Amino acid transport pathways in the small intestine of the neonatal rat.

Abstract

Extract: The activity of amino acid transport pathways in the small intestine of the 2-day-old rat was investigated. Transport was determined by measuring the uptake of 1 mM concentrations of various amino acids by intestinal segments after a 5- or 10-min incubation and it was expressed as intracellular accumulation. The neutral amino acid transport pathway was well developed with intracellular accumulation values for leucine, isoleucine, valine, methionine, tryptophan, phenylalanine, tyrosine, and alanine ranging from 3.9–5.6 mM/5 min. The intracellular accumulation of the hydroxy-containing neutral amino acids threonine (essential) and serine (nonessential) were 2.7 mM/5 min, a value significantly lower than those of the other neutral amino acids. The accumulation of histidine was also well below the level for the other neutral amino acids (1.9 mM/5 min). The basic amino acid transport pathway was also operational with accumulation values for lysine, arginine and ornithine ranging from 1.7–2.0 mM/5 min. Accumulation of the essential amino acid lysine was not statistically different from that of nonessential ornithine. Accumulation of aspartic and glutamic acid was only 0.24–0.28 mM/5 min indicating a very low activity of the acidic amino acid transport pathway. Accumulation of sarcosine and betaine was about 0.5 mM/5 min, which indicates a low activity for the imino acid-glycine pathway. The presence of 1 mM alanine significantly (P < 0.01) increased the accumulation of lysine by 28.5%, which suggests the presence of an exchange transport system. These results show that by operation of the neutral and basic amino acid transport systems, the neonatal rat can effectively absorb all the essential amino acids; however, accumulation of individual amino acids appears to be governed by structural rather than nutritional considerations. Speculation: The data presented demonstrate the differential ability of the neonatal rat to absorb various ammo acids. This finding is of potential importance in the formulation and evaluation of synthetic protein diets.