Mechanisms of glucagon secretion in mouse pancreatic alpha-cells

Abstract

Under hypoglycemic conditions, glucagon is secreted from α-cells, within pancreatic islets of Langerhans, to stimulate hepatic glucose output and, therefore, to restore proper glycemia. Once normoglycemia is reestablished, glucagon release is inhibited. Two general models have been proposed to account for this suppression: direct inhibition by glucose or indirect inhibition by paracrine factors released in the islet. To rigorously identify α-cells in the intact islet, we took advantage of transgenic mice expressing fluorescent proteins specifically in this cell-type. α-cell NAD(P)H responses to glucose demonstrate that α-cells metabolize glucose; glucokinase being the likely rate-limiting enzyme. Glucagon secretagogues such as arginine and pyruvate also enhance α-cell metabolic redox state, indicating that such an elevation is not sufficient to inhibit secretion. Importantly, glucose stimulates glucagon output from pure populations of flow-sorted α-cells. These observations argue against a direct effect of glucose and support the paracrine inhibition model. Pharmacological modulations of ion channels under low glucose conditions indicate that activation of L-type voltage-gated calcium channels is integral for α-cell calcium oscillations and glucagon secretion. In addition, α-cell [Ca2+]i and glucagon release are affected by KATP channel activity in a manner similar to insulin-secreting α-cells. Closure of KATP leads to greater [Ca2+]i and hormone output, whereas opening has the opposite effect. As a result, modulation of KATP channel activity could constitute a possible mechanism for regulating glucagon secretion. In particular, paracrine inhibitors could potentially suppress α-cell secretory activity by opening KATP channels and reducing [Ca2+]i. Because glucagon release from islets is inhibited by glucose, one would naively expect α-cell [Ca2+]i to drop concomitantly. However, our calcium imaging studies in intact islets reveal that glucose slightly elevates α-cell [Ca2+]i. Application of candidate paracrine inhibitors (insulin, zinc, GABA, and somatostatin) inhibits glucagon secretion but does not reduce α-cell calcium activity either. Taken together, the data indicate that [Ca2+]i and glucagon secretion are uncoupled at inhibitory concentrations of glucose, and that suppression occurs downstream from α-cell calcium signaling, presumably at the level of vesicle trafficking or exocytotic machinery.