Increased glomerular permeability to negatively charged Ficoll relative to neutral Ficoll in rats

Abstract

It is established that the glomerular filter sieves macromolecules based on their size, shape, and charge. Anionic proteins are thus retarded compared with their neutral or cationic counterparts. However, recent studies have indicated that charge effects are small, or even "anomalous," for polysaccharides. We therefore investigated the impact of charge on the glomerular permeability to polysaccharides by comparing sieving coefficients (theta; primary urine-to-plasma concentration ratio) for negatively charged, carboxymethylated (CM) FITC-Ficoll and FITC-dextran with their neutral counterparts. For these probes, theta were determined in anesthetized Wistar rats [269 +/- 2.7 g (+/-SE; n = 36)], whose ureters were cannulated for urine sampling. The glomerular filtration rate was assessed using FITC-inulin. Polysaccharides were constantly infused, and after equilibration, urine was collected and a midpoint plasma sample was taken. Size and concentration determinations of the FITC-labeled polysaccharides were achieved by size-exclusion HPLC (HPSEC). For CM-Ficoll, theta was significantly increased (32 times at 55 A) compared with that of uncharged Ficoll. A small increase in theta for CM-dextran compared with neutral dextran was also observed (1.8 times at 55 A). In conclusion, negatively charged Ficoll relative to neutral Ficoll was found to be markedly hyperpermeable across the glomerular filter. Furthermore, negatively charged Ficoll was observed to be larger on HPSEC compared with its neutral counterpart of the same molecular weight. It is proposed that the introduction of negative charges in the "dendrimeric," cross-linked Ficoll molecule may alter its configuration, so as to make it more extended, and conceivably, more flexible, thereby increasing its glomerular permeability.

Molecular conformation and filtration properties of anionic Ficoll

“…We do not report a "degree of substitution" as other investigators have, because we do not have data allowing estimation of charge-to-mass ratio of CM-Ficoll (3,20). The degree of substitution measured by titration in a polydisperse solution only yields an aggregate sense of how charged the solutes are.…”

“…Opposing that coherent body of theory and experiment are other data suggesting that some of the apparent charge selectivity may be artifactual, due to cellular processing of charged solutes (7). More recent papers demonstrate both enhanced transport of anionic tracers compared with neutral ones and in vivo studies in which reducing the charge of the basement membrane gel had negligible effect on permselectivity to albumin (3,6,20,22,35). At least one group, Rippe and colleagues (3), has suggested that enhanced transport of carboxymethylated Ficoll might be explained by conformational changes instead of electrostatics; that is, the modification process by which an anionic charged Ficoll is produced causes the Ficoll to become less compact and more flexible.…”

Influence of molecular shape, conformability, net surface charge, and tissue interaction on transscleral macromolecular diffusion

“…27 This is consistent with reports showing the ficoll becomes more open when a negative charge is added. 28 This is most likely due to the opposing negative electrostatic charges repelling each other, forcing the structure to open. Given that neutral dextrans exist in a relatively open structure, they may have been less likely to change their configuration in response to an altered surface charge.…”

“…Asgeirsson et al demonstrated an increased permeability of the glomerular capillary wall to negatively charged carboxymethylated ficoll, relative to neutral ficoll. 28 This occurs because the negatively charged ficoll becomes larger, 28 and whilst the increased molecular radius might be anticipated to slow diffusion across tissue, the molecule instead becomes less dense and consequently more flexible (compressible), making it hyperpermeable across the glomerular barrier. By contrast, dextran did not show this marked increase in hyperpermeability when carboxymethylated.…”

Continuum of historical controversies regarding the structural-functional relationship of the glomerular ultrafiltration unit

“…This may be of some concern especially when one is dealing with modified dextrans or Ficoll since the modifications are amenable to alter the tertiary configuration of these macromolecules and thereby . In this regard, an anomalously "increased" transglomerular passage of "anionic" Ficoll was observed relative to "neutral" Ficoll, which in theory if this situation is applied to the globular proteins should have a "decreased" GUU value (1). In view of these concerns, Sverrisson et al cautiously used "conformationally unchanged" or "intact Ficoll" with varying hydrodynamic radii as functional tracers for their experiments; however, minor changes in the tertiary conformation may still not be detectable.…”