Efflux of taurine from renal brush border membrane vesicles: is it adaptively regulated?

Abstract

The uptake of the beta-amino acid taurine by rat renal brush border membrane vesicles (BBMV) adapts to changes in dietary sulfur amino acid intake. Initial rate Vmax "upregulates" after ingestion of a low methionine and taurine diet (LTD) and "downregulates" after a high taurine diet (HTD). This is reflected in vivo by hypotaurinuria after a LTD (90% reduction in excretion) and an 18-fold increase in urine taurine after a HTD. This study was performed to determine whether taurine efflux from BBMV is adaptively regulated by external taurine concentration or by diet. Vesicles were preloaded with varying concentrations of radiolabelled and unlabelled taurine and a 150 mM concentration of various salts. Efflux conditions were: taurine and 150 mM salt inside and 150 mM salt outside. The efflux of five concentrations of taurine (10-500 microM) was linear over 6 min, reached equilibrium by 21 min, and was dependent upon intravesicular taurine content. The kinetic characteristics of efflux (E) were significantly different from influx (I): Km = 109.8 +/- 5.8 (E) versus 23.6 +/- 4.2 (I), P < 0.001 [time of linearity = 360 s (E) vs. 20 s (I)]. Efflux of taurine was dependent on the presence of both sodium and chloride in the system, but neither external taurine content (0.100 microM, 1,000 microM) nor external beta-alanine altered initial efflux. Feeding rats a normal diet, LTD, or fasting altered taurine uptake but not efflux. Efflux does not appear to play a role in the adaptive regulation of taurine transport found in all mammalian species.