Interactive effects of pH and temperature on N-methyl-D-aspartate receptor activity in rat cortical brain slices.

Low extracellular pH decreases the activity of the N-methyl-D-aspartate (NMDA) glutamate receptor, and may thus limit neuronal calcium overload during cerebral ischemia. During induced hypothermia, alkaline pH ("alphastat regulation") is often used to preserve cardiac and enzymatic function. The purpose of this study is to measure the functional activity of cerebral cortex NMDA receptors over the range of temperatures used in profound hypothermic cardiopulmonary bypass (20-37 degrees C). Extracellular pH was varied over a broad range relevant to both alphastat and pH stat acid-base management (7.0-7.8). Change in cytosolic free calcium evoked by 50 microM NMDA in brain slices was used as an index of NMDA receptor activity. Cortical slices (300 microns thick) were loaded with fura-2 Aspartate Methyl for study in a fluorometer. At 37 degrees C, a change in extracellular pH from 7.1 to 7.8 increased the NMDA-evoked change in cytosolic calcium in brain slices by a factor of 4 (p < 0.05). In contrast, at 20 degrees C there was minimal effect of changing extracellular pH from 7.1 to 7.8 (27% increase). We conclude that hypothermia results in decreased pH sensitivity of the NMDA receptor. The results predict that different strategies of pH management during induced hypothermia may have limited impact on NMDA receptor-mediated processes, such as neuronal calcium overload.