Localization and distribution of anionic charges in the glomerular mesangium of normal and nephrotic rats

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Abstract
Sulfated glycosaminoglycans and sialoglycoproteins are thought to play a pivotal role in the glomerular capillary wall barrier to filtration since these anionic charged elements are important in the maintenance of capillary wall integrity and constitute a charge-selective filter. The development of proteinuria in puromycin aminonucleoside (PAN) nephrosis is associated with polyanion loss from the glomerular capillary wall structures. Since in PAN nephrosis the permeability of the mesangial area to plasma proteins and tracer substances has also been shown to be increased, the purpose of this study was to analyse the localization and distribution of anionic charges in the glomerular mesangium in this experimental model. Glycosaminoglycans were labeled by perfusion of the kidneys with ruthenium red solution (RR). Electron microscopic examination revealed the presence of distinct small RR granules (“anionic sites”) in the mesangial intercellular matrix substance and in the laminae rarae of the glomerular basement membrane (GBM). The center-to-center spacing of the granules was measured and a frequency distribution of intervals in different interval classes was constructed. In normal glomeruli the anionic sites in the mesangial matrix showed a distribution pattern identical to the GBM with a maximal interval incidence at the 31–40 nm class. In nephrotic rats anionic site distributions in matrix and GBM did not change significantly. Sialoglycoproteins were labeled with colloidal iron (CI). In PAN nephrosis a decrease of CI binding was observed at the epithelial-basement membrane junction of the glomerular capillary wall. However, CI labeling of the mesangial matrix and mesangial cell membranes did not differ from that of normal glomeruli. In conclusion, the increased mesangial permeability in PAN nephrosis appears not to be associated with significant loss or altered configuration of mesangial anionic charged moieties.