Acute Regulation of the Betaine/GABA Transporter BGT-1 Expressed in Xenopus Oocytes by Extracellular pH

Abstract

Besides uptake of Na⁺ and Cl^–, mammalian cells counteract osmotic cell shrinkage also by Na⁺-coupled uptake of osmolytes, e.g., myo-inositol, taurine or betaine. The expression of the corresponding transporters is transcriptionally regulated by the ambient pH and osmolarity and is increased upon cell shrinkage, a process requiring hours. The present study has been performed to disclose rapid regulation by pH of osmolyte transport via BGT-1. Transport of GABA was investigated by using the two-electrode voltage-clamp technique with BGT-1 expressing Xenopus oocytes. GABA was used as a substrate, because of the low oocyte endogenous transport activity. Extracellular acidification to pH 5.5 reversibly decreased and extracellular alkalinization to pH 8.5 increased GABA-induced currents. Kinetic analysis revealed that extracellular alkalinization increases the affinity for Cl^– as reflected by a decrease of the apparent K_m-value for Cl^– from >500 mM to 55.8 ± 4.7 mM upon an increase of the pH from 7.0 to 8.5. The apparent K_m- values for Na⁺ and GABA remained unaltered in the pH range from 6.0 to 8.5. Instead, alkalinization increased the maximal current induced by saturating Na⁺ and GABA concentrations. The results are compatible with a model of interference of H⁺ ions with Cl^– binding and a pH-dependent reduction of V_max for Na⁺ and GABA.