Determinants of the transglomerular passage of macromolecules.

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.