Glomerular sieving of anionic and neutral bovine albumins in proteinuric rats

To characterize the defect in glomerular permeability leading to albuminuria in rats made nephrotic acutely by infusion of hexadimethrine (HDM) or chronically by administration of Adriamycin (doxorubicin) (Adria), we developed and validated a tissue accumulation method for simultaneous determination of the glomerular sieving coefficients (GSC) of anionic 131I-labeled bovine albumin (BSA-pI 4.9) and 125I-labeled charge-modified neutral BSA (nBSA-pI 7.5 to 8.0). Total filtered marker was calculated by adding marker excreted in the urine to that filtered but reabsorbed by the tubules. The latter was determined by subtracting interstitial marker present in the left kidney, rendered non-filtering by ureteral ligation during mannitol diuresis, from the total marker accumulating within the right, filtering kidney. Experiments showed that markers circulated and were excreted intact and were neither degraded nor deiodinated during the period of the clearance studies. In control animals the GSC of nBSA (0.026 +/- 0.004) greatly exceeded that of BSA (0.0006 +/- 0.0002), demonstrating the normal charge dependence of permeability. Both proteinuric groups had marked increases in the GSC of BSA (HDM: 0.021 +/- 0.005; Adria: 0.025 +/- 0.004), which correlated with appearance of rat albumin in their urine. HDM rats also had a twofold increase in the GSC of nBSA (0.049 +/- 0.005), indicating alteration of the size dependence of permeability. The absolute increase of GSC of BSA and nBSA was similar, suggesting that albuminuria resulted from appearance of new "pores" in the glomerular filter that were not charge selective for proteins of the size of albumin. Thus, infusion of HDM, which binds to and neutralizes GBM anions, appears to produce albuminuria by inducing a structural change in the glomerular filter. Conversely, Adria rats had no significant increase in the GSC of nBSA (0.031 +/- 0.005), indicating no significant change in the size dependence of permeability for proteins of the size of albumin. In these animals, the GSC of the anionic BSA approached that of the neutral nBSA, indicating that Adriamycin induces albuminuria by markedly reducing the normal charge dependence of permeability.