Dietary acid and alkali loading do not alter taurine uptake by renal proximal tubule brush border membrane vesicles in kittens

Kittens adapted to a purified control diet containing 43.5% soy protein plus 0.15% taurine were randomly divided into three groups: control, acid-loaded or alkali-loaded. For dietary acid or alkali loadings, 2% NH₄Cl or 0.87% NaHCO₃ plus 0.87% KHCO₃ were added to the control diet. Acid-loaded, control and alkali-loaded kittents had venous blood pH of 7.33 ± 0.01, 7.37 ± 0.02 and 7.39 ± 0.02, and urine pH of 5.5 ± 0.1, 7.2 ± 0.1 and 8.1 ± 0.1, respectively. After 6 wk of the dietary treatment, the plasma taurine concentrations of acid-loaded, control and alkali-loaded kittens were 74 ± 4, 77 ± 7 and 87 ± 8 μmol/L (P > 0.05) respectively. Compared with the control group, dietary acid or alkali loading did not significantly change the taurine uptake by brush border membrane vesicles (BBMV) when BBMV were at the same in vitro pH (7.35). However, when the pH of the medium was changed to 8.0 for the alkali-loaded group or to 5.5 for the acid-loaded group, the initial taurine uptake was significantly elevated (55%) or decreased (31%) (P < 0.05), respectively, compared with the values for the control group at pH 7.35. When BBMV prepared from the same group were tested at different pH levels (8.0 vs. 7.35 vs. 5.5), the initial uptake significantly increased (30%) at pH 8.0 and decreased (37%) at pH 5.5 compared with that observed at pH 7.35 (P < 0.05), regardless of the acidity of the diet. These results suggest that the taurine transporter present in the brush border membrane of the proximal tubule is affected by the pH of the tubular environment in vitro. However, dietary acid and alkali loading do not alter the ability of the renal proximal tubule to transport taurine and renal taurine resorption in vivo.