Vol. 54, Issue 4, 639-646, October 1998

Activation of Glycine and Glutamate Receptors Increases Intracellular Calcium in Cells Derived from the Endocrine Pancreas

C. David Weaver, John G. Partridge, Tom L. Yao, J. Michael Moates, Mark A. Magnuson, and Todd A. Verdoorn

Departments of Pharmacology (C.D.W., T.A.V., J.G.P., T.L.Y.) and Molecular Physiology and Biophysics (J.M.M., M.A.M.), Vanderbilt University School of Medicine, Nashville, Tennessee 37232-6600

We studied calcium signaling in a newly described pancreatic cell line, GK-P3, that expresses functional amino acid neurotransmitter receptors. GK-P3 cells express the first strychnine-sensitive glycine receptors reported in a permanent cell line. In addition, GK-P3 cells express -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. Both types of amino acid receptors showed electrophysiological and pharmacological behavior similar to their neuronal counterparts. The glycine receptors were permeable to Cl and blocked by the selective antagonist strychnine. AMPA receptors showed limited permeability to Ca²⁺, were blocked by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline, and were potentiated by cyclothiazide. Interestingly, activation of either receptor type increased intracellular Ca²⁺ measured by digital imaging of Fura-2 fluorescence. These Ca²⁺ signals were completely blocked by 30 µM La³⁺, suggesting that the Ca²⁺ entered the cells largely through voltage-dependent Ca²⁺ channels. Alterations in the extracellular concentrations of Cl and/or HCO₃ had only marginal effects 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 conclude that activation of ligand-gated cation or anion channels depolarize GK-P3 cells sufficiently to activate their voltage-gated Ca²⁺ channels leading to increases in intracellular Ca²⁺ concentration. Thus, glycine and glutamate receptors may regulate Ca²⁺-dependent secretory mechanisms in islet cells by altering the membrane potential of these cells. Our data in GK-P3 cells support the growing weight of evidence for a role of amino acid neurotransmitters in pancreatic islets and introduce strychnine-sensitive glycine receptors as a novel target of amino acid neurotransmitter regulation in islets.