Glomerular sieving of high molecular weight proteins in proteinuric rats

Abstract

To characterize the permeability of the glomerular capillary wall to high molecular weight proteins in normal and proteinuric rats, we determined the glomerular sieving coefficients (GSC) of radioodinated marker proteins of known size and charge by means of a paired label, tissue accumulation method previously validated in this laboratory. In one group of rats (Series A) the GSCs of 125I-anionic IgG (aIgG - molecular weight [mol wt] 150,000, pI 4.9) and 131I-neutral IgG (nIgG - pI 7.4 to 7.6) were measured simultaneously. In Series B, the GSC of a second anionic marker, 131I-human ceruloplasmin (Crp - mol wt 137,000, pI 4.9) was compared to that of 125I-nIgG. As in the previous report, the labeled proteins were not degraded or deiodinated during the 20 minute clearance period for GSC determination. Within Series A and B, three subgroups of rats were studied: control saline-infused rats, rats made acutely proteinuric by infusion of the polycation hexadimethrine (HDM), and rats with chronic doxorubicin (Adriamycin® - Adria) nephrosis. In the control rats, GSCs for the anionic markers aIgG (Series A) or Crp (Series B) were significantly greater than that of nIgG (both series). These large proteins crossed the filtration barrier by a different pathway from that available to smaller neutral molecules the size of albumin, which in our previous study had a much higher GSC than a native, anionic albumin marker. In a third group of control rats only (Series C), the GSCs of native anionic bovine albumin (BSA) and nIgG were compared directly. The GSC of BSA (0.0029) was only slightly larger than the GSC of nIgG (0.0025), indicating that most of the native albumin crosses the glomerular capillary wall via a nonselective pathway similar to that available to nIgG. The results in the control groups are compatible with recently-described heteroporous models of glomerular size selectivity. In the HDM proteinuric rats of both series, the filtration of all three markers was significantly increased, although the GSCs of the anionic markers continued to exceed that of nIgG. The finding of increased nIgG filtration in HDM rats confirms our previous observation that HDM, although it binds to and neutralizes anionic sites in glomerular basement membrane, induces a structural change which leads to altered pore size distribution. Adria rats also had increased GSCs of all markers, although the increase was not as great as in HDM rats, and the GSCs of the anionic and neutral markers became equal. The finding of increased nIgG filtration in the Adria rats demonstrates that altered size dependence of permselectivity, shown by others using polysaccharide probes, can be detected with protein markers as well, if they are of large enough size.