GLOMERULAR-PERMEABILITY BARRIER IN THE RAT - FUNCTIONAL ASSESSMENT BYIN-VITRO METHODS

The formation of glomerular ultrafiltrate is dependent on the prevaili ng hemodynamic forces within the glomerular microcirculation and the i ntrinsic properties of the filtration barrier. However, direct assessm ent of the permeability barrier is difficult with most available techn iques. We used confocal microscopy to image 1-mum thick optical cross- sections of isolated intact glomeruli and glomeruli denuded of cells a nd quantitated dextran (70,000 mol wt) diffusion from the capillary lu men. Dextran permeance was 11 times greater for the acellular filtrati on barrier than the intact peripheral capillary. Consideration of the basement membrane and cells as series resistors demonstrated that cell s of the filtration barrier contribute 90% of the total resistance to macromolecular permeance. Using a different approach, dextran sieving coefficients for acellular glomeruli consolidated as a multilayer shee t in a filtration cell were similar to those for intact glomeruli in v ivo at radii 30-36 angstrom and approximately 50 times greater at a de xtran radius of 60 angstrom. The presence of cells significantly reduc ed hydraulic permeability determined on consolidated intact or acellul ar glomeruli in an ultrafiltration cell with 50 mmHg applied pressure. The glomerular basement membrane does restrict macromolecular permeab ility but cells are important determinants of the overall macromolecul ar and hydraulic permeability of the glomerulus.