Glomerular charge selectivity for horseradish peroxidase and albumin at low and normal ionic strengths

Abstract

The classical concept of a negative glomerular charge barrier has recently been questioned, mainly based on the somewhat high clearance for anionic horseradish peroxidase (HRP). The validity of using anionic HRP can be tested by changing the properties of the charge barrier. A rather unequivocal approach is to reduce the ionic composition of the perfusate and hence increase the Debye length. We determined the glomerular clearance for horseradish peroxidase and serum albumin, using isolated rat kidneys perfused at 8 degrees C to reduce the tubular modification of the primary urine. The perfusate contained trace amounts of the neutral 125I-nHRP and the anionic 131I-aHRP and were otherwise identical except for different ionic strengths, 152 mM and 34 mM, respectively. During control, the fractional clearance (theta) was 0.11 +/- 0.015 for nHRP and 0.045 +/- 0.010 for aHRP, with an average clearance ratio (n/a) of 2.8 +/- 0.24. Low ionic strength reduced theta for aHRP to 0.027 +/- 0.006, giving an increased clearance ratio for HRP of 4.2 +/- 0.44. The existence of a negative charge barrier is supported by the experiments. The result obtained during normal perfusion is compatible with a charge density (omega) of 34 mEq L-1, using a model of homogeneously charged membrane. Low ionic strength perfusion reversibly reduced the concentration of fixed charges to 12 mEq L-1, suggesting an almost threefold increase of the glomerular membrane volume. Thus, the glomerular charge barrier should be regarded to have a dynamic gel structure rather than being a rigid membrane.

Glomerular filtration rate dependence of sieving of albumin and some neutral proteins in rat kidneys

“…Much of the evidence in favor of charge selectivity of the glomerular filter has been based on comparisons between sieving data for uncharged and anionic dextran (dextran sulfate) (3). Although vivid arguments against glomerular charge selectivity have been raised during the last decade (5,26,32,42), strong evidence supporting the classic view was recently given by comparing neutral and anionic lactate dehydrogenase or neutral and anionic HRP in the IPK (13,31). These studies largely confirm the classic studies by Rennke et al (22) for differently charged HRP (22).…”

Functional and molecular alterations of the glomerular barrier in long-term diabetes in mice

“…More recently, focus has shifted to the podocytes, since several novel proteins in the podocyte slit diaphragm have been found to be important for the barrier function [1][2][3]. In addition, we have suggested a role for the glomerular endothelial cell glycocalyx [4][5][6][7][8][9][10], an idea supported also by others [11,12].…”

“…Alternatively, the tubular processes may be inhibited by toxins [24], but such drugs seem to affect permeability per se [25]. At present, one of the most reliable estimates of glomerular filtration is obtained by using the cooled isolated perfused kidney (cIPK) model, either in rats [4,8,26] or in mice [9].…”

Ficoll and dextran vs. globular proteins as probes for testing glomerular permselectivity: effects of molecular size, shape, charge, and deformability

“…Hence, in intact kidneys with a higher size selectivity to native proteins than in the fIPK, Lund et al (37) found a ratio of of ϳ7-10 between neutral and native (negatively charged) albumin, later confirmed by Bakoush et al (2). Similarly, high values were obtained for the ratio of the largely neutral probe LDH-5 to the negatively charged LDH-1 in the cIPK, the ratio averaging 7.59 (range 4.11-11.94) (61). On the other hand, studies comparing the clearances of neutral IgG 2 to anionic IgG 4 (pI 5.5-6.0) have yielded results indicating a lower degree of charge selectivity, with IgG 2 /IgG 4 ratios varying from 1.35 (21) and 4 (65) up to 10 (18).…”