Glomerular filtration in the isolated perfused kidney. I. Sieving of macromolecules.

The permeability of the glomerular capillary wall to neutral macromolecules was studied in isolated perfused rat kidneys. Pluronic F108 (BASF, Wyandotte, MI, USA), a polyoxyethylene-polyoxypropylene block copolymer of mol weight approximately 14,000, was used as plasma expander. Pore theory was applied to the fractional clearances of Pluronic F108 and dextran (mol weight 19,400) molecules measured both as a function of glomerular filtration rate. Using the pore model of Verniory et al. [30] the effective pore radius (60.9 A) and the ratio of total pore area and pore length (4.0 cm/nephron) were estimated, and a hydraulic permeability coefficient KF (0.036 nl/s . mm Hg) was calculated. There was no significant difference between the fractional clearance of Pluronic F108 obtained with different Pluronic F108 concentrations over the range 15-35 g/l, hence with largely differing osmotic pressures. It was concluded that the sieving properties of the glomerular membrane of the isolated perfused rat kidney are not detectably different from those in the intact rat, at least in the case of uncharged macromolecules.