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

Abstract

Venturoli, Daniele, and Bengt Rippe. Ficoll and dextran vs. globular proteins as probes for testing glomerular permselectivity: effects of molecular size, shape, charge, and deformability. Am J Physiol Renal Physiol 288: F605-F613, 2005; doi:10.1152/ajprenal.00171.2004.-Polydisperse mixtures of dextran or Ficoll have been frequently used as molecular probes for studies of glomerular permselectivity because they are largely inert and not processed (reabsorbed) by the proximal tubules. However, dextrans are linear, flexible molecules, which apparently are hyperpermeable across the glomerular barrier. By contrast, the Ficoll molecule is almost spherical. Still, there is ample evidence that Ficoll fractional clearances (sieving coefficients) across the glomerular capillary wall (GCW) are markedly higher than those for neutral globular proteins of an equivalent in vitro Stokes-Einstein (SE) radius. Physical data, obtained by "crowding" experiments or measurements of intrinsic viscosity, suggest that the Ficoll molecule exhibits a rather open, deformable structure and thus deviates from an ideally hard sphere. This is also indicated from the relationship between (log) in vitro SE radius and (log) molecular weight (MW). Whereas globular proteins seem to behave in a way similar to hydrated hard spheres, polydisperse dextran and Ficoll exhibit in vitro SE radii that are much larger than those for compact spherical molecules of equivalent MW. For dextran, this can be partially explained by a high-molecular-size asymmetry. However, for Ficoll the explanation may be that the Ficoll molecule is more flexible (deformable) than are globular proteins. An increased compressibility of Ficoll and an increased deformability and size asymmetry for dextran may be the explanation for the fact that the permeability of the GCW is significantly higher when assessed using polysaccharides such as Ficoll or dextran compared with that obtained using globular proteins as molecular size probes. We suggest that molecular deformability, besides molecular size, shape, and charge, plays a crucial role in determining the glomerular permeability to molecules of different species. capillary permeability; polysaccharides; macromolecules; reflection coefficient; transport POLYDISPERSE MIXTURES OF DEXTRAN, and more recently Ficoll, are frequently used as molecular probes in studies of glomerular permselectivity. After being filtered through the glomerular capillary wall (GCW), polysaccharides, unlike proteins, are left unreabsorbed by the proximal tubules. This implies that their sieving coefficients (), i.e., their filtrate-to-plasma concentration ratios, can be determined directly from their urinary clearances relative to that of a glomerular filtration rate marker (e.g., inulin). Infusing polydisperse mixtures of dextran or Ficoll and using chromatographic techniques to fractionate plasma and urine samples make it possible to simultaneously determine the for a wide spectrum of different-sized molecular probes, provided that proper size calibrations ...

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

“…Ventruli and Rippe reviewed the diffusion of dextrans, albumin and ficoll across the glomerular membrane of the kidney. 22 Unlike the results of the present study, they noted that ficoll and dextran diffused quickly across the glomerulus. They speculated that this was due to the linear shape of dextrans, with high molecular-size-asymmetry, and the deformability of both dextrans and ficoll.…”

“…This observation concurs with other researchers. 22 It is known that FITC holds an approximately -1mV charge, 45 hence FITC-labelled molecules are often slightly more anionic that unlabelled molecules, with the final charge depending on the ratio of labelling.…”

“…27 Dextrans are thought to be linear, flexible molecules, with a loose, open structure, and a high degree of deformability, which renders them more permeable than a more rigid globular protein such as albumin, and indeed even more permeable than ficoll, at least with respect to the glomerulus. 22 These studies indicate that, in combination, dextran, ficoll and albumin serve as useful molecules with which to study the effect of shape and conformability on diffusion across tissue.This study explores the transscleral diffusion of dextrans, ficoll, and albumin of the same MWT, and compares the results to diffusion in a free solution using fluorescence techniques.In particular, this study investigates factors other than MWT that might influence diffusion, namely net surface charge, molecular shape, and the degree of molecular conformability. By comparing transscleral diffusion to that in free solution it is possible to determine the intrinsic diffusional characteristics of each molecule, and how this might change when interacting with scleral tissue.…”

“…27 Dextrans are thought to be linear, flexible molecules, with a loose, open structure, and a high degree of deformability, which renders them more permeable than a more rigid globular protein such as albumin, and indeed even more permeable than ficoll, at least with respect to the glomerulus. 22 These studies indicate that, in combination, dextran, ficoll and albumin serve as useful molecules with which to study the effect of shape and conformability on diffusion across tissue.…”

“…When comparing dextrans and ficoll to other molecules of an equivalent in vitro Stokes-Einstein (SE) radius both molecules are hyperpermeable, and it has been proposed that this is due their relative conformable nature. 22 Lavrenko et al studied the mass of ficoll, its viscosity, and their effect on the diffusion coefficient in the kidney. 23 Bohrer et al used light scattering techniques and suggested that ficoll was shaped more like a sphere than a rod.…”