in the two tissues were observed at 4-degrees-C for 24 h of incubation, and its optimum pH ranged from 7.6 to 8.0. Under these conditions, at tracer concentration of porcine insulin (200 pg/ml), 10.4% and 6.6% of I-125-porcine insulin added to each reaction tube bound specifically to 10(5) X g-pellet fractions (microsomal membrane) from the cortical tissue (0. 3 mg of protein) and from the medullary tissue (2 mg of protein), respectively. I-125-porcine insulin binding was observed predominantly in the microsomal membrane from the bovine adrenal cortex, and in a 15,000 x g- pellet fraction(synaptosomal membrane) from the bovine adrenal medulla. Scatchard analysis of binding data yielded curvilinear plots in each tissue. Analysis of curvilinear plots based on two sites model revealed similar affinity constant between the cortex and medulla. Receptor concentration of the cortex was several times higher than that of the medulla. In the two bovine adrenal tissues, human proinsulin and insulin-like growth factor I (IGF-I) had about 1/100 potency compared to porcine insulin in displacing I-125-porcine insulin binding. Porcine glucagon added with concentration up to 10(-6)M did not inhibit I-125-porcine insulin binding to both the cortex and the medulla. These kinetic properties and specificity of insulin receptors in the two bovine adrenal tissues were similar to those of insulin receptors previously reported in mammalian target tissues.