Role of the Kidney in Insulin Metabolism and Excretion

Abstract

Removal of these organs from animals prolongs the half-life of injected labeled or unlabeled insulin. Similar findings, reversible by transplantation, are noted in patients with severe renal disease. After injection of insulin-I-131 into a peripheral vein, the concentration of radioactivity in the renal cortex of rats is 9 times greater than any other tissue and 21% of the administered dose is present in the kidneys at 15 min. In contrast to other organs, an increase in the injected dose results in a greater proportion being localized to the kidneys. The concentration of insulin in renal venous blood is 30 to 40% lower than the arterial level, and the quantity of insulin removed by the kidneys over 24 hr. is 6 to 8 U. The renal clearance of insulin in man is approximately 200 ml/min. There is both direct and indirect evidence that insulin is filtered at the glomerulus and almost completely reabsorbed and degraded by cells lining the proximal convoluted tubules. This mechanism accounts for 50 to 60% of the renal uptake of insulin, the remaining 40 to 50% being removed from the postglomerular peritubular capillaries. The amount of insulin excreted in the urine is less than 2% of the filtered load and the urinary clearance is 0.1-0.5 ml/min. This clearance is constant over a wide range of serum levels and is thus a useful reflection of the mean serum level over a period of time. These observations explain the fall in insulin requirements of diabetic patients who develop renal failure. Furthermore, the severe hypoglycemia which occasionally occurs in elderly subjects with uremia following the administration of oral sulfonylureas may be partly due to this mechanism.