Dynamic alterations of glomerular charge density in fixed rat kidneys suggest involvement of endothelial cell coat

In a previous paper, we found that low ionic strength (I) reversibly reduced the glomerular charge density, suggesting increased volume of the charge-selective barrier. Because glutaraldehyde makes most structures rigid, we considered the isolated, perfusion-fixed rat kidney to be an ideal model for further analysis. The fixed kidneys were perfused with albumin solutions containing FITC-Ficoll at two different Is (I = 151 and 34 mM). At normal I, the fractional clearance (0) for albumin was 0.0049 (SE -0.0017, +0.0027, n = 6), whereas 0 for neutral Ficoll_35.5Å of similar size was significantly higher 0.104 (SE 0.010, n = 5, P < 0.001). At low I, θ for albumin was 0.0030 (SE -0.0011, +0.0018, n = 6, not significant from θ_albumin at normal I) and θ for Ficoll _35.5Å was identical to that at normal I, 0.104 (SE 0.015, n = 6, P < 0.01 compared with θ_albumin at low I). According to a heterogeneous charged fiber model, low I reduced the fiber density from 0.056 to 0.0315, suggesting a 78% gel volume expansion. We conclude that 1) there is a significant glomerular charge barrier. 2) Solutions with low I increase the volume of the charge barrier even in kidneys fixed with glutaraldehyde. Our findings suggest that polysaccharide-rich structures, such as the endothelial cell coat, are key components in the glomerular barrier.