Naomi Gusowski

Renal cortex taurine content regulates renal adaptive response to altered dietary intake of sulfur amino acids.

Rats fed a reduced sulfur amino acid diet (LTD) or a high-taurine diet (HTD) demonstrate a renal adaptive response. The LTD results in hypotaurinuria and enhanced brush border membrane vesicle (BBMV) accumulation of taurine. The HTD causes hypertaurinuria and reduced BBMV uptake. This adaptation may relate to changes in plasma or renal cortex taurine concentration. Rats were fed a normal-taurine diet (NTD), LTD, or HTD for 14 d or they underwent: (a) 3% beta-alanine for the last 8 d of each diet; (b) 3 d of fasting; or (c) a combination of 3% beta-alanine added for 8 d and 3 d of fasting. Each maneuver lowered the cortex taurine concentration, but did not significantly lower plasma taurine values compared with controls. Increased BBMV taurine uptake occurred after each manipulation. Feeding 3% glycine did not alter the plasma, renal cortex, or urinary taurine concentrations, or BBMV uptake of taurine. Feeding 3% methionine raised plasma and urinary taurine excretion but renal tissue taurine was unchanged, as was initial BBMV uptake. Hence, nonsulfur-containing alpha-amino acids did not change beta-amino acid transport. The increase in BBMV uptake correlates with the decline in renal cortex and plasma taurine content. However, since 3% methionine changed plasma taurine without altering BBMV uptake, it is more likely that the change in BBMV uptake and the adaptive response expressed at the brush border surface relate to changes in renal cortex taurine concentrations. Finally, despite changes in urine and renal cortex taurine content, brain taurine values were unchanged, which suggests that this renal adaptive response maintains stable taurine concentrations where taurine serves as a neuromodulator.

Factors affecting the transport of β-amino acids in rat renal brush-border membrane vesicles. The role of external chloride

The effect of a variety of ions and other solutes on the accumulation of the beta-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na+ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K+ or Li+ do not increase taurine accumulation more than Na+-free mannitol, except that the combination of external K+ and Na+ in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN- and NO3-) or less permeant (SO4(2-)), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl- stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO3 and LiSO4. Internal LiCl, regardless of the final Na+ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO3 or LiSO4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl- in taurine uptake in addition to Na+ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H+ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na+/H+ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na+ concentrations. Taurine uptake is blocked only by other beta-amino acids and in a competitive fashion. D-Glucose and p-aminohippurate at high concentrations (greater than 10(-3) M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of D-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal beta-amino acid transport system in brush-border vesicles and indicate a role for external Cl- in this uptake system.

Developmental aspects of renal beta-amino acid transport. V: Brush border membrane transport in nursing animals--effect of age and diet.

ABSTRACT. This study examines the Na+-dependent accumulation of the β-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 μM 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.

Developmental Aspects of Renal |[beta]|-Amino Acid Transport. IV. Brush Border Membrane Response to Altered Intake of Sulfur Amino Acids

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.

Developmental Aspects of Renal |[beta]|-Amino Acid Transport. VI. The Role of Membrane Fluidity and Phospholipid Composition in the Renal Adaptive Response in Nursing Animals

ABSTRACT. Accumulation of the β-amino acid taurine is higher in adult rat renal brush border membrane vesicles than in nursing animals, which relates to a higher initial rate Vmax. A low sulfur amino acid diet increases and a 3% taurine diet reduces the Vmax of Na+ -taurine cotransport in brush border membrane vesicles at all ages after 7 days as compared to values on a normal diet. To determine if changes in membrane fluidity account for these developmental and adaptive events, the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene was used to measure fluorescence anisotropy. A two-component, single break curve fit the data over the range 4 to 56° C. Values for the break temperature centered around 23° C, not different than a break temperature determined in adult membranes. The values for membrane polarization range from 0.311 to 0.329 in membranes from 7-, 14-, and 21-day-old pups exposed to each of the three diets, significantly lower than the value in adult membranes (p < 0.02). The slopes of each component, equivalent to they apparent energy of activation, did not differ in relation to diet. The ontogenic changes in taurine uptake by brush border membrane vesicles related to the exposure to different dietary sulfur amino acid levels are not related to changes in membrane fluidity using 1,6-diphenyl-1,3,5-hexatriene as a probe. However, there is a decrease in fluidity with age. Total phospholipid content falls postweaning, and the percent of total content of phosphatidyl choline and glycerol phosphate fall, and phosphatidyl serine and ethanolamine rise as the rat ages. The results indicate that alterations in the phospholipid composition occur during the process of maturation and that changes in the fluidity of brush border membranes may account for maturational differences in Na+-dependent taurine transport.

Studies on renal adaptation to altered dietary amino acid intake: reduced renal cortex taurine content increases the Vmax of taurine uptake by brush border membrane vesicles

Rats were placed on a normal taurine diet (NTD), low taurine diet (LTD) or a high taurine diet (HTD) for 14 days. β-Alanine was fed to half of the animals in each group and resulted in a lowered renal cortex taurine content. Brush border membrane vesicle (BBMV) uptake of taurine was higher after β-alanine feeding and was associated with an increase in Vmax of uptake. β-Alanine feeding to HTD animals also altered the Km of uptake, possibly since the load of sulfur amino acids (6% of diet) was high. As a control, glycine (3%) feeding for 8 days along with each diet did not alter the plasma or renal cortex content; BBMV uptake as well as Km and Vmax of taurine accumulation were minimally altered. Accordingly, ingestion of a non-sulfur-containing α-amino acid did not change β-amino acid transport. This study provides evidence that whole body taurine homeostasis is maintained in the presence of a taurine-depleting agent (β-alanine feeding) by an increase in the number of Na+-taurine uptake sites.

Cyclic AMP does not alter taurine accumulation by rat renal brush border membrane vesicles

Secondary hyperparathyroidism has been attributed to be responsible for the generalized aminoaciduria and phosphaturia of vitamin D deficiency. Since PTH acts in the kidney to generate cAMP, we explored the possibility that its synthetic analog, dbcAMP, would alter the renal transport of taurine (an amino acid lost in the urine in vitamin D deficiency) and Pi. Exposure of renal BBMV prepared from normal and vitamin D-calcium-deficient rats to dbcAMP at concentrations ranging between 10(-4) and 10(-7) M did not alter taurine uptake by these vesicles. Higher dbcAMP concentrations blunted uptake, but these concentrations reduced intravesicular volume, thus representing an artifact of osmolarity. Preincubation of BBMV with dbcAMP for times between 0 and 60 min at 0 or 25 degrees C also did not alter taurine accumulation. Hypotonic lysis of BBMV, allowing entry of the cyclic nucleotide, followed by isotonic resealing did not influence taurine uptake. The addition of potassium fluoride (to inhibit phosphodiesterase activity) and ATP (as an energy source) did not alter taurine accumulation at 60 sec. The uptake of Pi, which is influenced by PTH, was decreased by 25% following exposure to dbcAMP on the internal surface of the vesicle. These data indicate that the taurinuria observed in vitamin D deficiency is unlikely to be related to a PTH-induced increase in intracellular cAMP, unlike the changes in Pi transport, which is sensitive to cyclic nucleotides.

Renal Taurine Transport — Recent Developments

The renal proximal tubule, with its brush border surface is the site of the active accumulation of all amino acids, including taurine (19). Following filtration at the glomeru-lus, taurine resides in the ultrafiltrate of plasma which passes by the apical or brush border surface of the tubule. As in many other biological membranes, the transfer of taurine across the brush border membrane is Na+-dependent (5,20) and requires external Cl− (6). This transport of taurine can best be studied by employing isolated renal brush border membrane vesicles (BBMV) which permit an evaluation of factors which influence the uptake of taurine both in vitro by changing the chemical composition of the incubation medium and in vivo by exposure of animals to various manipulations (4–5). Our group has been interested in establishing how renal amino acid transport mechanisms adapt to changes in the level of taurine or its precursors in the diet. To carry out these studies, a rat model has been used and the accumulation of taurine by BBMV has been measured.

Membrane fluidity and phospholipid composition in relation to sulfur amino acid intake in brush border membranes of rat kidney.

ABSTRACT. The transport of ions and solutes across biological membranes may relate to changes in the lipid microenvironment of the membrane which could alter the activity or configuration of transport sites. Changes in the sulfur amino acid content of diets fed to young rats results in an increase in Na+-taurine symport in brush border membranes isolated from animals fed a low sulfur amino acid diet and a reduction in symport after a high taurine diet in comparison to uptake by membranes from normally fed animals (“the renal adaptive response”). We explored the possibility that diet-induced changes in brush border membrane symport relate to altered membrane fluidity and phospholipid composition in response to diet. An Arrhenius plot of initial rate (15 s) taurine uptake in breakpoint at 22° C, but no change in relation to dietary alteration. Fluorescence polarization data employing the probe 1-6-diphenyl-l,3,5-hexatriene best fits a two-phase linear model employing a computer model fitting program. Dietary manipulations did not change the breakpoint upper segment slope or lower segment slope after incubating fresh membranes with DPH over the temperature range 4–56° C. No change was evident in membrane phospholipid composition in relation to diet. This study indicates that the changes in initial rate Na+-taurine symport in relation to diet are less likely to be due to changes in the configuration of the transporter from an alteration of the lipid microenvironment of the membrane.

Studies on the Renal Handling of Taurine: Changes during Maturation and After Altered Dietary Intake

The main route for the removal of taurine from the body is by urinary excretion. As a consequence, the renal tubular transport and handling of taurine is important in regulating the taurine content in the remainder of the body. In our previous report, we have described the use of stop-flow, free-flow micropuncture, continuous microperfusion and endogenous clearance as techniques to examine the renal handling of taurine (6). This chapter also covered our studies on the accumulation of taurine by thin cortex slices of mouse (4) and rat (5) kidney. These studies indicate that under normal conditions taurine excretion is 4 to 10% of the filtered load and that uptake by slices occurs by two processes: a low-Km, high-affinity uptake system and a high-Km, low-affinity uptake system. The kinetics of taurine accumulation are determined by Lineweaver-Burk analysis and the Eadie-Augustinson transformation. Other characteristics of taurine accumulation include transport by the β-amino acid transport system (such that β-amino acids but not α-amino acids will block uptake), sodium dependency of uptake, dependency on oxidative metabolism for active accumulation and enhanced efflux of taurine when incubated in a solution containing other β-amino acids.