Vesicular Inhibitory Amino Acid Transporter Is Present in Glucagon-Containing Secretory Granules in αTC6 Cells, Mouse Clonal α-Cells, and α-Cells of Islets of Langerhans

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Abstract
Islets of Langerhans contain γ-aminobutyrate (GABA) and may use it as an intercellular transmitter. In β-cells, GABA is stored in synaptic-like microvesicles and secreted through Ca2+-dependent exocytosis. Vesicular inhibitory amino acid transporter (VIAAT), which is responsible for the storage of GABA and glycine in neuronal synaptic vesicles, is believed to be responsible for the storage and secretion of GABA in β-cells. However, a recent study by Chessler et al. indicated that VIAAT is expressed in the mantle region of islets. In the present study, we investigated the precise localization of VIAAT in rat islets of Langerhans and clonal islet cells and found that it is present in α-cells, a minor population of F-cells and αTC6 cells, and clonal α-cells but not in β-cells, δ-cells, or MIN6 m9-cells (clonal β-cells). Combined biochemical, immunohistochemical, and electronmicroscopical evidence indicated that VIAAT is specifically localized with glucagon-containing secretory granules in α-cells. ATP-dependent uptake of radiolabeled GABA, which is energetically coupled with a vacuolar proton pump, was detected in digitonin-permeabilized αTC6 cells as well as in MIN6 m9 cells. These results demonstrate that functional neuronal VIAAT is present in glucagon-containing secretory granules in α-cells and suggest that the ATP-dependent GABA transporter in β-cells is at least immunologically distinct from VIAAT. Because glucagon-containing secretory granules also contain vesicular glutamate transporter and store l-glutamate, as demonstrated by Hayashi et al., the present results suggest more complex features of the GABAergic phenotype of islets than previously supposed.