NA+ INFLUX-INDUCED DECREASE OF (NA+ + K+)-ATPASE ACTIVITY IN RAT-BRAIN SLICES - ROLE OF CA-2+

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 Ca2+ 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 Ca2+-free medium containing 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. The effect of monensin at a low concentration on enzyme activity was antagonized by amiloride (1 mM), bepridil (5-mu-M), quinacrine (30-mu-M) or verapamil (30-mu-M), but not by nifedipine (1-mu-M) or omega-conotoxin (1-mu-M). Furthermore, the inhibitory effect of monensin at a high concentration under Ca2+-free conditions was blocked by BAPTA-AM (30-mu-M) and by bepridil (100-mu-M) or diazepam (500-mu-M), inhibitors of mitochondrial Na+-Ca2+ exchange. Inhibitors of calmodulin, protein kinase C, phospholipase A2 and calpain did not affect the monensin-induced decrease of enzyme activity. Dithiothreitol (10 mM) blocked the effect of monensin on enzyme activity but did not affect the ionophore-induced influx of Ca2+ in the slices.