Reabsorption of native and glycated albumin by renal proximal tubular epithelial cells

The proximal tubule epithelial handling of native and glycated albumin was evaluated by quantitative immunocytochemistry. Native bovine serum albumin (BSA) and its glycated form (gBSA), tagged to different haptens, were simultaneously injected in anesthetized mice and maintained in circulation for 10 or 60 min. Both albumins were localized within the capillary lumen, in glomerular and peritubular basement membranes, the urinary space, and cellular compartments of the proximal tubular epithelial cells. In these cells, both forms of albumin were concomitantly found within the same endocytic-lysosomal system. Morphometric evaluations have indicated higher proportions of gBSA in the urinary space, reflecting probably a significant glomerular filtration of this form of albumin combined to a lesser reabsorptive clearance. Indeed, higher proportions of native BSA were found in the endocytic compartment of the tubular epithelial cells, suggesting its preferential reabsorption. The present study thus supports a preferential glomerular filtration of gBSA with a facilitated filtration of native BSA in the presence of the glycated one. It also demonstrates the tubular reabsorption of BSA and gBSA through a common endocytic pathway, in which the native BA is preferentially reabsorbed with respect to its glycated form.