ABSTRACT
Preparations of kidney tubules were isolated from rat kidney cortex and were demonstrated to possess specific binding sites for insulin. The binding was time-and temperature-dependent and the label was displaced by bovine insulin, A1-B29 dodecoyl insulin, proinsulin and insulin A- and B-chains in proportion to their relative activity. Cell-associated degradation was studied by incubating tubules in the presence of fatty-acid-free albumin. The tubules showed high insulin-degrading activity, which was dependent on temperature, time and cell concentration. The number and affinity of insulin receptors on tubules isolated from kidneys taken from streptozotocin-diabetic rats was not significantly different from tubules isolated from untreated control or insulin-treated diabetic rats. Diabetes did not alter the kinetics of insulin degradation by the tubules. This lack of response by the tubules to changes in the concentration of circulating insulin supports the hypothesis that the kidneys do not play an active role in modulating the rate of insulin removal from the circulation.
