Size-selectivity of a synthetic high-flux and a high cut-off dialyzing membrane compared to that of the rat glomerular filtration barrier

The aim of the present study was to investigate the size-selectivity of two different synthetic dialyzing membranes, having widely differing sieving properties, with respect to their handling of polydispersed fluorescein isothiocyanate (FITC)-Ficoll, FITC-dextran and of proteins, i.e. 125I-human serum albumin (RISA) and 125I-myoglobin (Myo). Are Ficoll and dextran, compared to proteins, “hyperpermeable” across synthetic dialyzing membranes, similar to their behavior across the glomerular filtration barrier (GFB)? A high-flux membrane (HF-Revaclear®; n = 12) and a high cut-off membrane (HCO; n = 14) in capillary mini-dialyzers were perfused with diluted horse serum. The perfusate contained polydisperse FITC-Ficoll 70/400 or FITC-dextran (mol radius 13–80 Å), FITC-Inulin, and, in some experiments, RISA/Myo. After a priming period, sampling of filtrate occurred, and a midpoint plasma sample taken. Filtrate-to-plasma concentration ratios (θ) vs. molecular radius (ae) were assessed using HPLC for Ficoll and dextran. Size-selectivity for Ficoll increased in the order: HF-Revaclear® < rat glomerulus < HCO. Although the HCO filter showed the highest cut-off, this occurred at the expense of a high permeability to albumin and large Ficoll molecules and a high degree of dispersity of (small) pore radii, as assessed using a log-normal + shunt distributed pore model. According to a two-pore model, the fractional hydraulic conductance accounted for by large pores (αL) was 8.58 ± 0.93 × 10−3 and 1.51 ± 0.88 × 10−3 for the HCO and the HF-Revaclear®, respectively, compared to 4.1 ± 0.80 × 10−5 for the rat glomerulus. In conclusion, the HCO filter investigated showed a high θ for myoglobin, similar to that of the GFB. However, the number of large pores was markedly higher and the pore size heterogeneity markedly larger than for the GFB. Membrane permeability was dependent on molecular species and increased in the order: proteins < Ficoll < dextran.