On the mechanism of folate transport in isolated intestinal epithelial cells.

The mechanism of folic acid (FA) uptake was studied in isolated intestinal epithelial cells prepared from 2- to 6-wk-old chicks. The cells accumulated FA, reaching a level of three- to fivefold that at equilibrium. In the presence of the metabolic inhibitors, NaN3 or KCN, FA was taken up only until equilibration while accumulation of FA was inhibited. Addition of these inhibitors at a steady state of FA accumulation caused a release of intracellular FA. The kinetics of FA uptake were found to be saturable (Km = 3.5 x 10(-5) M), indicating a carrier-mediated mechanism. The steady-state level of FA accumulation was higher as the concentration of NA+ in the medium increased from 0 to 120 mM. This stimulation of FA uptake by Na+ was not due to the stimulation of glucose uptake, because in experiments carried out in the presence of phlorizin, a glucose-transport inhibitor, FA accumulation was not diminished. It is suggested that FA is taken up by a Na+-coupled transport system.