Insulin-stimulated phosphorylation and insulin binding in canine renal basolateral membranes

To characterize the interaction of insulin with the renal proximal tubular cell, we measured insulin-stimulated phosphorylation in basolateral membranes and brush border membranes isolated from canine renal cortex. Insulin stimulated the specific phosphorylation of a 92,000 Mr protein band demonstrable on autoradiograms of sodium dodecyl sulfate-polyacrylamide gels of basolateral membranes. Dephosphorylation of the 92,000 Mr band occurred over time. Insulin-stimulated phosphorylation was concentration dependent, being clearly detectable at 10(-9) M insulin and maximal at 10(-6) M. The phosphorylated 92,000 Mr protein band from detergent-solubilized basolateral membranes was immunoprecipitated using serum from a patient with anti-receptor antibodies. No insulin-stimulated phosphorylation was detected in brush border membranes. Binding of insulin to membranes was highly specific for native hormone and was severalfold greater in basolateral membranes than in brush border membranes. These observations are consistent with the asymmetrical distribution of insulin-stimulated protein kinase as well as specific insulin binding sites in the proximal tubular cell. The data suggest that insulin exerts physiological effects on the cell through binding to specific basolateral membrane receptors and phosphorylation of those receptors.