Absorption of folic acid by everted segments of rat jejunum.

1. Everted rings of rat intestine were used to study the initial uptake rate of folic acid at various concentrations and incubation temperatures in vitro.2. Folic acid was accumulated linearly for periods of at least 30 min.3. The initial uptake rate was found to reach a constant value as the concentration of folic acid in the incubation medium was increased above 5 x 10(-6)M.4. Reducing the temperature of incubation from 37 to 27 degrees C gave small Q(10) values at either end of the concentration range.5. Transfer of folic acid into the serosal compartment of everted sacs was shown to undergo a rate reduction in the same concentration range.6. A mechanism for folic acid transport is suggested in which folic acid is converted to the neutral species at the mucosal surface in an acid microclimate.