Ontogenesis of intestinal transport of 5-methyltetrahydrofolate in the rat

Developmental aspects of the intestinal transport of 5-methyltetrahydrofolate (5-CH₃H₄-PteGlu) were studied in suckling (14-day-old), weanling (22-day-old), and adult (90-day-old) rats by use of the intestinal everted-sac technique. Mucosal-to-serosal transport of 0.5 μM 5-CH₃H₄PteGlu was linear with time for 40-min incubation and occurred at a rate of 0.035, 0.032 and 0.010 nmol·g initial tissue wet wt^-1 for suckling, weanling, and adult rats, respectively. The transport of 5-CH₃H₄PteGlu in all age groups was pH dependent (maximal at pH 6) and was higher in the jejenum than in the ileum. In all age groups the transport of 5-CH₃H₄PteGlu occurred by an active carrier-mediated system. The system was saturable; energy, temperature, and Na dependent; inhibited by structural analogues; and capable of accumulating the substrate against a concentration gradient. Kinetic parameters of the transport process, however, showed some difference. A progressive decrease in V(max) was observed from suckling to weanling to adult rats (5.1, 3.7 and 0.8 nmol·g initial tissue wet wt^-1·30 min^-1, respectively), while apparent K(t) was similar (2.2, 1.73, and 1.79 μM, respectively). This study demonstrates that the transport system of 5-CH₃H₄PteGlu in the rat is fully developed at the suckling age. The results also suggest that the activity and/or the number, but not the affinity, of the transport carriers decrease with maturation.