Relations between substrate affinities and charge equilibration rates in the rat GABA cotransporter GAT1.
The relations between apparent affinity for substrates and operating rates have been investigated by two-electrode voltage clamp in the GABA transporter rGAT1 expressed in Xenopus oocytes. We have measured the transport current induced by the presence of GABA, as well as the charge equilibration rate in the absence of the neurotransmitter, in various experimental conditions known to affect the transporter characteristics. The apparent affinities for GABA and for Na(+) were also determined in the same conditions. Two pharmacological actions and three mutated isoforms have been examined. In all cases significant correlations were found between the charge equilibration rates and apparent affinities for both substrates. In particular in the transport process, the apparent affinity for GABA appears to be inversely related to the sum of the unidirectional charge equilibration rates (alpha+beta), while the Na(+) apparent affinity is directly related to their ratio (beta/alpha). Together these observations suggest a kinetic basis for GABA affinity with higher turnover rates resulting in lower affinity, and indicate that an efficient uptake requires a compromise between these two parameters.
Soragna
Andrea
A
Department of Structural and Functional Biology, University of Insubria, Via Dunant 3, 21100 Varese, Italy.
Bossi
Elena
E
Giovannardi
Stefano
S
Pisani
Rossana
R
Peres
Antonio
A
D000818:Animals; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D004594:Electrophysiology; D018682:GABA Agents / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins; D008564:Membrane Potentials / Q000502:physiology; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D009557:Nipecotic Acids / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D017354:Point Mutation; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D012996:Solutions; D014635:Valproic Acid / Q000494:pharmacology; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D018076:DNA, Complementary; D018682:GABA Agents; D050485:GABA Plasma Membrane Transport Proteins; D026901:Membrane Transport Proteins; D009557:Nipecotic Acids; D012333:RNA, Messenger; D011994:Recombinant Proteins; C493415:Slc6a1 protein, rat; D012996:Solutions; D005680:gamma-Aminobutyric Acid; D014635:Valproic Acid; C045598:N-(4,4-diphenyl-3-butenyl)nipecotic acid; D012964:Sodium