Laminin in glomerular basement membranes of aminonucleoside nephrotic rats. Increased proteinuria induced by antilaminin immunoglobulin G.

The amount and distribution of the glycoprotein laminin was investigated in the glomerular basement membranes (GBM) of rats made nephrotic by 10 daily subcutaneous injections of the aminonucleoside of puromycin (AMN). Affinity-purified, sheep antilaminin immunoglobulin G (S alpha L) was injected intravenously into AMN rats, and kidney-bound S alpha L was compared with normals. Photometric measurements of glomerular-bound S alpha L showed that intravenous injections of 1.5 mg of S alpha L saturated laminin in normal glomeruli. The same amount of glomerular S alpha L was present in 10-day AMN nephrotic rats after injection of a saturating dose. In nephrotic rats, approximately 4.4% of a dose of 0.1 to 0.9 mg of 125I-S alpha L bound in the kidneys as compared with approximately 3.8% in normals. By immunofluorescence microscopy, S alpha L bound in a linear pattern to the GBM in nephrotic and normal rats and remained similarly bound throughout all stages of nephrosis. Conjugates of S alpha L and horseradish peroxidase (S alpha L-HRP) injected into normal and 10-day nephrotic rats bound to all three layers of the GBM, to fibrillar structures within the laminae rarae, and to the plasma membranes of epithelial cells below the slit diaphragms. In nephrotic rats, S alpha L-HRP was also bound to the epithelial plasma membrane where it had detached from the GBM. Rats given S alpha L before the induction of AMN nephrosis (S alpha L-AMN) developed significantly greater proteinuria on day 10 than rats given AMN alone (372 versus 274 mg/24 hours, p less than 0.05) and on day 12 (603 versus 453 mg/24 hours, p less than 0.05). In addition, there was greater epithelial detachment from the GBM in S alpha L-AMN rats than simple AMN rats. We conclude that (a) large amounts of laminin are neither lost nor redistributed during AMN nephrosis, (b) laminin is present as fibrils within the GBM as well as on the epithelial plasma membrane adjoining the GBM, and (c) GBM-bound S alpha L promotes proteinuria during late stages of AMN nephrosis, possibly by enhancing epithelial detachment.