Fate of [125I]insulin removed from the peritubular circulation of isolated perfused rat kidney

Differing effects of antiinsulin serum and antiinsulin receptor serum on 123I-insulin metabolism in rats

Anesthetized rats were treated with saline, antiinsulin receptor serum, or antiinsulin serum, and the biodistribution of high pressure liquid chromatography-purified 123I-Tyr A14-insulin was studied by scintillation scanning. Time activity curves over organs of interest were calibrated by sacrificing the rats at the end of the experiment and directly determining the radioactivity in the blood, liver, and kidneys. Saline-treated rats exhibited normal insulin biodistribution. The highest concentration of 123I-insulin was found in the liver, and reached 30% of total injected dose between 3 and 5 min after injection. After this peak, activity rapidly decreased with a t1/2 of 6 min. Activity of 123I-insulin in kidney showed a more gradual rise and fall and was approximately 15% of injected dose at its maximum. In rats treated with antiinsulin antiserum, insulin biodistribution was markedly altered. Peak liver activity increased with increasing antibody concentration with up to 90% of injected dose appearing in the liver. In addition, there was no clearance of the liver 123I-insulin over 30 min. Autoradiographic studies demonstrated that in contrast to the normal rats in which radioactivity was associated with hepatocytes, in rats passively immunized with anti-insulin serum, 125I-insulin was associated primarily with the Kuppfer cells. In contrast, antibodies to the insulin receptor markedly inhibited 123I-insulin uptake by the liver. Kidney activity increased, reflecting the amount of free 123I-insulin that reached this organ. This is similar to the pattern observed when insulin receptors are saturated with a high concentration of unlabeled insulin. Thus, both insulin antibodies and anti-receptor antibodies alter the distribution of insulin, but with very different patterns. The use of 123I-insulin and scintillation scanning allows one to study specific alterations in insulin distribution in animal models of insulin-resistant states, and should also be useful in human disease states.

The renal metabolism of insulin

The kidney plays a pivotal role in the clearance and degradation of circulating insulin and is also an important site of insulin action. The kidney clears insulin via two distinct routes. The first route entails glomerular filtration and subsequent luminal reabsorption of insulin by proximal tubular cells by means of endocytosis. The second involves diffusion of insulin from peritubular capillaries and subsequent binding of insulin to the contraluminal membranes of tubular cells, especially those lining the distal half of the nephron. Insulin delivered to the latter sites stimulates several important processes, including reabsorption of sodium, phosphate, and glucose. In contrast, insulin delivered to proximal tubular cells is degraded to oligopeptides and amino-acids by one of two poorly delineated enzymatic pathways. One pathway probably involves the sequential action of insulin protease and either GIT or non-specific proteases; the other probably involves the sequential action of GIT and lysosomal proteases. The products of insulin degradation are reabsorbed into the peritubular capillaries, apparently via simple diffusion. Impairment of the renal clearance of insulin prolongs the half-life of circulating insulin by a number of mechanisms and often results in a decrease in the insulin requirement of diabetic patients. Much needs to be learned about these metabolic events at the subcellular level and how they are affected by disease states. Owing to the heterogeneity of cell types within the kidney and to their anatomical and functional polarity, investigation of these areas will be challenging indeed.