ABSTRACT. This study examines the Na+-dependent accumulation of the &amino acid, taurine, by brush border membrane vesicles isolated from nursing animals compared to uptake in adult animals. The diets fed to the mothers nursing these pups were altered so as to provide a low sulfur amino acid intake or a high taurine diet as well as conventional sulfur amino acid intake. Taurinuria is greater in nursing animals than in adult controls, but animals of all ages respond to exposure to the low sulfur amino acid intake by conservation of taurine and to the high taurine diet by hyperexcretion of taurine. Taurine uptake at 10 pM by brush border membrane vesicles is influenced by age in all groups and by diet in 14-and 21-day-old animals. A precession of uptake is seen both in terms of initial and peak rate of uptake with the lowest values in 7-day-old animals to the highest in adult. Greater brush border membrane vesicle uptake is found in 14-and 21-day-old rats after exposure to the low sulfur amino acid intake and reduced uptake after the HTD, whereas no dietary influence on uptake was found in 7-day-old rats. Neither the pattern of the time course of uptake nor the uptake values at equilibrium (45 min) are affected by age or diet. Kinetic analyses of concentration dependent uptake show that the maturational process involves a change in the Vmax of initial uptake. Kinetic analysis of the adaptive response reveals an increase in Vmax after low sulfur amino acid intake and a decline after high taurine diet in 14-and 21-day-old pups, but not in 7-day-old pups. Uptakes at high taurine concentrations (5 mM) which are 10-fold higher than the Km are uninfluenced by age or diet. This study indicates that the physiologic taurinuria of immature rats may relate, in part, to a lower rate of uptake at the brush border surface, but that after 1 wk and before 2 wk of age the kidney can adapt to changes in sulfur amino acid intake. (Pediatr Res 202390-894,1986 The renal tubular epithelium is involved in the reabsorption of various ions and nutrients, including amino acids that arise by virtue of filtration across the glomerular basement membrane. A "physiologic" aminoaciduria is evident in the young of all mammalian species. On the surface, aminoaciduria seems inappropriate in view of the need for a young, rapidly growing animal to conserve compounds that are important in growth (I, 2). Our lab has been concerned with those factors that contribute to the development of amino acid transport processes in the maturing animal and we hope to understand more fully whether maturation of the transport process can be accelerated by various physiologic manipulations (3-7). We have used as a transport probe, the p-amino acid, taurine, since one can manipulate the dietary intake of sulfur amino acids which results in a renal adaptive response for taurine (5,(8)(9)(10)(11)(12)(13).Full grown rats fed a limited sulfur amino acid intake reduce their urinary taurine excretion, a finding which is paralleled in vitro by augmented uptake of taurine by c...