The extent to which the glomerular basement membrane (GBM) contributes to the charge selectivity of the glomerular capillary wall has been controversial. To reexamine this issue, the size and charge selectivity of filters made from isolated rat GBM were assessed, using polydisperse Ficoll and Ficoll sulfate as test macromolecules. Ficoll sulfate, a novel tracer with spherical shape synthesized for this purpose, exhibited little or no binding to serum albumin, thereby avoiding a major difficulty that has been reported with dextran sulfate. The sieving coefficients of Ficoll sulfate were not different from those of Ficoll at physiological ionic strength, although the values for Ficoll sulfate were depressed at low ionic strength. These results confirm that the GBM possesses fixed negative charges but suggest that its charge density is insufficient to confer significant charge selectivity under physiological conditions, where electrostatic interactions are relatively well screened. The sieving coefficients of Ficoll sulfate and Ficoll were elevated significantly and by similar amounts when bovine serum albumin (BSA) was present in the retentate at 4 g/dl. This could be explained as the combined effect of two nonspecific physical factors, namely, the reduction in filtration velocity due to the osmotic pressure of BSA and the effect on macromolecular partitioning of repulsive solute-solute interactions. The view that BSA does not affect the intrinsic properties of the GBM is supported also by the absence of an effect on the hydraulic permeability of isolated GBM. The sieving coefficient of BSA was roughly half that of Ficoll or Ficoll sulfate of similar Stokes-Einstein radius. Given the finding of negligible charge selectivity, this difference may be attributed to the nonspherical shape of albumin. The results suggest that, to the extent that isolated GBM is similar to GBM in vivo, the charge selectivity of the glomerular capillary wall must be due to the endothelial and/or epithelial cell layers.

The glomerular basement membrane (GBM) is an integral structural component of the glomerular filter, but its contribution to the hydraulic and macromolecular permeability properties of the glomerulus has been the subject of much controversy. We have modified previously reported methods to develop a technique with which to study filtration properties of microgram quantities of isolated GBM in vitro at physiological pressures. Rat glomeruli were sieved, and cells were removed with N-laurylsarcosine and DNase. GBM (150 micrograms; greater than 95% pure) were added to a mini-ultrafiltration cell and consolidated under pressure to form a continuous filter at the base of the cell. Water flux was identical to inulin clearance at applied pressures less than 150 mmHg and increased with progressive increments in the transmembrane pressure. Hydraulic conductivity of GBM was inversely related to the prevailing transmembrane pressure gradient. The hydraulic conductivity depended on albumin concentration in a manner that was not monotonic, with the conductivity being lower at 4 g/dl albumin than at 0 or 8 g/dl. When plasma was utilized as the retentate, the fractional clearance of albumin was over twice that of immunoglobulin G, and the fractional clearance of each protein was much higher than that in the intact glomerulus. On the basis of these results, both the hydraulic and macromolecular permeability of an individual layer of GBM are much greater than that reported for the intact glomerulus. The large quantitative differences between GBM permeability and that of intact glomeruli suggest a major contribution of cellular elements to glomerular permeability properties.

A theoretical model was developed to relate the size selectivity of the glomerular barrier to the structural characteristics of the individual layers of the capillary wall. Thicknesses and other linear dimensions were evaluated, where possible, from previous electron microscopic studies. The glomerular basement membrane (GBM) was represented as a homogeneous material characterized by a Darcy permeability and by size-dependent hindrance coefficients for diffusion and convection, respectively; those coefficients were estimated from recent data obtained with isolated rat GBM. The filtration slit diaphragm was modeled as a single row of cylindrical fibers of equal radius but nonuniform spacing. The resistances of the remainder of the slit channel, and of the endothelial fenestrae, to macromolecule movement were calculated to be negligible. The slit diaphragm was found to be the most restrictive part of the barrier. Because of that, macromolecule concentrations in the GBM increased, rather than decreased, in the direction of flow. Thus the overall sieving coefficient (ratio of Bowman’s space concentration to that in plasma) was predicted to be larger for the intact capillary wall than for a hypothetical structure with no GBM. In other words, because the slit diaphragm and GBM do not act independently, the overall sieving coefficient is not simply the product of those for GBM alone and the slit diaphragm alone. Whereas the calculated sieving coefficients were sensitive to the structural features of the slit diaphragm and to the GBM hindrance coefficients, variations in GBM thickness or filtration slit frequency were predicted to have little effect. The ability of the ultrastructural model to represent fractional clearance data in vivo was at least equal to that of conventional pore models with the same number of adjustable parameters. The main strength of the present approach, however, is that it provides a framework for relating structural findings to the size selectivity of the glomerular barrier.

Compensatory renal and glomerular hypertrophy accompanies the many functional and structural changes associated with a reduction in functional renal mass. Increased levels of dietary sodium supplementation ranging from deficient to 2.3% Na+ (38-fold above the minimal daily requirement for the rat) in rats with subtotal nephrectomy were associated with a progressive rise in proteinuria and renal size without any significant change in arterial pressure. To further define these relationships, groups of rats on two intermediate levels of sodium intake were studied in detail. Single-nephron filtration rate and glomerular capillary pressure were similar in subtotally nephrectomized rats fed the 0.06 and 0.46% Na+ diets. Both the volume fraction and absolute volume of periodic acid-Schiff staining mesangial lesions in the glomerulus were greater in the 0.46% Na+ group. Glomerular volume and the mean glomerular capillary radius was larger in the 0.46% Na+ group. Increased glomerular tension, as predicted by the Laplace law, may represent a final common pathway by which compensatory growth and/or glomerular hypertension result in glomerular injury.

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