Tryptophan transport by isolated newborn rabbit jejunum.

Segments of newborn or adult rabbit jejunum were mounted as flat sheets in flux chambers. Mucosal-to-serosal (Jm leads to s) and serosal-to-mucosal (Js leads to m) fluxes of tryptophan (Try), phenylalanine (Phe), and alanine (Ala) were measured in the absence of electrochemical gradients. In the presence of Na, J Try s leads to m increased linearly with concentration, whereas the relationship of J Try m leads to s and concentration contained both linear and saturating components. The relationship of net Try flux and concentration was in accord with Michaelis-Menten kinetics. Ouabain (10(-3) M) abolished net Na fluxes in both newborn and adult rabbit jejunum and abolished net Try flux in the adult, but did not reduce net Try flux to zero in the newborn. In the presence of Na-free solutions small net fluxes of Try, Phe, and Ala occurred in the newborn. Addition of 10(-4) M 2,4-dinitrophenol abolished net Try flux in ouabain-treated newborn rabbit jejunum that was bathed by either Na-free or Na-containing solutions. In the presence of Na, Phe and Ala reduced J Try m leads to s, and Try reduced the fluxes of Phe and Ala. Phe inhibited J Try m leads to s in Na-free solutions, and Try reduced J Phe m leads to s. These results suggest that the newborn rabbit jejunum resembles the adult in that Try is absorbed primarily by a Na-dependent system. These results show that the newborn differs from the adult in that Try is absorbed by a system that is independent of Na but requires metabolic energy.