Jejunal uptake of triated folic acid in the rat studied by in vivo perfusion

The jejunal uptake of tritiated folic acid was measured in male Wistar rats by in vivo perfusion. The uptake of ³H folic acid was linear relative to concentrations between 4 nM and 1 μM, but between concentrations of 1 μM and 4.6 μM, uptake was similar, suggesting a saturable uptake process. A double reciprocal plot (Lineweaver Burk) yielded a Km value of 1.57 μM with a Vmax value of 0.654 μm/cm/hr. Exposure of the lumen to 11 μM methotrexate, an inhibitor of dihydrofolate reductase, prior to and during perfusion of 1.1 μM ³H folic acid resulted in a significant decrease of jejunal dihydrofolate reductase (folic acid reductase, EC 1.5.1.3) with no change in the jejunal uptake of ³H folic acid. There was no observable difference in jejunal dihydrofolate reductase nor of the uptake of ³H folic acid among rats fed normal diet, normal diet plus ethanol, a protein deficient diet, or a protein deficient diet plus ethanol. These studies suggest that the initial uptake of ³H folic acid by the jejunal mucosa is rate limited between concentrations of 1 and 4.6 μM, does not require metabolic reduction, and is not affected by protein deficiency or ethanol.