Intestinal amino acid absorption in development

Abstract

1. An in vitro preparation was employed to investigate uptake of a number of neutral and basic amino acids by the developing rat small intestine. 2. The kinetics of uptake of these substrates were determined from birth to maturity and changes described in the affinity for and rate of uptake by the jejunum particularly around the time of weaning. All the amino acids had a higher affinity for uptake and were accumulated to a greater extent by the jejunum during the first three weeks of life as compared with older animals. Similar kinetic profiles for certain amino acids suggested that they shared common uptake systems. 3. The specificity of neutral and basic amino acid uptake was examined developmentally by employing inhibition kinetics. The results suggested that while distinct neutral and basic uptake systems were present at birth, there was some functional overlap : neutral amino acids were able to use basic systems and vice versa. After three weeks of age, this picture changed and the uptake systems were much more defined. 4. Uptake of amino acids was found to depend to a large extent on energy derived from oxidative phosphorylation even at birth. This energy expenditure was directed toward maintaining a Na+ gradient across the enterocyte membrane and co-transport of neutral amino acids with Na+ thus promoted accumulation. The basic amino acids, however, were found not to be dependent on the existence of such an ion gradient but they did require the presence of Na+ ions for carrier binding, and their uptake demanded energy expenditure at some stage. 5. Precociously inducing the appearance of a functionally distinct absorptive cell was found to change the kinetics of basic amino acid uptake in the jejunum, and promote such specificity of uptake as was seen in older animals. 6. Changes in the developing small intestine itself have led to problems in the expression of results and these have been discussed at length in terms of their relation to these and other findings. 7. These results on the development of amino acid uptake systems are discussed in the context of morphological changes that occur postnatally in the small intestine, with particular reference to the appearance of a functionally distinct absorptive cell around the time of weaning, and are related to the physiological requirements of the neonatal animal.