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The influence of molecular configuration on the filtration of macromolecules across glomerular capillary walls was examined by comparing fractional clearances of two uncharged polysaccharides of distinctly different molecular configuration in the Munich-Wistar rat. The macromolecules employed were dextran, a slightly branched polymer of glucopyranose, and ficoll, a highly cross-linked copolymer of sucrose and epichlorohydrin. Differences in effective shape between these two polymers were determined from measurements of several physical properties of aqueous solutions containing either dextran or ficoll. It was found that dextran is best represented as a prolate ellipsoid with axial ratios of 4, 9, and 16 for molecules with Stokes-Einstein radii of 22, 32, and 40 A, respectively. On the other hand, ficoll is more closely approximated as spherical since the axial ratio was found to be between 1 and 2 for all molecular sizes. Fractional clearances of dextran and ficoll ranging in effective radius from 18 to 44 A were determined in each of seven Munich-Wistar rats. Fractional clearances of dextran were found to be greater than those of ficoll, the difference being significant for molecular radii ranging from 24 to 44 A. In addition, as shown previously for dextran, ficoll was found to be neither secreted nor reabsorbed by the renal tubules. These results, therefore, suggest that in addition to molecular size and charge, molecular configuration is also a determinant of the filtration of macromolecules across the glomerular capillary wall.

To investigate the mechanism(s) of increased filtration of serum proteins after glomerular injury, polydisperse samples of uncharged [(3)H]dextran (D) or anionic [(3)H]dextran sulfate (DS) were infused into 14 control and 16 puromycin aminonucleoside- (PAN) treated Munich-Wistar rats. Fractional clearances of D or DS ranging in radius from 18 to 42A were determined in these rats, together with direct measurements of the forces governing the glomerular filtration rate of water. Whole kidney and single nephron glomerular filtration rates were approximately 40% lower in PAN-treated rats, relative to controls, due mainly to a marked reduction in the glomerular capillary ultrafiltration coefficient and, to a lesser extent, to a small reduction in glomerular plasma flow rate as well. In PAN-treated rats, as in normal controls, inulin was found to permeate the glomerular capillary wall without measurable restriction, and both D and DS were shown to be neither secreted nor reabsorbed. Fractional clearances of uncharged D were reduced after PAN administration, falling significantly for effective D radii from 22 to 38A. Utilizing a theory based on macromolecular transport through pores, these results indicate that in PAN-treated rats, effective pore radius is the same as in controls, approximately 44A. In PAN nephrosis, however, the ratio of total pore surface area/pore length, a measure of pore density, is reduced to approximately one-third that of control, due very likely to a reduction in filtration surface area. In contrast to the results with uncharged D, fractional clearances of DS were found to increase after PAN administration for all DS radii studied. These results with D and DS suggest that proteinuria in PAN nephrosis is due, not to an increase in effective pore radius or number of pores, but rather to a diminution of the electrostatic barrier function of the glomerular capillary wall, thereby allowing increased passage of polyanions such as DS and albumin.

To investigate the mechanism(s) of angiotensin II-induced proteinuria, polydisperse [3H]dextran (D) (radius = 18-42 A) was infused into seven Munich-Wistar rats before and during intravenous infusion of angiotensin II (AII), 0.35 microgram/kg per min. During AII infusion, UprotV rose approximately twofold, and the fractional clearances of D [(U/P)D/(U/P)In] increased significantly for dextrans with radii greater than 22 A. Single nephron filtration fraction increased, due to a measured rise in the glomerular transcapillary hydraulic pressure difference from 34 to 43 mmHg. Near constancy of single nephron glomerular filtration rate resulted, however, from the offsetting effect of a decrease in glomerular plasma flow rate from 83 to 60 nl/min. These measured hemodynamic changes were found, by the use of pore theory, to account to a large extent for the measured increases in (U/P)D/(U/P)In. In seven other rats, fractional clearances of polyanionic dex-ran sulfate (a more reliable marker of albumin filtration than D) were also found to increase significantly with AII, suggesting that the proteinuria induced by AII can be explained, in large part, by hemodynamic factors.

Developments in the hydrodynamic theory of solute transport through porous membranes are reviewed, with emphasis on their application to macromolecule movement across capillary walls. A model that treats the capillary wall as a barrier containing uniform cylindrical pores, and permeating solutes as hard spheres, is shown to be successful in describing the size-selectivity of the glomerulus. Influences of various hemodynamic perturbations on solute transport are also accounted for by this approach. Possible extensions and modification of the theory, to account for the influence of molecular charge and other factors on glomerular permeability properties, are discussed.

In addition to molecular size, at least two other factors influence the transglomerular passage of macromolecules. The hemodynamic determinants of glomerular filtration rate affect macromolecule transport by altering the volume flux through the glomerular capillary wall and the profile of macromolecule concentrations along a glomerular capillary. The electrostatic properties of the glomerular capillary wall markedly restrict the passage of circulating polyanions, relative to neutral macromolecules of similar size. In the normal animal, the combined effects of these various mechanisms ensure that only very small quantities of plasma proteins are filtered. In experimental proteinuric conditions such as nephrotoxic serum nephritis and puromycin-induced nephrosis, reduction in the fixed negative charges on the glomerular capillary wall appears to be largely responsible for the increased transglomerular passage of anionic macromolecules such as albumin. Other evidence suggests that hemodynamic factors may contribute to proteinuria under certain circumstance. The possibility of an increase in number and/or radius of glomerular "pores" being responsible for proteinuria, perhaps the most intuitively obvious and widely held view, has yet to receive direct experimental support.

We have quantified the concentration profile of 125I-labeled rabbit albumin in the avascular intima and media of the rabbit descending thoracic aorta following intravenous injection under control and acute hypoxic conditions in vivo. Our purpose was to determine if alterations occurred in the transmural concentration profiles which could be attributed to hypoxia-induced changes in the permeability of the intimal endothelium to plasma-borne macromolecules. The profiles were obtained with frozen serial sections of the aorta from experiments of 30 min duration. Acute hypoxia was induced by addition of nitrogen to the breathing mixture. The hypoxia resulted in arterial pO2 values of 23--32 mm Hg while the arterial pO2 in the control animals ranged from 80 to 88 mm Hg. All animals were under sodium pentobarbital anesthesia. The results revealed no detectable changes in the concentration profile in the inner media accompanying hypoxia. However, increases in the label concentration in the outer media of the hypoxic animals suggested either dilation or increased permeability of the adventitial blood vessels.

To examine the electrostatic effects of fixed negative charges on the glomerular capillary wall, polydisperse [(3)H]DEAE dextran, a polycationic form of dextran, was infused into 10 Munich-Wistar rats. Fractional clearances of DEAE ranging in radius from 18 to 44A were determined in these rats, together with direct measurements of the forces and flows governing the glomerular filtration rate of water. These results were compared with data previously obtained in Munich-Wistar rats receiving tritiated neutral dextran (D) and polyanionic dextran sulfate (DS). Measured values for the determinants of the glomerular filtration rate of water in rats given DEAE were found to be essentially identical to those in rats given either D or DS. In addition, DEAE was shown to be neither secreted nor reabsorbed. Fractional clearances of polycationic DEAE were increased relative to both D and DS, the increase relative to D being significant for effective molecular radii ranging from 24 to 44A. Fractional DEAE clearances were also measured in a separate group of six Munich-Wistar rats in the early autologous phase of nephrotoxic serum nephritis (NSN). Fractional DEAE clearances in NSN rats were reduced significantly, relative to values measured in normal rats, for effective DEAE radii ranging from 18 to 42A. Moreover, in NSN rats, fixed negative charges on the glomerular capillary wall were greatly reduced, relative to non-NSN rats, as evidenced by a reduction in intensity of colloidal iron staining. Thus, in NSN rats, DEAE clearances were essentially indistinguishable from values obtained with both neutral D and polyanionic DS.