Concentration dependence of protein permeability across the canine visceral pleura

Water and protein movement across the pulmonary endothelial-visceral pleural membrane of spontaneously breathing anesthetized dogs was analyzed to determine if the protein concentration at the microvascular membrane (Cpro) influences microvascular permeability. The left lung was enclosed in a water-impermeable membrane, creating a visceral pleural space (VPS); fluid and solute fluxes were determined as the filtration or reabsorption of water and protein in the VPS. The plasma protein concentration was experimentally varied by plasmapheresis with saline replacement while the pleural fluid protein concentration was varied by introducing different concentrations of plasma mixed with saline into the VPS. Hydrostatic pressures were maintained within a physiologic range (pulmonary capillary pressure 12.1-16.1 mm Hg). The plasma protein concentration fell as low as 1.98 g/dl, and Cpro, calculated as the mean of the plasma and pleural fluid protein concentrations, ranged from 1.73 to 6.23 g/dl. The relationship between Cpro and the apparent homoporous diffusional permeability for protein (Phs), Phs(cm/sec X 10(-6] = 0.95 Cpro (g/dl) + 2.28, was highly significant (r = 0.87, P less than 0.01). In contrast, the hydraulic conductivity was not affected by a reduction in Cpro to this level (r = 0.21, P greater than 0.4). Although the solute concentration at the endothelial membrane should be considered when evaluating changes in protein permeability, under most experimental conditions the magnitude of this effect will be small.