Specific insulin receptors in rat renal glomeruli.

The kidney is a major site of pathologic complications in diabetes mellitus including those affecting the basement membrane. Although several hypotheses of the etiology of renal diabetic microangiopathy involve effects of insulin on basement membrane metabolism, little is known about the localization or properties of insulin receptors in isolated renal glomeruli. We have incubated isolated glomeruli and tubules with 125I-insulin and examined the binding of the hormone to these purified renal subfractions. Glomeruli and tubules exhibited specific binding of 125I-insulin at 22 degrees C which increased with time and protein concentration. Nonspecific binding accounted for 20-35% of the total binding observed. At a concentration of 1 ng/ml 125I-insulin specific binding reached a plateau within 60-90 min at 22 degrees C. Unlabeled insulin competitively displaced 125I-insulin from its binding sites with 2.8 x 10(-9) M insulin inhibiting half of the initial binding to glomeruli while a 10-fold higher concentration of unlabeled insulin was required to achieve a similar inhibition of tubular binding. Insulin analogues were less potent than insulin itself in inhibiting the binding of 125I-insulin to both glomeruli and tubules in the order of relative potency: insulin greater than despentapeptide insulin greater than proinsulin indicating the specificity of the binding. Scatchard analysis of the competition binding data of 125I-insulin to glomeruli and tubules was consistent with two distinct binding sites and/or negative cooperativity between one class of receptor sites. The binding of 125I-insulin to the glomerular receptor(s) exhibited a higher affinity than that to the tubular receptor(s) which had a greater binding capacity. These two subunits of the nephron, therefore, contain insulin receptors with different binding properties which may serve different functions.