Effects of Calcium on the GABA<sub>A</sub>-Coupled CI<sup>-</sup>/HCO<sub>3</sub><sup>-</sup> -ATPase from Plasma Membrane of Rat Brain

Abstract

The work is a study of the influence of Ca 2+ (0.01-1 mM) on neuronal Cl − , 3 HCO −-ATPase complex: an enzyme that is a Cl −-pump which is functionally and structurally coupled to GABAA-receptors. It is found that influence of Ca 2+ on the multifunctional complex starts at concentration of 50 µM and at concentration of 0.1 mM, it reduces the "basal" one and increases the Cl − , 3 HCO −-stimulated Mg 2+-ATPase activities. GABA (0.1-100 µM) activates the "basal" Mg 2+-ATPase activity in the absence of calcium. The effect of GABA on the enzyme in the presence of 0.01 µM Ca 2+ does not change. At the same time, 1 mM Ca 2+ eliminates the GABA effect on the "basal" Mg 2+-ATPase activity. Competitive blocker of GABAA-receptors bicuculline (5-20 µM) in the absence of Ca 2+ ions eliminates the stimulation of the "basal" Mg 2+-ATPase by anions. When 0.25 mM Ca 2+ is added to the incubation medium the inhibitory bicuculline effect on the enzyme does not appear. We found that 0.1 mM o-vanadate (protein tyrosine phosphatase blocker) reduces the GABA-activated ATPase activity. At the same time, 0.1 mM genistein (a protein tyrosine kinase blocker) has no effect on enzyme activity. In the presence of Ca 2+ (0.25 mM), the effect of o-vanadate on the "basal" and Cl − , 3 HCO −-ATPase activities does not appear. It is shown for the first time that high concentrations of Ca 2+ prevent the action of GABAA-ergic ligands on the study ATPase. It is assumed that there is the involvement of protein kinases and protein phosphatases in the modulation of the enzyme activity by calcium. The observed effect of calcium on the ATPase may play an important role in the study of the mechanisms of epileptogenesis and seizure activity.