Ficoll is not a rigid sphere

Abstract

Polydisperse Ficoll mixtures have been used to explore glomerular sieving. Ficoll appears to be neither absorbed nor secreted by the renal tubule, and so urinary Ficoll concentrations reflect only the glomerular filtration barrier. The literature is contradictory regarding Ficoll's behavior as an idealized spherical solute. Further definition of Ficoll transport will inform interpretation of in vivo results. Flat-sheet membranes comprising a uniform array of slit pores measuring 8 nm by 45 mum were perfused with FITC-labeled Ficoll 70 and BSA. Ficoll and BSA concentrations were quantified by gel-permeation chromatography and Bradford assay, respectively. BSA and Ficoll molecules with diameters equal to approximately half of the slit pore width displayed hindered transport in agreement with modeled rigid sphere transport through slit-shaped pores. Ficoll molecules larger than approximately 0.65 slit width displayed transport rates in excess of predictions. Ficoll molecules with Stokes-Einstein diameters greater than the pore dimension were observed in permeate samples. We present data for Ficoll filtration through a novel array of well-defined pores, which illustrate that Ficoll is well modeled as an ideal sphere in one size domain, but the model breaks down as molecular diameter approaches pore size. These data inform the present debate regarding glomerular filtration and affect conclusions drawn from the use of Ficoll as a tracer molecule. The apparent hyperpermeability of Ficoll through slit-shaped pores suggests that further modeling incorporating deformation of the molecule is necessary when using Ficoll solutions to characterize membranes.

Molecular conformation and filtration properties of anionic Ficoll

“…In brief, a silicon membrane of extremely uniform slit pores did not retain Ficolls with hydrodynamic radii larger than the pore size of the membrane (19). Further investigations suggested that transport of globular proteins through the membrane was similar to that predicted from first principles by Dechadilok and Deen (11), leading to the conclusion that Ficoll, as had previously been suggested by Rippe, was not a sphere and was possibly not rigid (19).…”

“…The solution was then incubated at 80°C for 2 h. After conjugation with AMC, labeled Ficoll was recovered by a PD-10 column. Labeled samples were analyzed by size-exclusion chromatography as previously described, except that the excitation and emission wavelengths of the fluorescence detector were set to the appropriate wavelengths for each fluorophore (17,19).…”

“…In brief, a silicon membrane of extremely uniform slit pores did not retain Ficolls with hydrodynamic radii larger than the pore size of the membrane (19). Further investigations suggested that transport of globular proteins through the membrane was similar to that predicted from first principles by Dechadilok and Deen (11), leading to the conclusion that Ficoll, as had previously been suggested by Rippe, was not a sphere and was possibly not rigid (19). Using a combination of size-exclusion chromatography with online multiangle light scattering (SEC-MALS) and viscometry, we concluded that Ficoll's conformation in aqueous solution was intermediate between a sphere and a random coil, but we were unable to distinguish conformational flexibility from dispersion in shape (18).…”

“…Our own work in engineering of ultrafiltration membranes led us to explore the properties of Ficoll, a globular polymer of sucrose and epichlorohydrin (17)(18)(19). In brief, a silicon membrane of extremely uniform slit pores did not retain Ficolls with hydrodynamic radii larger than the pore size of the membrane (19).…”

Letter to the Editor: “Can early plasma elimination rate be used to quantify renal clearance of macromolecules?”

“…In addition, the permeability for Ficoll both in the GFB and synthetic membranes apparently lies somewhere between that of globular proteins and dextran (1,3,13,20,28). For example, if the plasma clearance of a bolus dose of albumin was only due to renal excretion, this would imply a plasma half-life of ϳ2,000 h, corresponding to a TER alb of Ͻ0.04% (i.e., Ͼ50 times less than that measured by Vuchkova et al) (29).…”