EXCESSIVE INTRACELLULAR CA-2+ INHIBITS GLUTAMATE-INDUCED NA+-K+ PUMP ACTIVATION IN RAT HIPPOCAMPAL-NEURONS

1. The effects of increased intracellular Ca2+ concentration ([Ca2+]i) on Na+-K+ pump activity in CA1 pyramidal neurons of rat hippocampal slices were investigated. The postglutamate hyperpolarization (PGH), which follows glutamate (GLU)-induced depolarization (GD), was used as an index of Na+-K+ pump activity, as was a ratio of PGH area to the preceding GD area (PGH ratio). 2. Perfusion of slices with saline containing Ca2+ ionophore (A23187, 10-mu-M) inhibited the PGH without producing apparent signs of cell deterioration. A 60-100% (85 +/- 15%, mean +/- SD) reduction in the PGH ratio occurred after 20-50 min of A23187 superfusion in 12 of 18 neurons tested. Complete abolition of the PGH occurred in 8 of these 12 cells exposed to A23187 for 30-120 min. 3. Application of A23187 in Ca2+-free/high-Mg2+ solution did not abolish the PGH, although small (< 50%; 37 +/- 10%) reductions in the PGH ratio were observed after perfusion of 50 min or longer in five neurons tested. 4. Intracellular injection of the Ca2+ chelator bis-(omicron-amino-phenoxy)-N,N,N',N'-tetraacetic acid (BAPTA, 300-400 mM) blocked inhibition of the PGH by A23187. After 50 min of perfusion with Ca2+ ionophore, no reduction of the PGH ratio was observed in five neurons tested. 5. Rundown of the PGH without apparent change in membrane properties was observed in three neurons that were stable for > 2-3 h, allowing repetitive GLU applications. 6. Block of the PGH produced by a Na+-K+-adenosinetriphosphatase (ATPase) inhibitor (strophanthidin) prolonged the duration of GDs because of a delay in repolarization. The PGH terminates GDs and allows cells to repolarize to resting potential even during a train of GDs, however, repetitive GLU applications after abolition of the PGH resulted in electrophysiological signs of cell deterioration. 7. These results suggest that sustained increases in [Ca2+]i, such as might be caused by excessive neuronal excitation, inhibit Na+-K+ pump activity in hippocampal CA1 neurons. Further accumulation of [Ca2+]i would result from secondary impairment of Na+-Ca2+ exchange due to reduction in the Na+ gradient. Such a pathophysiological positive feedback may be a critical factor contributing to neurotoxicity such as occurs during repeated epileptiform seizures and hypoxia/ischemia.