Abstract
In order to identify insulin receptors in the bovine adrenal cortex and medulla, we have studied 125I-porcine insulin binding to the membrane preparations from the bovine adrenal cortex and medulla. 125I-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 125I-porcine insulin binding in the two tissues were observed at 4°C for 24h of incubation, and its optimum pH ranged from 7.6 to 8.0. Under these conditions, at tracer concentration of porcine insulin (200pg/ml), 10.4% and 6.6% of 125I-porcine insulin added to each reaction tube bound specifically to 105×g-pellet fractions (microsomal membrane) from the cortical tissue (0.3mg of protein) and from the medullary tissue (2mg of protein), respectively. 125I-porcine insulin binding was observed predominantly in the microsomal membrane from the bovine adrenal cortex, and in a 15, 000×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 125I-porcine insulin binding. Porcine glucagon added with concentration up to 10-6M did not inhibit 125I-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.
