Pyridoxine transport across the turtle, rat and rabbit intestines

Abstract

1. The mucosal-to-serosal and serosal-to-mucosal fluxes of pyridoxine were equal when equal low concentrations of pyridoxine bathed the serosal and mucosal surfaces of the isolated intestines of the turtle and rabbit. At higher concentrations however the fluxes were not solely explainable on the basis of simple passive diffusion. 2. The presence of alanine in the incubation media did not alter the transmural pyridoxine flux rates across the turtle or rabbit intestine. 3. The initial uptake rate of pyridoxine in turtle mucosal strips was linear with physiological medium pyridoxine concentration, and curvilinear with concentrations above 5 mM. 4. Pyridoxine did not accumulate in the epithelial cells of turtle, rat or rabbit intestine. The intracellular to extracellular distribution ratio at low concentrations of the vitamin was about unity but at higher concentration intracellular pyridoxine remained lower than extracellular in spite of extended periods of incubation. 5. Amino acids had no effect on the steady-state uptake of pyridoxine by the epithelial cells of turtle, rat or rabbit intestine. 6. The results indicate that pyridoxine at low concentrations is transported by simple passive diffusion but at higher concentrations the vitamin limits its own permeation through the cell membrane. 7. The transport of pyridoxine into the cells of the turtle, rat or rabbit intestine, is not altered by amino acids. This disproves the possibility of a complete interdependence between the transport of the vitamin and these solutes.