ACTIVATION OF GLYCINE AND GLUTAMATE RECEPTORS INCREASES INTRACELLULARCALCIUM IN CELLS DERIVED FROM THE ENDOCRINE PANCREAS

We studied calcium signaling in a newly described pancreatic cell line , GK-P3, that expresses functional amino acid neurotransmitter recepto rs. GK-P3 cells express the first strychnine-sensitive glycine recepto rs reported in a permanent cell line. In addition, GK-P3 cells express lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type g lutamate receptors. Both types of amino acid receptors showed electrop hysiological and pharmacological behavior similar to their neuronal co unterparts. The glycine receptors were permeable to Cl- and blocked by the selective antagonist strychnine. AMPA receptors showed limited pe rmeability to Ca2+, were blocked by 6-cyano-2,3-dihydroxy-7-nitroquino xaline and were potentiated by cyclothiazide. Interestingly, activatio n of either receptor type increased intracellular Ca2+ measured by dig ital imaging of Fura-2 fluorescence. These Ca2+ signals were completel y blocked by 30 mu M La3+, suggesting that the Ca2+ entered the cells largely through voltage-dependent Ca2+ channels. Alterations in the ex tracellular concentrations of Cl- and/or HCO3- had only marginal effec ts on glycine-evoked Ca2+ signals. However, increases in intracellular Ca2+ mediated by AMPA receptors were absent when the extracellular Na + was replaced with an impermeant cation, N-methyl-D-glucamine. We con clude that activation of ligand-gated cation or anion channels depolar ize GK-P3 cells sufficiently to activate their voltage-gated Ca2+ chan nels leading to increases in intracellular Ca2+ concentration. Thus, g lycine and glutamate receptors may regulate Ca2+-dependent secretory m echanisms in islet cells by altering the membrane potential of these c ells. Our data in GK-P3 cells support the growing weight of evidence f or a role of amino acid neurotransmitters in pancreatic islets and int roduce strychnine-sensitive glycine receptors as a novel target of ami no acid neurotransmitter regulation in islets.