Postnatal amino acid uptake by the rat small intestine. Energetics of membrane transport systems for amino acids in the developing jejunum.

The energetics of amino acid uptake by the developing small intestine was investigated in vitro. L-valine, L-leucine, L-phenylalanine, L-methionine, L-lysine and L-arginine were all actively transported by the newborn rat jejunum. Metabolic inhibitors (e.g. 2,4-dinitrophenol) significantly reduced uptake of all amino acids but uptake against a concentration gradient was not totally abolished. Uptake of all amino acids was reduced at low[Na+]. Inhibition of transport of neutral amino acids by reduced luminal [Na+] was greater than that of basic amino acids, and the tissue was barely able to concentrate the neutral amino acids. [Na+] affected the Michaelis constant (Km) of neutral transport systems for their substrates; for the basic amino acids Km values were unaffected by the presence or absence of Na+. Ouabain significantly inhibited neutral amino acid uptake but had no effect on L-lysine or L-arginine uptake. These results are discussed in terms of the Na+ gradient hypothesis for amino acid transport, and the site of energy input to active transport. The role of glycolysis in providing energy for intestinal transport in the neonatal rat and the efficiency of Na+ dependent and independent transport mechanisms are considered. It is concluded that the energetics of amino acid transport systems in neonatal and adult rats are essentially similar.