Direct and indirect tests of pore size and charge selectivity in nephrotic syndrome.

We studied direct and indirect methods of measuring membrane charge by detecting fixed anionic sites with polyethylenimine (PEI) on the glomerular basement membrane (GBM) and Alcian blue on red blood cell (RBC) membrane (ABRBC), respectively, in 40 children with nephrotic syndrome (NS). Size selectivity of the GBM was measured indirectly by fine analysis of urinary proteins with sodium dodecyl sulfate-polyacrylamide gel electrophoresis in 22 of these children. Correlation between ABRBC and PEI was strongest (r = 0.79; p = 0.0037) in 11 children with steroid-responsive NS (SRNS), moderate (r = 0.31) in 10 children with focal glomerulosclerosis (FGS), and absent in 14 children with hepatitis B antigen membranous nephropathy (MGN) and 5 with mesangioproliferative glomerulonephritis (MPGN). ABRBC and PEI were reduced in the group as a whole as compared with their controls (ABRBC: 44.53 +/- 9.81 vs 71.54 +/- 12.14, p < 0.05; PEI: 16.31 +/- 4.34 vs 33.3 +/- 1.09, p < 0.005). This reduction was greater in SRNS (26.35 +/- 7.15 p = 0.004) but was also detected in the remainder of the group taken together (52.31 +/- 26.07, p < 0.001). Excretion of glomerular proteins was restricted by size (< or = 80 kd) in SRNS but unrestricted (< or = 80 kd plus > 80 kd) in FGS, MGN, and MPGN. The main cause of proteinuria is likely to be depletion of negative charge on the GBM in SRNS, and distortion of capillary pore size in MGN and MPGN, with probable overlap of these mechanisms in each disease, especially in FGS. Basement membrane injury appears widespread in SRNS but confined to the kidney in MGN and MPGN.