Am J Physiol Renal Physiol 301: F708–F712, 2011.
First published July 20, 2011; doi:10.1152/ajprenal.00183.2011.
Translational Physiology
Reduced diffusion of charge-modified, conformationally intact anionic Ficoll
relative to neutral Ficoll across the rat glomerular filtration barrier in vivo
Josefin Axelsson,1 Kristinn Sverrisson,1 Anna Rippe,1 William Fissell,2 and Bengt Rippe1
1
Department of Nephrology, Lund University, Lund, Sweden; and 2Biomedical Engineering, Cleveland Clinic,
Cleveland, Ohio
Submitted 5 April 2011; accepted in final form 15 July 2011
capillary permeability; sieving coefficient; glomerular basement
membrane; FITC
TRADITIONALLY, THE MAMMALIAN glomerular filtration barrier
(GFB) is envisioned as being negatively charged. Evidence in
favor of its charge selectivity is based on comparisons of
glomerular sieving data for uncharged vs. anionic dextran and
for uncharged vs. cationic dextran in rats (6, 7, 21). Thus
sulfated, negatively charged dextran was found to be markedly
retarded compared with neutral dextran, which in turn was less
permeable across the glomerular barrier than polycationic
(DEAE) dextran. However, these studies have been criticized
on the ground that sulfated dextran can bind to plasma proteins
(14) and cell membranes (28) and may be desulfated by
glomerular cells before reaching the primary urine (30). This
Address for reprint requests and other correspondence: J. Axelsson, Dept. of
Nephrology, Univ. of Lund, Univ. Hospital of Lund, S-221 85 Lund, Sweden
(e-mail: [email protected]).
F708
implies that glomerular sieving coefficients (␪) for sulfated
dextran might have been erroneously underestimated. Hence,
Schaeffer et al. (29) were not able to find any differences in rat
glomerular ␪ for carboxymethylated (CM), anionic (nonsulfated) dextran vs. neutral dextran. They also assessed glomerular sieving curves for negatively charged hydroxy ethyl starch
(HES). Anionic HES molecules showed lower ␪ for any given
in vitro Stokes-Einstein molecular radius (ae) than did anionic
dextran. Since both tracers were about equally negatively
charged, the authors concluded that the glomerular barrier
restricts the transport of polysaccharide molecules as a function
of their size and configuration, but not due to their charge. In
addition, a number of previous in vitro studies have demonstrated that the mammalian GBM does not seem to contribute
to the charge selectivity of the entire GFB (4, 5). Also, it was
recently shown that glomerular filtration of albumin is normal
in the absence in the GBM of both agrin and perlecan-heparan
sulfate (HS), both being negatively charged, and that reductions of anionic sites in the GBM by heparanase does not lead
to proteinuria (12, 17, 31).
Using charge-modified, but conformationally altered, polyanionic CM-Ficoll, we previously noted an increased fractional
clearance of anionic Ficoll relative to uncharged Ficoll for the
rat GFB in vivo (2). However, it seemed that the charge
modification had significantly increased the molecular radius
of the Ficoll molecules for all molecular weights (MW), in that
CM-Ficoll eluted earlier than neutral Ficoll on high-performance size-exclusion chromatography (HPSEC), semiquantitatively confirmed using single-angle quasi-elastic light-scattering (2). Since molecules with an increased “frictional ratio”
(ratio of ae over the radius of an ideally compact spherical
molecule of identical MW) generally show increases in glomerular permeability (1, 24), we ascribed the enhanced transport of CM-Ficoll to a conformational difference between
neutral and anionic Ficoll molecules, making the latter less
“compact” and more flexible, and thereby, hyperpermeable
across the GFB (32). A similar explanation can be given to the
results of Guimarães et al. (15), who also found a markedly
increased glomerular permeability for large CM-Ficoll molecules compared with conventional Ficoll for the rat GFB. This
is in contrast to a number of previous studies demonstrating a
negative charge of the GFB for differently charged protein
probes, as recently reviewed (8, 16, 32).
In testing glomerular charge selectivity, it is thus of utmost
importance that charge-modified probes, compared with their
neutral counterparts, show identical conformation in terms of
e.g., frictional ratio (32). Recently, Fissell and coworkers (13)
succeeded in charge modifying FITC-Ficoll-70 and FITCFicoll-400 in a way that resulted in negligible conformational
changes, as checked by multiangle light scattering and size-
1931-857X/11 Copyright © 2011 the American Physiological Society
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Axelsson J, Sverrisson K, Rippe A, Fissell W, Rippe B. Reduced
diffusion of charge-modified, conformationally intact anionic Ficoll
relative to neutral Ficoll across the rat glomerular filtration barrier in
vivo. Am J Physiol Renal Physiol 301: F708 –F712, 2011. First
published July 20, 2011; doi:10.1152/ajprenal.00183.2011.—The glomerular filtration barrier (GFB) is commonly conceived as a negatively charged sieve to proteins. Recent studies, however, indicate that
glomerular charge effects are small for anionic, carboxymethylated
(CM) dextran vs. neutral dextran. Furthermore, two studies assessing
the glomerular sieving coefficients (␪) for negative CM-Ficoll vs.
native Ficoll have demonstrated an increased glomerular permeability
for CM-Ficoll (Asgeirsson D, Venturoli D, Rippe B, Rippe C. Am J
Physiol Renal Physiol 291: F1083–F1089, 2006; Guimarães M,
Nikolovski J, Pratt L, Greive K, Comper W. Am Physiol Renal
Physiol 285: F1118 –F1124, 2003.). The CM-Ficoll used, however,
showed a larger Stokes-Einstein radius (ae) than neutral Ficoll, and it
was proposed that the introduction of negative charges in the Ficoll
molecule had made it more flexible and permeable. Recently, a
negative FITC-labeled CM-Ficoll (CMI-Ficoll) was produced with a
conformation identical to that of neutral FITC-Ficoll. Using these
probes, we determined their ␪:s in anesthetized Wistar rats (259 ⫾ 2.5
g). After blood access had been achieved, the left ureter was cannulated for urine sampling. Either polysaccharide was infused (iv)
together with a filtration marker, and urine and plasma were collected.
Assessment of ␪ FITC-Ficoll was achieved by high-performance
size-exclusion chromatography (HPSEC). CMI-Ficoll and native Ficoll had identical elugrams on the HPSEC. Diffusion of anionic Ficoll
was significantly reduced compared with that of neutral Ficoll across
the GFB for molecules of ae ⬃20 –35 Å, while there were no charge
effects for Ficoll of ae ⫽ 35– 80 Å. The data are consistent with a
charge effect present in “small pores,” but not in “large pores,” of the
GFB and mimicked those obtained for anionic membranes in vitro for
the same probes.
Translational Physiology
THE GLOMERULAR FILTRATION BARRIER IS NEGATIVELY CHARGED
exclusion chromatography (SEC), here denoted CMI-Ficoll-70
and CMI-Ficoll-400, respectively, where “I” stands for “intact”
or “identical” (with respect to conformation). Using these
probes in sieving experiments under well-controlled conditions
in vivo, we demonstrate that anionic CMI-Ficoll was significantly retarded compared with neutral Ficoll across the rat
GFB in a way mimicking the results recently obtained with
these probes in vitro for precision made anionic synthetic
membranes (13).
MATERIALS AND METHODS
AJP-Renal Physiol • VOL
kidney was collected for 5 min, with a midpoint (2.5 min) collection
of a plasma sample.
Plasma and urine samples were assessed on a HPSEC system
(Waters, Milford, MA) with an Ultrahydrogel 500 column (Waters)
and calibrated as described in detail previously (2). The mobile phase
was driven by a pump (Waters 1525), and fluorescence was detected
with a fluorescence detector (Waters 2475) with an excitation wavelength at 492 nm and an emission wavelength at 518 nm. The samples
were loaded to the system with an autosampler (Waters 717 plus), and
the system was controlled by Breeze Software 3.3 (Waters). Ficoll ␪ were
obtained by analyzing HPSEC curves from the plasma and urine sample
for each experiment. The ␪ of Ficoll was determined as the fractional
clearance, according to the formula ␪ ⫽ (CFU ⫻ CIP)/(CFP ⫻ CIU), where
CFU represents the urine Ficoll concentration, CIP represents the inulin
concentration in plasma, CFP the Ficoll concentration in plasma, and CIU
the inulin concentration in urine.
Glomerular filtration rate. Glomerular filtration rate (GFR) was
measured in the left kidney during the experiment, using 51Cr-EDTA.
A bolus dose of 51Cr-EDTA (0.3 MBq in 0.2 ml iv, Amersham
Biosciences, Buckinghamshire, UK) was administered and followed
by a constant infusion (10 ml·kg⫺1·h⫺1) of 51Cr-EDTA (0.37 MBq/ml
in 0.9% NaCl) throughout the experiment, which yielded a stable
plasma concentration of 51Cr-EDTA over time. Urine was collected
from the left ureter repeatedly during the experiment, and blood
samples, using microcapillaries (25 ␮l taken for assessment of GFR),
every 10 min. Radioactivity in blood and urine was measured in a
gamma counter (Wizard 1480, LKP, Wallac, Turku, Finland). Hematocrit was assessed throughout the experiments to convert blood
radioactivity (51Cr-EDTA) into plasma radioactivity. During the
FITC-Ficoll sieving periods, GFR was also assessed from the urine
clearance of FITC-inulin. The urinary excretion of 51Cr-EDTA and
FITC-inulin per minute (Ut ⫻ Vu) divided by the concentration of
tracer in plasma (Pt) was used to calculate GFR, where Ut represents
the tracer concentration in urine, and Vu is the flow of urine per
minute. Since the variability (coefficient of variation) for FITC-inulinassessed GFR was slightly higher than that for 51Cr-EDTA-assessed
GFR, we present the latter consistently throughout the study.
Two-pore analysis. A two-pore model (20, 25) was used to analyze
the ␪ data for Ficoll (molecular radius 15– 80 Å). A nonlinear
least-squares regression analysis was used to obtain the best curve fit,
using scaling multipliers, as described at some length previously (20).
Statistics. Values are presented as means ⫾ SE. Differences among
groups were tested using nonparametric analysis of variance with the
Kruskal-Wallis test and post hoc tested using the Mann-Whitney
U-test. Bonferroni corrections for multiple comparisons were made.
Significance levels were set at *P ⬍ 0.05, **P ⬍ 0.01 and ***P ⬍
0.001. All statistical calculations were made using SPSS 18.0 for
Windows (SPSS, Chicago, IL).
RESULTS
Hemodynamic parameters. Animals showed a stable mean
arterial pressure (MAP) over time (Fig. 1). Also, GFR was
stable in both groups, but with a small tendency to decline
with time. For neutral Ficoll experiments, GFR decreased
from 0.69 ⫾ 0.04 at the start to 0.65 ⫾ 0.04 ml·min⫺1·g⫺1
(not significant; ns) at the end of the experiment (25 min),
and for CMI-Ficoll experiments this slight decrease was
from 0.67 ⫾ 0.04 to 0.58 ⫾ 0.04 ml·min⫺1·g⫺1 (ns).
Sieving of FITC-Ficoll and FITC-CMI-Ficoll. Figure 2 demonstrates ␪ vs. ae for Ficoll vs. CMI-Ficoll (10 – 40 Å) plotted
on a linear scale. For CMI-Ficoll, there was a reduced ␪ for
molecules of radius ⬃20 –35 Å compared with neutral Ficoll in
the same size range. For neutral Ficoll of radius 28 Å, ␪ was
0.56 ⫾ 0.01, and for CMI-Ficoll it was 0.47 ⫾ 0.02 (P ⬍ 0.05).
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Animals and surgery. Experiments were performed in 16 male
Wistar rats (Möllegard, Lille Stensved, Denmark) with an average
body weight of 259 ⫾ 2.5 g. The rats were given water and standard
chow ad libitum. The animal Ethics Committee at Lund University
approved the animal experiments. Anesthesia was induced with pentobarbital sodium (60 mg/kg ip). Body temperature was kept at 37°C
by a thermostatically controlled heating pad, and a tracheotomy was
performed to facilitate breathing. The tail artery was cannulated
(PE-50 cannula) for continuous monitoring of arterial pressure and
registration of heart rate on a polygraph (model 7B; Grass Instruments, Quincy, MA), and for maintenance administration of anesthesia. The left carotid artery was cannulated (PE-50 cannula) for blood
sampling and the left jugular vein for infusion purpose. Access to the
ureter was obtained through a small abdominal incision. The left
ureter was dissected free and cannulated (PE-10 cannula) for continuous urine sampling. Furosemide (Furosemide Recip 0.375 mg/kg
body wt) was administered in the tail artery to increase urine production and facilitate cannulation of the ureter.
FITC-CMI-Ficoll. Carboxymethylation of Ficoll was performed
using strong NaOH and monochloroacetic acid (MCA) in aqueous
solution at elevated temperature. One gram of Ficoll 70 and 400
(catalog nos. 46326 and 46327, respectively, Fluka, St. Louis, MO)
were each mixed with 26.5 ml of deionized water and stirred for 30
min at 40°C, after which 20 ml 10 M NaOH was slowly added. MCA
was then added dropwise into the mixture. The mixture was stirred at
40°C for 3 h and then neutralized with 5 M HCl and dialyzed against
distilled water for 4 days. The Z-potential for CMI-Ficoll was measured by a Brookhaven ZetaPlus (Brookhaven Instruments, Holtsville,
NY). The (charge) characterization of the CMI-Ficoll (⫺20 charges/
molecule) has been presented previously (13). For comparison, the
anionic CM-Ficoll used in a previous study had a charge of either ⫺40
or ⫺95/molecule, respectively (2).
Labeling of CMI-Ficoll with FITC was performed according to
Ohlson et al. (22). CMI-Ficoll was dissolved in a solution of AMC in
DMSO-glacial acetic acid-sodium cyanoborohydride and incubated at
80°C for 2 h. After conjugation with AMC, CMI-Ficoll was recovered
by a PD-10 column. Labeled samples were analyzed using HPSEC as
described in previous papers (9, 10) with the excitation and emission
set at 492 and 518 nm, respectively.
FITC-Ficoll/FITC-CMI-Ficoll ␪. FITC-Ficoll-70, FITC-Ficoll400, and FITC-inulin were bought from TdB Consultancy (Uppsala,
Sweden). FITC-CMI-Ficoll-70/400 was produced as referred to
above. Experiments were performed with either FITC-Ficoll (Ficoll, n ⫽ 7) or FITC-CMI-Ficoll (CMI-Ficoll, n ⫽ 9). A mixture of
Ficoll-70 and Ficoll-400, or CM-Ficoll-70 and CM-Ficoll-400, in a
1:24 relationship was administered together with FITC-inulin as a
bolus dose followed by constant infusion. The bolus contained 40 ␮g
Ficoll-70 or CMI-Ficoll-70, 960 ␮g Ficoll-400 or CMI-Ficoll-400,
500 ␮g FITC-inulin, and 0.3 MBq 51Cr-EDTA and was followed by
a constant infusion (10 ml·kg⫺1·h⫺1) containing 20 ␮g/ml Ficoll-70
or CMI-Ficoll-70, 0.48 mg/ml Ficoll-400 or CMI-Ficoll-400, 0.5
mg/ml FITC-inulin, and 0.3 MBq/ml 51Cr-EDTA. Ficoll was allowed
to circulate in the animal for at least 20 min before sieving measurements to reach a stable concentration, after which urine from the left
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THE GLOMERULAR FILTRATION BARRIER IS NEGATIVELY CHARGED
For larger molecules (35– 80 Å), there were no significant
differences between the groups (Fig. 3).
Figure 4, A and B, demonstrates elugrams for Ficoll-70 vs.
CMI-Ficoll-70 and Ficoll-400 vs. CMI-Ficoll-400, respectively. For Ficoll-70, there was no difference in elution time
between the two differently charged probes. Figure 4A, however, demonstrates a very small (ns) difference between Ficoll400 and CMI-Ficoll-400 in elution time, indicating that a slight
increase in molecular radius had occurred for the anionic prob.
Pore parameters. The best curve fits of ␪ vs. ae for Ficoll
according to the two-pore model were obtained using the
parameters listed in Table 1. The major finding is that there
was a significant reduction in diffusive transport, i.e., the
unrestricted pore area over unit diffusion path length (A0/⌬X)
in the CMI-Ficoll experiments compared with the Ficoll experiments. For both experiments, the small-pore radius was 45
Å, with a very slight (⫹0.9%), but significant, increase in the
apparent small-pore radius for the CMI-Ficoll group.
Fig. 3. ␪ vs. ae plotted on a semilogarithmic scale. There was no difference in
␪ for Ficoll35– 80Å vs. ␪ for CMI-Ficoll35– 80Å. Solid line, Ficoll; hatched line,
CMI-Ficoll.
glomerular capillary wall exhibits low discrimination ability
with respect to differently charged, nonsulfated dextrans (2,
29). Hence, in two rather recent studies anionic CM-Ficoll was
found to show an increased, not a decreased, glomerular
permeability, compared with that for native Ficoll molecules.
However, Asgeirsson et al. (2) noted that the CM-Ficoll molecules tested showed higher in vitro ae (relative to their
molecular weight) compared with neutral Ficoll and suggested
that the reduced “density” of the anionic Ficoll molecules
DISCUSSION
The charge-selective properties of the GFB are highly controversial. Generally, anionic proteins are retarded in the GFB
compared with neutral or cationic proteins of equivalent size,
as recently reviewed (8, 16, 32). However, it seems that the
Fig. 2. Sieving coefficients (␪) vs. Stokes-Einstein radii (ae) for Ficoll (solid
line) and CMI-Ficoll (hatched line). There was a slight, but significant,
reduction in ␪ for CMI-Ficoll of radius 20 –35 Å.
AJP-Renal Physiol • VOL
Fig. 4. A and B: elugrams of Ficoll-70 and CMI-Ficoll-70 (A; n ⫽ 2), and
Ficoll-400 and CMI-Ficoll-400 (B; n ⫽ 2). Solid lines, Ficoll; hatched lines,
CMI-Ficoll.
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Fig. 1. Mean arterial pressure vs. time in Ficoll and negative FITC-labeled
carboxymethylated Ficoll (CMI-Ficoll) groups stayed constant throughout the
experiment. Solid line and filled squares, uncharged Ficoll; hatched line and
filled circles, CMI-Ficoll.
Translational Physiology
THE GLOMERULAR FILTRATION BARRIER IS NEGATIVELY CHARGED
Table 1.
Two-Pore Parameter
Ficoll
CMI-Ficoll
rs, Å
rL, Å
␣L ⫻ 105
JvL/GFR ⫻ 105
A0 /⌬X cm/g ⫻ 10⫺5
44.7 ⫾ 0.12
138.1 ⫾ 4.59
3.97 ⫾ 0.56
11.6 ⫾ 1.56
8.93 ⫾ 0.16
45.1 ⫾ 0.08*
142.1 ⫾ 12.1
4.36 ⫾ 0.83
12.9 ⫾ 2.59
4.02 ⫾ 0.49†
Values are means ⫾ SE. CMI-Ficoll, negative FITC-labeled carboxymethylated (CM)-Ficoll; rs, small-pore radius; rL, large-pore radius; ␣L, fractional
ultrafiltration coefficient accounted for by large pores; JvL/GFR, fractional
fluid flow through large pores; GFR, glomerular filtration rate; A0/⌬X, effective
pore area over unit diffusion path length. Statistical differences between Ficoll
and CM-Ficoll: *P ⬍ 0.05, †P ⬍ 0.001.
AJP-Renal Physiol • VOL
organized than the mammalian GBM, thereby conceivably
acting as a negatively charged interstitial gel. It has long been
known that the interstitium in the systemic blood-lymph barrier
acts as a positive charge barrier, although the capillary wall
proper appears to be negatively charged (11, 25). Obviously,
negatively charged proteoglycans and glycosaminoglycans in
the interstitium endow the tissue with ion-exchange column
properties, causing negatively charged macromolecules to
move faster than uncharged macromolecules across the meshwork of negatively charged gel elements, at least during
non-steady-state conditions (1, 11). The much thicker interstitial gel-like tissue in the GFB of the salamander obviously
shows properties similar to those of the systemic blood-lymph
interstitial barrier, yielding anomalous electrical (positive
charge barrier) behavior.
The present results are in qualitative agreement with previous studies on the charge selectivity of the rat GFB in the
cooled, isolated perfused kidney model. Ohlsson et al. (23)
compared ␪ of neutral Ficoll36Å with that of negatively charged
native albumin (ae 36 Å), and based on that, they calculated the
fixed negative charge of the GFB to be 30 meq/l. However,
comparing proteins with polysaccharides may be problematic,
because polysaccharides generally show a higher “frictional
ratio” (molecular extension compared with that of an ideally
compact and spherical molecule) than do proteins, and thus can
be predicted to be more permeable across the GFB (32). Hence,
comparing proteins with polysaccharides could have influenced the large permeability differences seen. Thus, as extensively reviewed previously (32), polysaccharides, such as dextran (or Ficoll), should not be compared with proteins based on
their in vitro as alone, except for large Ficoll molecules (as
50 – 80 Å), passing through “large pores” (shunt pathways) of
the GFB (26). In the present study both Ficoll36Å (cf. albumin)
and CMI-Ficoll36Å showed a marked hyperpermeability (␪
⬃0.06) compared with albumin (␪ ⬃0.0001). This hyperpermeability of both Ficoll36Å and CMI-Ficoll36Å (⫺20 charges/
molecule) is likely to be due to the effects of molecular
conformation alone (26). A parallel can be drawn to the
marked glomerular hyperpermeability of the very asymmetric
glycoprotein bikunin, showing identical ae and net charge to
albumin in vitro, but being ⬃1,000-fold more permeable than
albumin across the rat GFB in vivo (1, 19).
In view of the recent results that the GBM does not appear
to show any charge selectivity in vitro (4, 5) and that the total
absence of the negatively charged GBM proteoglycans agrin
and perlecan does not seem to affect the permeation of albumin
across the GFB (12, 17), we speculate that the negative charge
of the GFB may reside in the endothelial glycocalyx (16, 23).
The charge-selective properties of the GFB seem, however,
much less pronounced than previously conceived. However,
for albumin being severely restricted in its passage across the
small-pore equivalent (or through a “tight” fiber matrix) by size
restriction alone, a negative charge may be critical in that it
may normally totally exclude albumin from the small pore
pathway. We have previously determined ␪ for neutralized
albumin to be ⬃0.005 (20, 27), and mostly dependent on
size-restricted clearance across the small pores. Hence, we
speculate that total abolition of charge-selective properties
in the small pores would increase ␪ for albumin from
normally 2–3 ⫻ 10⫺4 (3) (large-pore transport) to 5 ⫻10⫺3
(large-pore ⫹ small-pore transport). Thus, to the extent that
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could have made the molecules less compact and more flexible,
explaining their relative “hyperpermeability” (32). The results
of the present study seem to corroborate these findings, because conformationally intact anionic CM-Ficoll of unperturbed ae, i.e., CMI-Ficoll (9), was found to be retarded
compared with neutral Ficoll across the rat GFB.
The present data agree, at least qualitatively, with the in vitro
data obtained using the same probes for precision-made anionic membranes (13). According to the double-layer hypothesis, for which charge screening occurs in a field extending 8
Å away from the surface of the molecule and also 8 Å from the
similarly charged membrane (or pore) surface, one would
expect almost no charge effects in large pores, as also noted in
this study. For small pores, however, charge effects would be
most dominant at solute radii approaching the pore radius and
decrease as the solute radius decreases. Both for the rat GFB
and the precision-made in vitro anionic membranes mentioned
above (13), the charge effects were about similar for Ficoll
molecules of relative radius ranging from 0.3 to 1.1 of the pore
radius (cf. from ⬃12 to ⬃40 Å for the rat GFB). Thus, neither
in the present study in vivo nor in the mentioned in vitro
membrane study did charge effects follow the double-layer
hypothesis. This may indicate that charge-restriction effects are
much less in magnitude and more complex than can be predicted by the simple double-layer hypothesis. The most conspicuous effect in the present study of the impact of negative
charge was a 50% reduction in the diffusive transport (A0/⌬X)
of CMI-Ficoll vs. neutral Ficoll, whereas the differences in ␪
were relatively small. Indeed, the charge effects peaked at 28
Å, the difference between ␪ for Ficoll and CMI-Ficoll being
only ⬃20%. The (average) GFR during the CMI-Ficoll experiments was fortuitously slightly lower (averaging 0.62 ml/min)
than that in the Ficoll experiments (averaging 0.67 ml/min).
Adjusting for the impact of this GFR effect using the two-pore
model, i.e., by lowering GFR from 0.67 to 0.62 in the Ficoll
group, would theoretically increase ␪ for uncharged Ficoll28Å
from 0.56 to 0.585, which would increase the separation of
CMI-Ficoll from uncharged Ficoll by ⬃4% (to ⬃24%).
The present data are seemingly at variance with recently
published data on the electric behavior of the salamander
(Necturus) GFB. Across the Necturus GFB, Hausmann et al.
(18) measured a positive (Bowman’s space negative) transmembrane potential directly proportional to the filtration pressure, suggesting the presence of a positively charged bloodurine barrier. The reason for this “anomalous” electrical behavior of the Necturus GFB is not clear. However, the GBM in
the Necturus GFB is at least 10-fold thicker and much less well
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THE GLOMERULAR FILTRATION BARRIER IS NEGATIVELY CHARGED
13.
14.
15.
16.
17.
18.
ACKNOWLEDGMENTS
Kerstin Wihlborg is gratefully acknowledged for skillfully typing the
manuscript.
19.
GRANTS
This study was supported by the Swedish Research Council (Grant 08285),
the Heart and Lung Foundation, and the Medical Faculty at Lund University
(ALF Grant).
20.
DISCLOSURES
21.
No conflicts of interest, financial or otherwise, are declared by the authors.
22.
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negative charges prevent small-pore permeation of albumin,
pore charge may be critically essential in restricting albumin
permeation across the GFB.
In conclusion, comparing ␪ and diffusion properties across
the rat GFB of conformationally identical Ficoll molecules
having different electrical charges clearly indicates that the
mammalian GFB is negatively charged. Yet, molecular conformation and flexibility, besides size, seem to be of greater
importance than charge in determining the permeation properties of most macromolecules in the mammalian GFB. Nevertheless, a negative charge in the small-pore equivalent of the
GFB can be predicted to critically affect the permeation of
albumin from blood to urine under normal conditions, of
importance for understanding the mechanisms of microalbuminuria.