Cl^– affects the function of the GABA cotransporter rGAT1 but preserves the mutal relationship between transient and transport currents

Abstract.

The effects of reducing external Cl^– on the electrophysiological properties of the Na⁺/Cl^–-dependent GABA transporter rGAT1 expressed in Xenopus oocytes were investigated. In agreement with a recently proposed kinetic scheme, the effects of Cl^– are complex but preserve the mutual relationship that links the transport-associated current, I _tr, measured in saturating GABA concentration, and the transient current, I _pre, recorded in the absence of GABA following a voltage step from the holding potential V _h to V. In particular, I _tr (V) – I _tr (V _h) = r ∫ I _pre (V) dt, where r is the relaxation rate of I _pre at the same membrane potential and Cl^– concentration. The model also predicts a relationship between charge relaxation rate and apparent affinity for GABA, which is also verified in the presence of lowered Na⁺ or Cl^– concentrations. In these conditions, the binding rate of GABA to the transporter is increased. All these effects are consistent with the hypothesis that interaction of the organic substrate with rGAT1 induces a conversion from a capacitive to a conductive mode of operation without strongly altering either the amount or the rate of charge movement.