Involvement of glutamate receptors of the NMDA type in the modulation of acetylcholine and glutamate overflow from the guinea pig ileum during in vitro hypoxia and hypoglycaemia

The involvement of NMDA glutamate receptors in the effects of glucose/oxygen deprivation (in vitro ischaemia) on spontaneous endogenous acetylcholine and glutamate overflow from the guinea pig ileum was studied. Neurotransmitter overflow was measured by HPLC. Deprivation of glucose in the medium slightly reduced acetylcholine overflow, and did not significantly influence glutamate overflow. During oxygen deprivation and glucose/oxygen deprivation, acetylcholine overflow augmented with a biphasic modality: an early peak was followed by a long lasting increase, whereas glutamate overflow increased with a rapid and sustained modality. The effects of glucose/oxygen deprivation on both acetylcholine and glutamate overflow were abolished after reperfusion with normal oxygenated medium.Acetylcholine and glutamate overflow induced by glucose/oxygen deprivation were significantly reduced in the absence of external Ca2+ as well as by the addition of the mitochondrial Na+–Ca2+ exchanger blocker, CGP 37157, and of the endoplasmic reticulum Ca2+/ATPase blocker, thapsigargin. ±AP5, an NMDA receptor antagonist, and 5,7-diCl-kynurenic acid, an antagonist of the glycine site associated to NMDA receptor, markedly depressed glucose/oxygen deprivation-induced acetylcholine and glutamate overflow as well.Our results suggest that in vitro simulated ischaemia evokes acetylcholine and glutamate overflow from the guinea pig ileum, which is partly linked to an increase in intracellular Ca2+ concentration dependent on both Ca2+ influx from the extracellular space and Ca2+ mobilization from the endoplasmic reticulum and mitochondrial stores. During glucose/oxygen deprivation, ionotropic glutamate receptors of the NMDA type exert both a positive feedback modulation of glutamate output and contribute to increased acetylcholine overflow.