Insulin regulates carbohydrate metabolism, and water, sodium, potassium, and phosphate reabsorption in the kidney by binding to specific receptors. Insulin receptors have been identified in the kidney using membrane preparations obtained from both glomeruli and tubules. In this study, an autoradiographic technique was used to characterize insulin receptors in the rat kidney. Frozen tissue sections were preincubated to remove endogenously bound insulin, incubated in a buffer containing 200 mu M I-125-Tyr-insulin, washed, and dried before exposure on Ultrofilm. Binding density was assessed by computerized microdensitometry. In the cortex, binding density was comparable in glomeruli and tubules. In the medulla, bound radioligand was found primarily in longitudinal structures traversing the outer portion, presumably corresponding to vascular bundles, and in the inner portion. Scatchard analysis of competition binding data resulted in curvilinear profiles, indicating either two classes of receptors with different affinity or the presence of a single class of receptors with a negative cooperative hormone-receptor interaction. Data analyzed for a two-site model showed one receptor site with Kd of 0.39+/-0.14 nmol/l and Bmax of 3.5+/-1.0 x 10(10) receptors/mm(3) and another site with Kd of 0.30+/-1.1 pmol/l and a Bmax of 3.2 x 10(13) receptors/mm(3). Thus, in situ autoradiography can be used to determine distribution and binding characteristics of insulin receptors in rat kidney and might be employed in receptor studies on rat models of human disease.
