Comparison of in vitro jejunal uptake of L-valine and L-lysine in the rat during maturation.

The maturational characteristics and patterns of absorption of the neutral amino acid L-valine were compared with those of the basic amino acid L-lysine using in vitro preparations of everted sacs obtained from the jejunum of suckling (2 weeks old), weanling (3 weeks old), and adolescent (6 weeks old) rats. Absorption rates determined as net transport into sac fluid and mucosal uptake (mumol/h/g protein or weight of mucosal scrapings) for both valine and lysine were greater in the suckling than in the weanling or adolescent rats. This appeared to be true for both the carrier-mediated and the diffusive processes for absorption, determined from the relationship between the observed absorption rates and the corresponding mucosal incubation medium amino acid concentrations. The major decline in rate of absorption occurred before the period of weaning. Between the period of suckling and adolescence the carrier-mediated absorption decreased by 55% for valine and by 90% for lysine, suggesting that during maturation of the jejunum the number of the carrier macromolecules responsible for absorption of valine and lysine per unit protein or weight of mucosa decreases with increasing age.