Glomerular size selectivity during protein overload in the rat.

Differential glomerular permeability to macromolecules as a function of their size (size permselectivity) is altered in experimental models of proteinuric renal disease. Size permselectivity during protein overload proteinuria was examined using urinary clearances of neutral dextrans in anesthetized Wistar-Furth rats. The animals were studied 3-4 h after the last of six twice-a-day intraperitoneal injections of either bovine serum albumin (BSA) or ovalbumin (OA) or of vehicle alone (controls). Glomerular filtration rates did not differ significantly among the three groups. OA-treated (n = 4) and control (n = 5) rats had virtually identical fractional dextran clearances over almost the entire molecular size range from 18 to 58 A Stokes-Einstein radius. In contrast, BSA-treated rats (n = 5) had elevated fractional clearances for medium-sized dextrans with the increases reaching statistical significance at radii of 40 A (+22.7% vs. control) and 44 A (+20.4%). Fractional clearances for BSA-treated rats returned to control values for larger dextrans. These findings demonstrate a significant size permselectivity defect in BSA overload, although not in OA overload.