Effect of HCO3(-) ions on the ATP-dependent GABA(A) receptor-coupled Cl(-) channel in rat brain plasma membranes

Dual character of GABA action on Cl--transport by the reconstituted Cl-/[Formula: see text]-ATPase from rat brain

The Cl^-/[Formula: see text]-ATPase from the plasma membranes of animal brains is an ATP-consuming Cl^--transporting ATPase P-type that is structurally coupled to GABA_A receptors. The aim of work was to study the GABA effect on Cl^- transport across liposomal membranes by the reconstituted ATPase under various conditions (i.e. in the absence and in the presence of ATP in the incubation medium). We reconstituted the affinity-purified enzyme in the liposomes with a fluorescent dye for Cl^-. The Cl^--transport in proteoliposomes was evaluated from variations in fluorescence. Native-PAGE of the purified enzyme preparation, as well as western blot analysis with an antibody against the GABA_AR β3 subunit, showed one band with a molecular mass of 300 kDa. The addition of GABA (100 μM) quickly resulted (1-3 s) in an increase in the Cl^- influx into proteoliposomes. The application of ATP (3 mM) resulted in a gradual increase in the Cl^- inflow into the proteoliposomes at 0.5-4 min. Vanadate (10 μM) did not change the GABA-induced Cl^- inflow into the liposomes, but completely inhibited the ATP-dependent Cl^- input. Under conditions where Cl^- was previously loaded into proteoliposomes by ATP-dependent Cl^- input, the addition of GABA to the incubation medium caused a short-lasting Cl^- efflux from the proteoliposomes. However, an additional long-time incubation of these proteoliposomes resulted in the restoration of Cl^- accumulation. Pentobarbital, propofol, or diazepam elevate and picrotoxin or furosemide eliminate the effect of GABA on the Cl^--transport processes. These findings indicate a dual nature of GABA action on the Cl^-/[Formula: see text]-ATPase.

Biochemical properties of the sensitivity to GABAAergic ligands, Cl-/HCO3--ATPase isolated from fish (Cyprinus carpio) olfactory mucosa and brain

This paper presents a comparative study of the roles of Cl^- and HCO₃^- in the functioning of the GABA_AR-associated Cl^-/HCO₃^--ATPase of the plasma membranes of the olfactory sensory neurons (OSNs) and mature brain neurons (MBNs) of fish. The ATPase activity of OSNs and its dephosphorylation were increased twofold by Cl^-(15-30 mmol l^-1), whereas the enzyme from MBNs was not significantly affected by Cl^-. By contrast, HCO₃^-(15-30 mmol l^-1) significantly activated the MBN enzyme and its dephosphorylation, but had no effect on the OSN ATPase. The maximum ATPase activity and protein dephosphorylation was observed in the presence of both Cl^-(15 mmol l^-1)/HCO₃^-(27 mmol l^-1) and these activities were inhibited in the presence of picrotoxin (100 μmol l^-1), bumetanide (150 μmol l^-1), and DIDS (1000 μmol l^-1). SDS-PAGE revealed that ATPases purified from the neuronal membrane have a subunit with molecular mass of ~ 56 kDa that binds [³H]muscimol and [³H]flunitrazepam. Direct phosphorylation of the enzymes in the presence of ATP-γ-³²P and Mg²⁺, as well as Cl^-/HCO₃^- sensitive dephosphorylation, is also associated with this 56 kDa peptide. Both preparations also showed one subunit with molecular mass 56 kDa that was immunoreactive with GABA_AR β3 subunit. The use of a fluorescent dye for Cl^- demonstrated that HCO₃^-(27 mmol l^-1) causes a twofold increase in Cl^- influx into proteoliposomes containing reconstituted ATPases from MBNs, but HCO₃^- had no effect on the reconstituted enzyme from OSNs. These data are the first to demonstrate a differential effect of Cl^- and HCO₃^- in the regulation of the Cl^-/HCO₃^--ATPases functioning in neurons with different specializations.

Isolation, purification, and partial characterization of a membrane-bound Cl-/HCO3--activated ATPase complex from rat brain with sensitivity to GABAAergic ligands

This study describes the isolation and purification of a protein complex with [Formula: see text]-ATPase activity and sensitivity to GABA_Aergic ligands from rat brain plasma membranes. The ATPase complex was enriched using size-exclusion, affinity, and ion-exchange chromatography. The fractions obtained at each purification step were subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE), which revealed four subunits with molecular mass ∼48, 52, 56, and 59 kDa; these were retained at all stages of the purification process. Autoradiography revealed that the ∼52 and 56 kDa subunits could bind [³H]muscimol. The [Formula: see text]-ATPase activity of this enriched protein complex was regulated by GABA_Aergic ligands but was not sensitive to blockers of the NKCC or KCC cotransporters.