title: Pre-steady-state and reverse transport currents in the GABA transporter GAT1.
abstract: The role of internal substrates in the biophysical properties of the GABA transporter GAT1 has been investigated electrophysiologically in Xenopus oocytes heterologously expressing the cotransporter. Increments in Cl(-) and/or Na(+) concentrations caused by intracellular injections did not produce significant effects on the pre-steady-state currents, while a positive shift of the charge-voltage (Q-V) and decay time constant (τ)-voltage (τ-V) curves, together with a slowing of τ at positive potentials, was observed following treatments producing cytosolic Cl(-) depletion. Activation of the reverse transport mode by injections of GABA caused a reduction in the displaced charge. In the absence of external Cl(-), a stronger reduction in the displaced charge, together with a significant increase in reverse transport current, was observed. Therefore, complementarity between pre-steady-state and transport currents, observed in the forward mode, is preserved in the reverse mode. All these findings can be qualitatively reproduced by a kinetic scheme in which, in the forward mode, the Cl(-) ion is released first, after the inward charge movement, while the two Na(+) ions can be released only after binding of external GABA. In the reverse mode, internal GABA must bind first to the empty transporter, followed by internal Na(+) and Cl(-).
authors.affiliation: Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Cherubino
authors.forename: Simone
authors.initials: S
authors.lastname: Bertram
authors.forename: Elena
authors.initials: E
authors.lastname: Bossi
authors.forename: Antonio
authors.initials: A
authors.lastname: Peres
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002712:Chlorides / Q000378:metabolism; D050485:GABA Plasma Membrane Transport Proteins / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology; D026901:Membrane Transport Proteins / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D014982:Xenopus laevis / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D002712:Chlorides; D050485:GABA Plasma Membrane Transport Proteins; D026901:Membrane Transport Proteins; D005680:gamma-Aminobutyric Acid; D012964:Sodium