Jejunal Uptake of Tritiated Folic Acid in the Rat Studied by in vivo Perfusion

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Abstract
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 Vmas value of 0.654 µmoles/cm/hour. Exposure of the lumen to 11 µM methotrexate, an inhibitor of dihydrofolate reductase, prior to and during perfusion of 1.1 µM3H-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.

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