Causes of calcium accumulation in rat cortical brain slices during hypoxia and ischemia: role of ion channels and membrane damage

Abstract

To better understand why neurons accumulate calcium during cerebral ischemia, the influence of specific ion channel inhibitors on the rise in cytosolic free calcium ([Ca 2+] c) during hypoxia or ischemia was evaluated in rat cerebrocortical brain slices. [Ca 2+] c was measured fluorometrically with the dye fura-2 during hypoxia (95% N 2/5% CO 2 or 100 μM NaCN), simulated ischemia (100 μM NaCN plus 3.5 mM iodoacetate), or 0.5–1.0 mM glutamate. Hypoxia or ischemia increased [Ca +2] c from 100–250 mM to 1,000–2,500 nM within 3–5 min. Greater than 85% of the calcium accumulation was influx from the extracellular medium. The non-competitive N-methyl- d-aspartate (NMDA) inhibitor MK-801 reduced [Ca 2+] c accumulation during hypoxia, but antagonism of α-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptors or voltage-gated sodium or calcium channels or Na +/Ca 2+ exchangers had no effect. During ischemia, combined antagonism of NMDA, AMPA and voltage-gated sodium channels slowed the rate of calcium accumulation, but not concentration at 5 min. Membrane damage, as indicated by leakage of lactate dehydrogenase into superfusate, occurred coincidentally with calcium influx and ATP loss during both hypoxia and ischemia. We conclude that cytosolic calcium changes during hypoxia or ischemia in cortical brain slices are due to multiple mechanisms, are incompletely inhibited by combined ion channel blockade, and are associated with disruption of cell membrane integrity.