Na + influx-induced decrease of (Na ++ K +) -ATPase activity in rat brain slices: role of Ca 2+

Abstract

Treatment of rat brain slices with veratrine and monensin decreased (Na + + K +)-ATPase activity in the membranes in a dose-dependent manner. The effect of monensin, like that of veratrine, was accompanied by a decrease of maximal binding sites for ouabain. The inhibitory effect of monensin on the enzyme activity was dependent on external Ca 2+ at low concentrations, but not at a high concentration. The decreased enzyme activity induced by monensin was restored by subsequent incubation of the slices in a Ca 2+-free medium containing 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), a chelator of intracellular Ca 2+ The effect of monensin at a low concentration on enzyme activity was antagonized by amiloride (1 mM), bepridil (5 μM), quinacrine (30 μM) or verapamil (30 ,μM), but not by nifedipine (1 μM) or ω-conotoxin (1 μM). Furthermore, the inhibitory effect of monensin at a high concentration under Ca 2+-free conditions was blocked by BAPTA-AM (30 μM) and by bepridil (100 μM) or diazepam (500 μM), inhibitors of mitochondrial Na +-Ca 2+ exchange. Inhibitors of calmodulin, protein kinase C, phospholipase A 2 and calpain did not affect the monensin-mduced decrease of enzyme activity. Dithiothreitol (10 mM) blocked the effect of monensin on enzyme activity but did not affect the ionophore-induced influx of Ca 2+ in the slices.