Restricted transport of cationic macromolecules across intestinal capillaries

The charge-selective properties of intestinal capillaries were investigated by measuring steady-state lymph-to-plasma concentration ratios (L/P) of endogenous and exogenous macromolecules of comparable molecular size but different charge. The steady-state L/P values for endogenous lactate dehydrogenase isoenzymes (LD1-LD5) decreased with increasing isoelectric point. The osmotic reflection coefficient for LD1, the most negative isoenzyme studied, was 0.71 +/- 0.01; that for the most positive isoenzyme (LD5) was 0.95 +/- 0.01. The steady-state L/P for an exogenous molecule, neutral dextran, was 0.51 +/- 0.04; the L/P for the positively charged dextran of similar size was 0.25 +/- 0.04. The results indicate that intestinal capillaries behave as a positively charged barrier that reduces blood-lymph exchange of cationic macromolecules and enhances the exchange of anionic molecules.