INTRACELLULAR CALCIUM LEVELS AND CALCIUM FLUXES IN THE CA1 REGION OF THE RAT HIPPOCAMPAL SLICE DURING IN-VITRO ISCHEMIA - RELATIONSHIP TO ELECTROPHYSIOLOGICAL CELL-DAMAGE

Five minutes of oxygen and glucose deprivation (termed ''in vitro isch emia'') causes long-term synaptic transmission failure (LTF) in the CA 1 region of the rat hippocampal slice. Dependence of LTF on cell calci um was tested by generating graded reductions in cell Ca. There was a strong correlation between the average level of exchangeable cell Ca i n CA1 during ischemia, and the extent of LTF. In standard buffer, exch angeable cell Ca in CA 1 increased by 35% after 3 min of ischemia and remained elevated for the entire 5 min of ischemia. Unidirectional Ca influx increased by 35% during the first 2.5 min of ischemia and remai ned at that level for the next 2.5 min. There were no changes in unidi rectional Ca efflux during this period. Thus, the accumulation results from increased influx of Ca. Ca influx during the first 2.5 min of is chemia depended entirely on NMDA channels; it was completely blocked b y the noncompetitive NMDA receptor antagonist MK-801. However MK-801 h ad no effect during the second 2.5 min. This inactivation of NMDA-medi ated influx during ischemia appears to result from dephosphorylation. Okadaic acid increased Ca influx during the second 2.5 min of ischemia and this increase was blocked by MK-801. The ischemia-induced Ca infl ux during the second 2.5 min of ischemia was attenuated 25% by nifedip ine (50 mum) and an additional 35% by the Na/Ca exchange inhibitor ben zamil (100 mum). The AMPA/kainate antagonist DNQX had no effect on the Ca influx. Antagonists were used to relate Ca influx to LTF. Blockade of enhanced Ca entry during ischemia in standard buffer (2.4 mm Ca) h ad no effect on LTF, consistent with total cell Ca prior to ischemia b eing adequate to cause complete LTF. However, MK-801 strongly protecte d against LTF when the buffer contained 1.2 mm Ca, a more physiologica l level. MK-801 combined with DNQX prevented transmission damage in st andard buffer. Thus, AMPA/kainate receptor activation contributes to i schemic damage, although not by enhancing Ca entry.