Regulation of glucagon secretion from pancreatic alpha cells
Sammanfattning: Glucagon secreted by pancreatic α-cells plays an important role in the regulation of blood glucose. In this thesis, different techniques such as electrophysiology, immunohistochemistry and hormone secretion assay were combined to explore the mechanisms by which glucagon secretion is regulated. Like pancreatic β-cells, which produce insulin, α-cells are electrically excitable and equipped with different ion channels. But unlike β-cells, α-cells electrical activity is only possible within a narrow window of low K(ATP) channel activity. At low glucose, the K(ATP) channels are almost fully – but not completely –inhibited. This allows firing of action potentials during which voltage-gated Na+ and Ca2+ channels open. The resultant Ca2+ influx through N-type Ca2+-channels triggers exocytosis of glucagon containing granules. We now provide evidence that glucose depolarizes the α-cell by closing K(ATP) channels in the α-cell. This depolarization in turn leads to (partial) inactivation of the channels involved in action potential firing. Thus, unlike what is the case in β-cells, further closure of K(ATP)-channels results in reduced electrical excitability of the α-cell and culminates in inhibition of glucagon secretion. The ability of low (micromolar) concentrations of the KATP-channel activator diazoxide to reverse the inhibitory effect of glucose supports this hypothesis. We also demonstrate that GLP-1 inhibits glucagon secretion by protein kinase A (PKA)-dependent inhibition of the N-type Ca2+-channels. This effect is through binding to classical GLP-1 receptors and mediated by low concentrations of cyclic AMP (cAMP). Adrenaline also acts on α-cells by cAMP-dependent mechanisms. However, unlike GLP-1, adrenaline stimulates glucagon secretion. This is because α-cells contain 100- to 1000-fold more β-adrenoreceptors than GLP-1 receptors. Exposure to adrenaline can therefore be expected to increase intracellular cAMP to high levels. Using knockout mice and selective agonists we show that adrenaline amplifies the secretory capacity of α-cells by activation of the low-affinity cAMP sensor Epac2 as well as PKA. In addition to PKA, protein kinase C (PKC) was also found to be involved in the regulation of glucagon secretion from both human and murine α-cells. Inhibition of PKC using a pharmacological antagonist suggested that tonic background activity of PKC is required to maintain a high exocytotic capacity of the α-cell. Interestingly, the complement of PKC isoforms present in human islets is rather different from that found in rodent islets. This illustrates the importance of verifying data obtained in rodent systems using human cells.
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