Effect of experimental diabetes on insulin binding by renal basolateral membranes

Removal of insulin from the peritubular vessels involves binding of insulin to specific receptors in the basolateral membranes (BLM); this is followed by phosphorylation of the receptor which may mediate the actions of the hormone. In most tissues receptor number is regulated by plasma insulin levels and is increased in insulinopenic diabetics. To determine whether cortical BLM insulin receptors are similarly regulated, we studied insulin binding to receptors in BLM from normal control rats and rats with streptozotocin diabetes of varying severity. Specific binding of insulin did not differ between control and modestly insulinopenic diabetics but was increased significantly in the severely insulinopenic diabetics. Insulin treatment returned binding to normal. Scatchard analysis suggested an increase in the binding capacity of the severe diabetic BLM rather than an increase in affinity for insulin. This latter was confirmed by competitive experiments in which similar displacement curves were obtained with control and diabetic membranes. Insulin removed by glomerular filtration binds to specific receptors in the luminal membranes but unlike BLM receptors, phosphorylation of these luminal receptors has not been observed. To determine whether luminal and BLM receptors differ structurally, binding sites in both membranes were affinity labelled with 125I-insulin and the cross linking agent, disuccinimidyl suberate, and subjected to SDS-polyacrylamide gel electrophoresis in the presence of a reducing agent. Autoradiograms revealed that the major specifically labelled subunit in both membranes is a 135,000 Mr species which is more abundant in the BLM. We conclude that insulin receptors in cortical BLM respond to severe insulinopenic diabetes as do receptors in most other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)