Ohlson, Maria, Jenny Sö rensson, and Bö rje Haraldsson. Glomerular size and charge selectivity in the rat as revealed by FITC-Ficoll and albumin. Am J Physiol Renal Physiol 279: F84-F91, 2000.-The fractional clearances () for FITC-Ficoll and albumin were estimated in isolated perfused rat kidneys in which the tubular activity was inhibited by low temperature (8°C) and/or 10 mM NH 4 Cl. The Ficoll data were analyzed according to a two-pore model giving small and large pore radii of 46 Å and

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... Å, respectively. The estimated negative charge density was 35-45 meq/l at 8°C. Perfusion with erythrocyte-free solutions of kidneys at 37°C reduced glomerular size and charge permselectivity. Thus the large pore fraction of the glomerular filtrate (f L ) was 1.64% at 37°C compared with 0.94% at 8°C. The for albumin was four times higher at 37°C than at 8°C (0.86% vs. 0.19%, respectively). NH 4 Cl caused further irreversible damage to the glomerular barrier. We conclude that there are no deleterious effects on the glomerular barrier of a reduction in temperature from 37°C to 8°C. Therefore our data seem to disprove the hypothesis of low glomerular permselectivity and transtubular uptake of intact albumin and support the classic concept of a highly selective glomerular barrier. capillary permeability; macromolecular transport; two-pore model THE MAINTENANCE OF AN INTACT glomerular barrier to macromolecules and in particular albumin plays a pivotal role for the electrolyte and fluid balance in the body. However, the properties of this intricate membrane are not yet completely understood, and even less is known about the disturbances leading to proteinuria. There are, for example, controversies concerning which glomerular layer constitutes the principal barrier: the endothelium, the basement membrane, or the podocytes. The classic dextran studies by Chang and associates (5-7) revealed that small molecules are freely filtered across the membrane, whereas the fractional clearance () for molecules with Stokes-Einstein radius (a SE ) Ͼ42 Å approaches zero (6). In this context, the term "Stokes-Einstein radius" is always used to describe molecular size, since it is derived from the free diffusion constant. Also, the passage of anionic molecules is restricted, whereas that of cationic ones seems to be enhanced (5). Recently, an alternative hypothesis of glomerular permselectivity was presented (29). Isolated kidneys, removed by en bloc dissection, were perfused with recirculated filtered 5% BSA in a Krebs-Henseleit buffer at 37°C. The tubular reabsorption was inhibited by the use of various drugs, i.e., NH 4 Cl. Dextrans with a SE of 26-50 Å were used to determine whether the glomerular permselectivity was unaffected by the drugs. The for albumin in the control situation was 0.75-0.9%, increasing to ϳ7% when the drugs were used. To explain these high values, the authors suggested that a new transtubular cell pathway must be responsible for the return of intact albumin from the tubular lumen to the blood (29). Also, the glomerular charge selectivity was suggested as being insignificant. However, the use of dextran as a transport probe for the ideal neutral sphere has been questioned. Rennke et al. (30) used the protein horseradish peroxidase (HRP) and found that the values for its cationic, neutral, and anionic forms were less than those for dextran with similar size and charge. Dextrans form random-coiled spheres in free solution and are vulnerable to deforming forces. It was suggested that dextran is subjected to unfolding during convectional transport across the glomerulus. Dextran would thus behave as if it was of smaller dimensions than assumed. Oliver et al. (27) found that Ficolls of various radii had a lower than dextrans of equal a SE . This implies that Ficoll might be a better probe for the measurement of the equivalent small and large pore radii. Also, Blouch et al. (3) found that over a molecular radius interval of 20-70 Å, for a given Ficoll was uniformly lower than the corresponding for a dextran of equal molecular radius, both in healthy and nephrotic humans. Solute shape is in fact highly important for its glomerular passage and may actually outweigh size and charge (22). One way of inhibiting the tubular activity is low temperature. Reduced temperature inhibits tubular function as well as energy consumption and myogenic tone (8, 12), and it reduces protease activity without detectable changes on capillary permeability (33).