Na(+)-dependent transport of taurine is found only on the abluminal membrane of the blood-brain barrier

Abstract

Luminal and abluminal plasma membranes were isolated from bovine brain microvessels and used to identify and characterize Na(+)-dependent and facilitative taurine transport. The calculated transmembrane potential was -59 mV at time 0; external Na(+) (or choline under putative zero-trans conditions) was 126 mM (T=25 °C). The apparent affinity constants of the taurine transporters were determined over a range of taurine concentrations from 0.24 μM to 11.4 μM. Abluminal membranes had both Na(+)-dependent taurine transport as well as facilitative transport while luminal membranes only had facilitative transport. The apparent K(m) for facilitative and Na(+)-dependent taurine transport were 0.06±0.02 μM and 0.7±0.1 μM, respectively. The Na(+)-dependent transport of taurine was voltage dependent over the range of voltages studied (-25 to -101 mV). The transport was over 5 times greater at -101 mV compared to when V(m) was -25 mV. The sensitivity to external osmolality of Na(+)-dependent transport was studied over a range of osmolalities (229 to 398 mOsm/kg H(2)O) using mannitol as the osmotic agent to adjust the osmolality. For these experiments the concentration of Na(+) was maintained constant at 50mM, and the calculated transmembrane potential was -59 mV. The Na(+)-dependent transport system was sensitive to osmolality with the greatest rate observed at 229 mOsm/kg H(2)O.