Abstract
The unidirectional influx of folic acid across the mucosal border of hamster duodenum and rat jejunum was determined. Influx follows saturation kinetics, is sodium-dependent, and is inhibited by methotrexate and is sodium-dependent, and is inhbited by methotrexate and folinic acid in the mucosal bathing solution. In hamster duodenum, the maximal influx is 1.2 nmol/(cm2.h), and the folic acid concentration required to give a half maximal influx (Km) is 7.2 micron.. At mucosal folic acid concentration of 1.5 micron, influx is reduced at least 65% by removal of sodium from the bathing solution. The influx process is significantly inhibited by cyanide and 2,4-dinitrophenol. The possibility was evaluated that the acidic microclimate at the brush border regulates the rate of folic acid transport and that inhibition of transmural transport by sodium-free media, ouabain, and methotrexate is brought about indirectly by an increase in pH of the microclimate. The data favor the alternative concept of a sodium-dependent carrier mechanism for entry of folic acid into the cells. The information presented is consistent with active transport of folic acid by a sodium-gradient mechanism, but additional information will be necessary to substantiate such a model.
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