Intrarenal Handling of Recombinant Human Interleukin-1α in Rats: Mechanism for Proximal Tubular Protein Reabsorption

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Abstract
The intrarenal distribution of recombinant human interleukin (IL)-1α was studied in Sprague-Dawley male rats by immunohistochemical staining. The effects of the concurrent administration of various proteins or synthetic polypeptides on the urinary excretion of IL-1α were also studied to clarify the mechanism(s) for the reabsorption of IL-1α in the renal tubules. Microscopic immunohistochemistry showed that IL-1α distributed to early proximal convoluted tubules but not to glomeruli, Henle's loops, distal tubules, or collecting ducts. Electron microscopic immunohistochemistry showed that IL-1α was taken up into the endocytic vesicle located close to the apical membrane of the proximal tubular epithelial cells, then accumulated in lysosomes. Urinary excretion of intravenous IL-1α at 500 μg/kg was extremely low, accounting for only 0.014% of the dose administered. The coadministration of intravenous human serum albumin did not affect the urinary excretion of IL-1α, whereas trypsinogen, myoglobin, and trypsin inhibitor dose-dependently produced an increase in the excretion of IL-1α, the potency of which was greatest in that order. Poly-L-lysine, but not poly-L-glutamic acid dose-dependently increased the urinary excretion of IL-1α. These results indicate that most of the glomerular filtrated IL-1α could be easily reabsorbed into the proximal tubular cells via endocytosis, and the reabsorption was inhibited by coadministered low molecular weight proteins, particularly basic proteins. This result suggests that scavengers with a negative charge and broad binding ability for glomerular filtered proteins exist on the surface of the apical membrane of proximal tubular cells and play an important role in the reabsorption of filtered proteins.