Secretory granule-mediated co-secretion of L-glutamate and glucagon triggers glutamatergic signal transmission in islets of Langerhans

L-Glutamate is believed to function as an intercellular transmitter in the islets of Langerhans. However, critical issues, i.e. where, when and how L-glutamate appears, and what happens upon stimulation of glutamate receptors in the islets, remain unresolved. Vesicular glutamate transporter 2 (VGLUT2), an isoform of the vesicular glutamate transporter essential for neuronal storage of L-glutamate, is expressed in α cells (Hayashi, M., Otsuka, M., Morimoto, R., Hirota, S., Yatsushiro, S., Takeda, J., Yamamoto, A., and Moriyama, Y. (2001) J. Biol. Chem. 276, 43400-43406). Here we show that VGLUT2 is specifically localized in glucagon-containing secretory granules but not in synaptic-like microvesicles in αTC6 cells, clonal α cells, and islet α cells. VGLUT1, another VGLUT isoform, is also expressed and localized in secretory granules in α cells. Low glucose conditions triggered co-secretion of stoichiometric amounts of L-glutamate and glucagon from αTC6 cells and isolated islets, which is dependent on temperature and Ca²⁺ and inhibited by phentolamine. Similar cosecretion of L-glutamate and glucagon from islets was observed upon stimulation of β-adrenergic receptors with isoproterenol. Under low glucose conditions, stimulation of glutamate receptors facilitates secretion of γ-aminobutyric acid from MIN6 m9, clonal β cells, and isolated islets. These results indicate that co-secretion of L-glutamate and glucagon from α cells under low glucose conditions triggers GABA secretion from α cells and defines the mode of action of L-glutamate as a regulatory molecule for the endocrine function. To our knowledge, this is the first example of secretory granule-mediated glutamatergic signal transmission.