Divergent membrane maturation in rat kidney: Exposure by dietary taurine manipulation

Taurinuria is characteristic of the immature rat. The excessive taurine loss could be the result of brush border or basal lateral membrane immaturity. The β-amino acid, taurine, and D-glucose were examined using isolated brush border membrane vesicles (BBMV), slices and tubules prepared from 28-day-old rats. In BBMV, 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 of taurine uptake, higher urinary taurine concentrations are found in younger rats. The efflux of taurine from slices and tubules was much slower than in adults and probably accounts for the taurinuria of young animals. A diet low in methionine and taurine (LTD) given for seven 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, despite ontogenic changes in taurine excretion, the adaptive response to dietary alteration is present at the brush border surface. The finding of adaptation to altered dietary amino acid intake in immature (28-day) rats indicates that animals can conserve or hyperexcrete an amino acid at a young age. This adaptation occurs despite increased taurine excretion in the basal state in immature animals and provides evidence that the immature kidney can respond to this environmental stress. Further, the brush border surface is the site of this adaptive response, even though the physiologic taurinuria of immaturity is not expressed at that surface, but rather represents an efflux block at the basolateral surface which alters with maturity.