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

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 −59mV at time 0; external Na+ (or choline under putative zero-trans conditions) was 126mM (T=25°C). The apparent affinity constants of the taurine transporters were determined over a range of taurine concentrations from 0.24μM to 11.4μM. Abluminal membranes had both Na+-dependent taurine transport as well as facilitative transport while luminal membranes only had facilitative transport. The apparent Km for facilitative and Na+-dependent taurine transport were 0.06±0.02μM and 0.7±0.1μM, respectively. The Na+-dependent transport of taurine was voltage dependent over the range of voltages studied (−25 to −101mV). The transport was over 5 times greater at −101mV compared to when Vm was −25mV. The sensitivity to external osmolality of Na+-dependent transport was studied over a range of osmolalities (229 to 398mOsm/kg H2O) 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 −59mV. The Na+-dependent transport system was sensitive to osmolality with the greatest rate observed at 229mOsm/kg H2O.