The gamma-aminobutyric acid transporter and its interaction with taurine in the apical membrane of the bovine retinal pigment epithelium.
The characteristics of gamma-aminobutyric acid (GABA) uptake were investigated in apical membrane vesicles prepared from the bovine retinal pigment epithelium. An inwardly directed NaCl gradient stimulated GABA uptake markedly, and the time course of uptake exhibited an overshoot phenomenon indicating the presence of an active transport mechanism for GABA in these membranes. Other monovalent cations were not capable of substituting for Na+. In addition to this obligatory requirement for Na+, the GABA uptake also exhibited a Cl(-)-dependence, evident from the observations that the uptake was negligible in the presence of NaF or sodium gluconate in place of NaCl. NO3- and SCN- could substitute for Cl- to some extent. The uptake process was electrogenic, with a Na+/Cl-/GABA stoichiometry of 2:1:1 or 3:1:1. Substrate-specificity studies showed that the beta-amino acids such as taurine, hypotaurine and beta-alanine interacted with the GABA uptake process. Uptake of GABA could be completely inhibited by an excess of taurine and, similarly, uptake of taurine could be completely inhibited by an excess of GABA, suggesting that common transport processes operate in the uptake of these two compounds. However, a number of compounds which are specific inhibitors of GABA uptake inhibited taurine uptake only to a maximum of 50%. Kinetic analysis of GABA uptake in the concentration range 0.1-10 microM revealed that the uptake occurred via a single system and that taurine was a competitive inhibitor of this system. The Michaelis-Menten constant (Kt) for GABA was 0.94 microM and the apparent inhibition constant (Ki) for taurine was 230 microM. On the contrary, even though the kinetic analysis of taurine uptake in the concentration range 25-150 microM revealed participation of a single system in the uptake process, the inhibition of taurine uptake by GABA was not competitive. The presence of GABA decreased the maximal velocity of the taurine uptake process and also decreased the Kt for taurine. Based on these data, it is proposed that: (i) there are two distinct transport systems, namely the GABA transporter and the taurine transporter, in these membranes which accept both GABA and taurine as substrates, (ii) the affinities of these systems for taurine are very similar and cannot be kinetically distinguished under the experimental conditions employed, and (iii) the difference between the affinities of these system for GABA is much greater than for taurine.