Glucagon stimulates exocytosis in mouse and rat pancreatic α-cells by binding to glucagon receptors

Glucagon, secreted by the pancreatic α-cells, stimulates insulin secretion from neighboring β-cells by CAMP- and protein kinase A (PKA)-dependent mechanisms, but it is not known whether glucagon also modulates its own secretion. We have addressed this issue by combining recordings of membrane capacitance (to monitor exocytosis) in individual α-cells with biochemical assays of glucagon secretion and cAMP content in intact pancreatic islets, as well as analyses of glucagon receptor expression in pure α-cell fractions by RT-PCR. Glucagon stimulated cAMP generation and exocytosis dose dependently with an EC₅₀ of 1.6-1.7 nM. The stimulation of both parameters plateaued at concentrations beyond 10 nM of glucagon where a more than 3-fold enhancement was observed. The actions of glucagon were unaffected by the GLP-1 receptor antagonist exendin-(9-39) but abolished by des-His ¹-[Glu⁹]-glucagonamide, a specific blocker of the glucagon receptor. The effects of glucagon on α-cell exocytosis were mimicked by forskolin and the stimulatory actions of glucagon and forskolin on exocytosis were both reproduced by intracellular application of 0.1 mM cAMP. cAMP-potentiated exocytosis involved both PKA-dependent and -independent (resistant to Rp-cAMPS, an Rp-isomer of cAMP) mechanisms. The presence of the cAMP-binding protein cAMP-guanidine nucleotide exchange factor II in α-cells was documented by a combination of immunocytochemistry and RT-PCR and 8-(4-chloro-phenylthio)-2′-O-methyl-cAMP, a cAMP-guanidine nucleotide exchange factor II-selective agonist, mimicked the effect of cAMP and augmented rapid exocytosis in a PKA-independent manner. We conclude that glucagon released from the α-cells, in addition to its well-documented systemic effects and paracrine actions within the islet, also represents an autocrine regulator of α-cell function.