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The anionic macromolecules at the glomerular endothelial cell surface are visualized only when stained with cationic stains. We investigated the arrangement and composition of this anionic matrix at the luminal surface. Rat kidneys were perfused with anionic ferritin (pI 4.5), ferritin (pI 7.4), or cationized ferritin (CF, pI 8.3). Anionic ferritin (pI 4.5) did not bind to the capillary wall, ferritin (pI 7.4) bound discontinuously only to the laminae rarae of the basement membrane, but cationized ferritin (CF, pI 8.3) bound as a thick continuous layer to the cell plasmalemma and bound to the anionic matrix in the fenestral spaces. These observations show that an anionic matrix lines the entire capillary lumen surface, fills the fenestrae, and is interposed between the blood and the basement membrane at the fenestrae. The anionic constituents at the capillary luminal surface were identified by in vivo digestion with specific enzymes. Absence of CF binding following digestion with specific enzymes was taken to indicate the presence of the particular glycoprotein known to be susceptible to the enzyme used. Neuraminidase digestion revealed that anionic sites over the surface plasmalemma are mainly from sialoproteins. In contrast, the matrix in fenestral channels contains heparan sulfate, hyaluronic acid, and sialoproteins. Papain digestion showed no glycolipids at the luminal surface. The functions of this continuous anionic layer located at the luminal surface of glomerular capillaries have not yet been established.

Previous micropuncture experiments have shown that gentamicin reduces the glomerular Kf even at low doses. Light and transmission electron microscopy have failed to provide evidence of structural alterations to explain the observed decreases in Kf. To elucidate and effects of gentamicin on glomerular ultrastructure, we used both scanning and transmission electron microscopy to study the glomeruli of rats given gentamicin at 4 and 40 mg/kg/day for 10 days, as well as 20 mg/kg/day for 4 weeks. Observations were also made during recovery from gentamicin-induced renal injury. Scanning and transmission electron microscopy revealed that gentamicin induced striking reductions in endothelial fenestral diameter, density, and area. These changes were not appreciated by light microscopy. By quantitative measurements, the reductions in endothelial fenestral area were found to be parallel to the previously observed reduction in Kf. During recovery, the morphological alterations improved pari passu with functional recovery. Contrary to observations in other models of acute renal failure, gentamicin-induced glomerular injury was unique in that the visceral epithelial surface was not affected. The mechanisms by which gentamicin induces alterations in glomerular ultrastructure remain to be defined.

In uranyl nitrate (UN)-induced acute renal failure (ARF) glomerular ultrafiltration coefficient (K(f)) decreases because of unknown reasons. Since transport of water across the glomerular capillary wall occurs predominantly extracellularly through the endothelial fenestrae (EF), a reduction in the diameter and/or the density of EF can reduce the extracellular filtration area and the glomerular K(f). To examine this possibility, ARF was induced in rats by intravenous administration of UN in low (15 mg/kg) and high doses (25 mg/kg). Fenestral density ( x+/-SEM) per 5 cm(2) from the scanning electron micrographs (x30,000) was 107+/-10, 103+/-9, and 101+/-11 at 2, 7, and 17 h after the intravenous administration of bicarbonate saline to the control rats. In the low-dose UN group the EF density was 91+/-2, 52+/-8, and 45+/-11 at 2, 7, and 17 h after the injection, whereas for the high-dose group at corresponding time intervals the EF density was 95+/-3, 54+/-9, and 44+/-10. Fenestral diameters, in Angstrom units ( x+/-SEM), were 751+/-53, 765+/-43, and 764+/-37 at 2, 7, and 17 h after the injection of bicarbonate saline to control rats. At corresponding intervals after the administration of UN, the fenestral diameters were 501+/-61, 472+/-28, and 438+/-98 for the low-dose group and 525+/-43, 470+/-39, and 440+/-56 for the high-dose group. 2, 7, and 17 h after the injection of UN, fenestral area of the low-dose group decreased to 52.1, 30.1, and 24.6% of the controls, whereas in the high-dose group, the fenestral area declined to 54.3, 30.2, and 23.6% of the controls. Administration of UN (15 mg/kg) to sodium-loaded rats did not alter renal function or endothelial cell morphology. It is suggested that in UN-induced ARF the morphological alterations in endothelial cells reduce the K(f) of glomerular capillaries by reducing the filtration area.