Acute regulation of the betaine/GABA transporter BGT-1 expressed in Xenopus oocytes by extracellular pH.
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.
Matskevitch
I
I
Department of Physiology, University of Tübingen, Germany.
Stegen
C
C
Wagner
C A
CA
Moschén
I
I
Bindels
R
R
Van Os
C
C
Bröer
S
S
Lang
F
F
D000818:Animals; D001622:Betaine / Q000378:metabolism*; D002352:Carrier Proteins / Q000096:biosynthesis*; D004285:Dogs; D050485:GABA Plasma Membrane Transport Proteins; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D014981:Xenopus; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; C072207:betaine plasma membrane transport proteins; D001622:Betaine; D005680:gamma-Aminobutyric Acid