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Regulation of glucagon secretion from pancreatic alpha cells

De Marinis, Yang LU (2009) In Lund University, Faculty of Medicine Doctoral Dissertation Series 2010:4.
Abstract
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+... (More)
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. (Less)
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author
supervisor
opponent
  • Professor Hoiriis Nielsen, Jens, University of Copenhagen, Denmark
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Epac2, glucagon secretion, pancreatic alpha cells, K-(ATP) channel, N-type Ca2+ channel, L-type Ca2+ channel, GLP-1, cAMP, PKA, Adrenaline, PKC
in
Lund University, Faculty of Medicine Doctoral Dissertation Series
volume
2010:4
pages
120 pages
publisher
Department of Clinical Sciences, Lund University
defense location
Medelhavet, Wallenberglaboratoriet, Malmö University Hospital, Malmö, Sweden
defense date
2010-01-22 13:15
ISSN
1652-8220
ISBN
978-91-86443-18-4
language
English
LU publication?
yes
id
a865e9cf-2770-425d-8edb-eadaf02b7f1a (old id 1517097)
date added to LUP
2010-01-05 14:39:08
date last changed
2016-09-19 08:44:47
@phdthesis{a865e9cf-2770-425d-8edb-eadaf02b7f1a,
  abstract     = {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. <br/><br>
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. <br/><br>
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). <br/><br>
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. <br/><br>
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.},
  author       = {De Marinis, Yang},
  isbn         = {978-91-86443-18-4},
  issn         = {1652-8220},
  keyword      = {Epac2,glucagon secretion,pancreatic alpha cells,K-(ATP) channel,N-type Ca2+ channel,L-type Ca2+ channel,GLP-1,cAMP,PKA,Adrenaline,PKC},
  language     = {eng},
  pages        = {120},
  publisher    = {Department of Clinical Sciences, Lund University},
  school       = {Lund University},
  series       = {Lund University, Faculty of Medicine Doctoral Dissertation Series},
  title        = {Regulation of glucagon secretion from pancreatic alpha cells},
  volume       = {2010:4},
  year         = {2009},
}