Amino Acid Transport Pathways in the Small Intestine of the Neonatal Rat

Abstract

ExtractThe 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 HIM 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. SpeculationThe 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. IntroductionThe quality and quantity of dietary protein which would result in optimal growth and development of infants at a reasonable cost is a subject that has been discussed by nutritionists for several decades. Such discussion stimulated investigation of intestinal transport at, or shortly after birth [4-6, 13, 15, 30].A recent article by the authors [15] characterized the 713

Developmental Pattern of Sugar and Amino Acid Transport in the Postnatal Rat Small Intestine

“…Although the high level of protein biosynthesis and the energy required for this synthesis can explain the necessity for an increased transport of amino acids and sugars in young animals, the manner in which the accelerated transport is mediated should be considered. Valine transport in the 2-day-old rats was found to be inhibited by leucine and galactose and dependent on the presence of sodium ions [13], indicating that the characteristics of the valine carrier system in young animals are similar to those found in adult animals [22,23]. Further evidence that the increased transport in young animals was not due to the transient presence of structurally modified transport carriers which may bind valine more efficiently comes from kinetic studies reporting that apparent K m for valine accumulation in 2-day-old rats and in adult rats was the same [26].…”

Metabolic Properties of Neonatal Transport

“…This may be explained by the changing specific activity of transport sites during postnatal maturation concomitant with increasing membrane area (1 5, 19). Two major transport systems for amino acids have been observed in intestines of 2day-old rats (16,17). The most active of these systems has the highest affinity for neutral L-amino acids with lipophilic side chains.…”

“…The second transport pathway has affinity for basic amino acids. The pathways mediating the transport of the acidic amino acids, imino acids, and N-substituted amino acids, however, have been shown t o have minor activity in the neonatal rat (17).…”

“…In continuing our study of transport in the 2-day-old rat intestine (16,17,26) we are now examining the metabolic properties of the increased intracellular accumulation of a prototype amino acid (L-leucine) and sugar (D-galactose) in order t o determine whether the transport properties of these metabolites in neonates are fundamentally different from those found in adult animals.…”