PROCESSING OF I-125 INSULIN BY POLARIZED CULTURED KIDNEY-CELLS

Renal clearance of insulin is achieved by glomerular filtration and by passage from the postglomerular peritubular circulation into the rena l interstitium, In the proximal tubule, filtered insulin binds to the apical membrane and is internalized and degraded, while insulin in the interstitium is taken up by receptor-mediated endocytosis and degrade d, To study these processes we have utilized cultured opossum kidney c ells. These cells have proximal-like features and process insulin in a manner consistent with that described in vivo. To study apical and ba solateral uptake and metabolism of insulin independently, cells were g rown on filters suspended in culture wells, Insulin was degraded to la rge insulin-size intermediates and low-molecular-weight products, This occurred whether the protein was internalized from the apical or baso lateral pole of the cell. Analysis of the intermediate products by rev erse-phase high-performance liquid chromatography revealed that produc ts formed after apical or basolateral internalization were similar. Si nce products were preferentially released from the side of uptake, it is likely that apically and basolaterally internalized insulin is degr aded in comparable organelles located in different regions of the cell , Most of the internalized insulin traversed the degradative pathway ; but some insulin followed a retroendocytic or minor transcytotic pathw ay, Degradation was inhibited by chloroquine, which also selectively i ncreased the release of internalized insulin from the apical pole irre spective of the side of uptake. Thus while the polar degradative proce sses appear to be similar in nature, the polar exocytotic processes ap pear to be different. (C) 1996 Academic Press, Inc.