Activation of glycine and glutamate receptors increases intracellular calcium 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 receptors. GK-P3 cells express the first strychnine-sensitive glycine receptors reported in a permanent cell line. In addition, GK-P3 cells express alpha-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 microM 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.
Weaver
C D
CD
Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-6600, USA. dweaver@bms.com
Partridge
J G
JG
Yao
T L
TL
Moates
J M
JM
Magnuson
M A
MA
Verdoorn
T A
TA
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D015640:Ion Channel Gating / Q000502:physiology; D007515:Islets of Langerhans / Q000502:physiology / Q000648:ultrastructure*; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D008822:Mice, Transgenic; D009474:Neurons / Q000648:ultrastructure; D051381:Rats; D018091:Receptors, AMPA / Q000502:physiology; D018079:Receptors, GABA / Q000502:physiology; D017470:Receptors, Glutamate / Q000502:physiology*; D018009:Receptors, Glycine / Q000502:physiology*; D012680:Sensitivity and Specificity; D013331:Strychnine / Q000494:pharmacology
D018091:Receptors, AMPA; D018079:Receptors, GABA; D017470:Receptors, Glutamate; D018009:Receptors, Glycine; D013331:Strychnine; D002118:Calcium