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Departments of 1 Physiology and 2 Nephrology, Göteborg University, SE-405 30 Gothenburg, Sweden
Modifying the ionic strength (I) is a gentle way to alter charge
interactions, but it cannot be done for studies of the glomerular sieving of proteins in vivo. We therefore perfused 18 isolated rat
kidneys with albumin solutions of different ionic strengths at a low
temperature (cIPK) to inhibit tubular uptake and protease activity.
Four anionic proteins were studied, namely albumin (Alb), orosomucoid
(Oro), ovalbumin (Ova), and anionic horseradish peroxidase (aHRP),
together with the neutral polymer Ficoll. With normal ionic strength of
the perfusate (152 mM), the fractional clearance (
) was 0.0018 ± 0.0003 for Alb, 0.0033 ± 0.0003 for Oro, 0.090 ± 0.008 for Ova, and 0.062 ± 0.002 for aHRP. These values were all
lower than for Ficoll of similar hydrodynamic size; e.g., Ficoll 36 Å was >20 times higher than for albumin. Low ionic strength (34 mM) increased size selectivity as for anionic proteins and Ficoll fell, suggesting a reduction in
small-pore radius from 44 ± 0.4 to 41 ± 0.5 Å,
P < 0.01. In contrast, low I reduced the charge
density of the membrane, 
, to one-quarter of the 20-50 meq/l
estimated at normal I. These dynamic changes in seem to be due to
volume alterations of the charged gel, fluid shifts that easily are
accounted for by the changes in electroosmotic pressures. The finding
that low ionic strength induces inverse effects on size selectivity and
charge density strongly suggests that separate structures of the
glomerular wall are responsible for the two properties.
capillary permeability; kidney glomerulus physiology; charge; endothelium
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