The effects of extracellular pH and calcium manipulation on protein synthesis and response to anoxia/aglycemia in the rat hippocampal slice

Abstract

Extracellular pH modulates the function of the N-methyl-D-aspartate (NMDA) receptor, which may influence pathophysiological responses to glutamate. While damage due to oxygen and glucose deprivation or glutamate exposure is attenuated by acidification of the incubating medium of cultured neurons, neuron damage is enhanced in vivo following ischemia in hyperglycemic animals. A persistent inhibition of protein synthesis (to less than 5% of normoxic levels) is a reliable index of damage to neurons both in vivo and in the rat hippocampal slice. We explored the influence of extracellular pH and calcium manipulation on protein synthesis inhibition and energy failure due to anoxia/aglycemia or exposure to N-methyl-D-aspartate in the rat hippocampal slice. Moderate acidification of the medium during anoxia/aglycemia did not reduce the damage to protein synthesis in hippocampal neurons (9% of normoxic levels) and did not alter basal ATP levels or the rate of ATP depletion during anoxia/aglycemia. However, when calcium levels were lowered during the acidification and following the anoxia/aglycemia, protein synthesis was almost completely protected (84% of normoxic levels). Calcium reduction itself also attenuated the protein synthesis inhibition due to anoxia/aglycemia (to 55.6% of normoxic controls), but the protection was not as complete. In contrast, moderate acidification of the medium significantly reduced the damage to protein synthesis due to a brief exposure to NMDA (37% of control with NMDA, 78.9% of control with acidification during NMDA), even in the presence of extracellular calcium. Alkalinization of the medium exacerbated the protein synthesis inhibition following anoxia/aglycemia, and significantly reduced basal ATP levels (to 52% of normoxic control levels). Thus, pHo changes influence neuronal metabolism and response to anoxia/aglycemia. In addition, while acidification can reduce the excitotoxic damage caused by direct exposure to NMDA, it cannot reduce damage due to anoxia/aglycemia unless calcium is lowered concomitantly. Thus, both NMDA receptor activation and calcium are involved in the damage due to oxygen and glucose deprivation in the slice.