Permeability of intestinal capillaries to endogenous macromolecules.

Steady-state lymphatic and plasma protein concentration (CL and CP) and lymph flows were analyzed in an autoperfused cat ileum preparation at venous outflow pressures of 0, 10, 20, and 30 mmHg. Albumin and nine endogenous protein fractions were analyzed in lymph and plasma by polyacrylamide gradient gel electrophoresis. The osmotic reflection coefficient (sigma d = 1--CL/CP when CL/CP is no longer influenced by capillary filtration rate. The sigma d values acquired increased as the molecular radius of the protein fraction increased. Permeability-surface area products were estimated for the various protein fractions by applying the acquired data to the Kedem-Katchalsky protein flux equation. A comparison of the permeability-surface area products at various venous pressures suggests a reduction in capillary surface area as venous pressure is increased to 20 and 30 mmHg. The results suggest that intestinal capillaries selectively restrict macromolecules to a greater degree than the continuous type of capillaries of many other organs.