EFFECT OF GLUTAMATE-RECEPTOR AGONISTS ON CATECHOLAMINE SECRETION IN BOVINE CHROMAFFIN CELLS

In this study, the effects of glutamate and glutamate receptor agonist s in cultured chromaffin cells from bovine adrenal medulla were invest igated. It was found that glutamate increases basal catecholamine (CA) secretion in a dose-dependent manner. This effect is mimicked by spec ific agonists of the four known glutamate receptors N-methyl-D-asparta te (NMDA), isqualate/(RS)-amino-3-hydroxy-5-methyl-4-isoxazol acid (AM PA), kainate (KA), and trans-(+)-1-amino-1,3-cyclopentane dicarboxylic acid (t-ACPD), which increased both basal and nicotine-evoked CA secr etion. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid, 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of KA and AMPA re ceptors, and L-(+)-2-amino-3-phosphonopropionic acid, an antagonist of the t-ACPD receptor, inhibited the stimulatory effect of related glut amate agonists. Hexamethonium, an antagonist of the nicotinic receptor , failed to influence glutamate agonists except for a 15% inhibition o f KA. The increase in CA secretion produced by a 100 mu M concentratio n of glutamate agonists was about 20-60% of that obtained with 10 mu M of nicotine, an agonist of the physiological stimulatory cholinergic receptor. The increase in CA secretion produced by glutamate was accom panied by both an increase in bisoxonol fluorescence, suggesting membr ane depolarization, and by an increase in intracellular Ca2+ concentra tions. Results obtained with image analysis on single cells indicated that the percentage of cells which respond to the stimulation of 50 mu M Of glutamate is 42%. From these results, we conclude that glutamate , through its four known glutamate receptors, can increase both basal and nicotine-evoked CA secretion in chromaffin cells by a process whic h involves membrane depolarization and an increase in intracellular ca lcium levels.