Developmental Aspects of Renal β-Amino Acid Transport. IV. Brush Border Membrane Response to Altered Intake of Sulfur Amino Acids

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Abstract
Summary: Taurinuria is characteristic of the immature rat. The capacity of the kidney to accumulate the β-amino acid taurine and D-glucose was examined using isolated brush border membrane vesicles (BBMV) prepared from 28-day-old rats. Taurine accumulation was inversely proportional to osmolarity, indicating uptake rather than binding, and taurine accumulation was Na+-dependent. BBMV from 28-day rats did not accumulate D-glucose to the same degree as in adult BBMV, and the initial rate of uptake was slower. Taurine uptake had a similar Km and Vmax in BBMV from immature rats. Despite similarities in the kinetics taurine uptake, higher urinary taurine concentrations are found in younger rats, suggesting that other factors, such as an efflux block, account for the taurinuria of young animals. A diet low in methionine and taurine (LTD) given for 7 days resulted in a lower excretion and fractional excretion of taurine than in animals fed a normal sulfur amino acid diet (NTD). A high taurine diet (HTD) causes excessive taurinuria. These patterns of excretion are reflected at the brush border membrane surface with greater uptake after the LTD and reduced uptake after the HTD. A kinetic analysis of adult and 28-day-old animal BBMV reveals that the Vmax of accumulation is altered by diet, whereas the Km remains unchanged. The Vmax is higher in BBMV from LTD animals and lower in BBMV from HTD animals. The kinetics of uptake are similar in adult and 28-day-old rat vesicles on a given diet. Thus, in addition to ontogenic changes in taurine excretion, there is an adaptive response to dietary alteration present at the brush border surface.