PROTECTIVE EFFECTS OF EXTRACELLULAR ACIDOSIS AND BLOCKADE OF SODIUM HYDROGEN ION-EXCHANGE DURING RECOVERY FROM METABOLIC INHIBITION IN NEURONAL TISSUE-CULTURE/

Acidosis is a universal response of tissue to ischemia. In the brain, severe acidosis has been linked to worsening of cerebral infarction. H owever, milder acidosis can have protective effects. As part of our in vestigations of the therapeutic window in our neuronal tissue culture model of ischemia, we investigated the effects of acidosis during reco very from brief simulated ischemia. Ischemic conditions were simulated in dissociated cortical cultures by metabolic inhibition with potassi um cyanide to block oxidative metabolism and 2-deoxyglucose to block g lycolysis. Lowering the extracellular pH (pH(e)) to 6.2 during metabol ic inhibition had no effect on injury, as measured by lactate dehydrog enase release from cultures after 24 h of recovery. Lowering the pH(e) during the first hour of recovery, in contrast, had profound protecti ve effects. When the duration of metabolic inhibition was lengthened t o 30 min, most of the protective effects of the NMDA receptor antagoni st MK-801 were lost. However, the protective effects of acidosis were unchanged. This suggested that the protective effects of extracellular acidosis could be due to more than blockade of NMDA receptors. Intrac ellular acidosis might be responsible. To test this, recovery of intra cellular pH (pH(i)) was slowed by incubation with blockers of Na+/H+ e xchangers at normal pH(e). The two compounds tested, dimethylamiloride and harmaline, had protective effects when present during recovery fr om metabolic inhibition. Measurements of pH(i) confirmed that the bloc kers slowed recovery from intracellular acidosis; more rapid pH(i) rec overy was correlated with injury. The protective effects of acidosis c ould be reversed by brief incubation with the protonophore monensin, w hich rapidly normalized pH(i). These results are the first demonstrati on of the protective effects of blocking Na+/H+ exchange in a model of cerebral ischemia. The protective effects of acidosis appear to arise either from suppressing pH-sensitive mechanisms of injury or from blo cking sodium entry due to Na+/H+ exchange.