Renal Transport of Endogenous Amino Acids

Abstract

In the late neonatal period of male Wistar rats (10 days old) concentrations in plasma were higher for 5 of 24 amino acids compared to adult animals (2 months old): β-alanine, tyrosine, glycine, histidine, and the dipeptide anserine. The plasma concentrations of tryptophan, valine and leucine were lower in young than in adult rats. The renal clearances of amino acids were lower in young rats, both in relation to 1 g b.w. and related to 1 g kidney weight. In the latter case the differences became more distinct because the relative kidney weight was higher in young than in adult animals (1.17 ± 0.07 vs. 0.82 ± 0.03 g/l00 g b.w.) and significant age differences in renal water content did not exist. The apparently more effective tubular reabsorption capacity in young rats can be explained as follows: Because of the significantly lower GFR in 10-day-old rats compared with adults (0.46 ± 0.03 vs. 1.10 ± 0.09 ml/ min/1 g kidney), the glomerularly filtered load of amino acids is generally lower in young rats (exceptions: tyrosine glycine). Therefore, the amino acid transporting carrier systems are able to reabsorb the absolutely lower amounts of amino acids from the ultrafiltrate in immature animals. This hypothesis is supported if one relates the renal amino acid clearance to the clearance of inulin. In this way it is possible to show that there are absolutely no differences between both age groups indicating mature transport systems for endogenous amino acids as early as in 10-day-old rats. The urinary excretion of amino acids in young and adult animals was less than 1-5% of the filtered load and age differences were not significant.