Paracellular transport of inorganic and organic ions across the rat ileum.

The properties of the absorption route via the tight junctions and the intercellular spaces (the paracellular pathway) in rat ileum were investigated with inorganic and organic ions. Isolated ileal epithelial sheets were used, mounted between two chambers. Biionic and diffusion potentials of the ions were measured with Ag/AgCl electrodes. From the Goldman-Hodgkin-Katz equation relative permeabilities were calculated and selectivity isotherms constructed. The paracellular pathway behaved as an aqueous pore with cation selectivity. The permeability order is K+ congruent to NH+4 greater than Na+ greater than Cl congruent to Li+ greater than tetramethylammonium+ greater than tetraethylammonium+ congruent to choline+ greater than carbachol+. TAP+ (2,4,6-triaminopyrimidinium) appeared, in contrast to the situation in other tissues, not to be an inhibitor of this pathway. The permeability for organic cations is high: compounds with a mol. wt of about 150 have a permeability relative to sodium of 0.45. Together with the results of previous studies, in which the transcellular aspects of organic cation transport were investigated, a model for organic cation absorption is developed.