Role of albumin and glomerular capillary wall charge distribution on glomerular permselectivity: studies on the perfused-fixed rat kidney model.

The charge-related determinants of albumin permeability are the subject of controversial discussion. To study this question we have developed an isolated perfused rat kidney model in which metabolic processes are eliminated by perfusion fixation with glutaraldehyde. The fixed kidneys were perfused with albumin solutions using the following approaches: 1. Modification of the charge of both the glomerular capillary wall (GCW) and albumin using different buffer systems in a pH range spanning the isoelectric points of albumin and the glomerular basement membrane (GBM), the extracellular matrix of the GCW. 2. Modification of the charge of the GCW by perfusing the isolated kidney with cations either before or after fixation. 3. Modification of the charge of albumin by cationization. In the model, the inulin "urine" to perfusate ratio was one. This shows that the tubules have no metabolic activity, that the glomerular filtration rate (GFR) is equal to "urine" flow rate and that the "urine" collected is identical to the ultrafiltrate. Therefore, sieving coefficients in this model can simply be calculated as the ratio between "urine" and perfusate protein concentrations. We could show that: 1. pH has a significant effect on the albumin sieving coefficient: it was maximally increased at pH 4.0 [(70.3 +/- 15.9) x 10(-3), n = 10 versus (8.7 +/- 3.7) x 10(-3), n = 11, at pH 7.4]. Only a pH as low as 4.0 should lead to a pronounced neutralization of the anionic charges of albumin and the GBM; the charge density of the GCW calculated with these data is 43 mEq/l at pH 7.4. 2. Modifying the ionic composition of the GCW with protamine before fixation with glutaraldehyde causes a bigger increase in the glomerular permeability for albumin [(51.2 +/- 22.5) x 10(-3), n = 10, glomerular charge density 21 mEq/l] than modifying the albumin charge by cationization. 3. Modifying the albumin charge by cationization increases the glomerular permeability for albumin [(20.0 +/- 6.7) x 10(-3), n = 8]. These findings support the hypothesis that at the onset of proteinuria changes in the charge and configuration of the GCW could be more important pathogenetic factors than changes in the charge of serum-derived proteins.