Characteristics and Functions of α-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionate Receptors Expressed in Mouse Pancreatic α-Cells

Abstract

Pancreatic islet cells use neurotransmitters such as l-glutamate to regulate hormone secretion. We determined which cell types in mouse pancreatic islets express ionotropic glutamate receptor channels (iGluRs) and describe the detailed biophysical properties and physiological roles of these receptors. Currents through iGluRs and the resulting membrane depolarization were measured with patch-clamp methods. Ca2+ influx through voltage-gated Ca2+ channels and Ca2+-evoked exocytosis were detected by Ca2+ imaging and carbon-fiber microamperometry. Whereas iGluR2 glutamate receptor immunoreactivity was detected using specific antibodies in immunocytochemically identified mouse α- and β-cells, functional iGluRs were detected only in the α-cells. Fast application of l-glutamate to cells elicited rapidly activating and desensitizing inward currents at −60 mV. By functional criteria, the currents were identified as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. They were activated and desensitized by AMPA, and were activated only weakly by kainate. The desensitization by AMPA was inhibited by cyclothiazide, and the currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Islet iGluRs showed nonselective cation permeability with a low Ca2+ permeability (PCa/PNa = 0.16). Activation of the AMPA receptors induced a sequence of cellular actions in α-cells: 1) depolarization of the membrane by 27 ± 3 mV, 2) rise in intracellular Ca2+ mainly mediated by voltage-gated Ca2+ channels activated during the membrane depolarization, and 3) increase of exocytosis by the Ca2+ rise. In conclusion, iGluRs expressed in mouse α-cells resemble the low Ca2+-permeable AMPA receptor in brain and can stimulate exocytosis.