Alterations of glomerular basement membrane (GBM) anionic (charge sites, CSs) in the development of proteinuria in a model of idiopathic nephrotic syndrome in man (puromycin aminonucleoside nephrotic syndrome [PAN] in the rat) were assessed quantitatively and sequentially early after disease induction. GBM CSs (known to consist mainly of heparan sulfate-rich proteoglycans) were stained in vivo and, in a separate group of animals by an in vitro method, with the cationic marker polyethyleneimine (PEI) studied by electron microscopic examination. Four hours after administration of PAN, there was a significant decrease in GBM lamina rara externa CSs: 18 +/- 0.7 versus 22.0 +/- 2.2 per 1000 nm GBM in controls by PEI injection and 17.2 +/- 2.7 versus 21.1 +/- 1.6 per 1000 nm GBM in controls by PEI in vitro staining. This CS alteration coincided with changes in glomerular epithelial cell morphologic characteristics (increased cytoplasmic organelles and rough endoplasmic reticulum) and preceded the detection of foot process broadening (at 24 hours) and increased urinary albuminuria (suggested at 12-24 hours, statistically significant at 36-48 hours). These results suggest that GBM CS-heparan sulfate proteoglycan alterations consisting of either decreased number and/or less anionic charge occur early in PAN and support a role for glomerular epithelial cell maintenance of GBM CS for normal glomerular function.