Voltage-dependent K(+) channels are positive regulators of alpha cell action potential generation and glucagon secretion in mice and humans.
OBJECTIVE
The regulation of glucagon secretion from alpha cells is poorly understood. Since action potential firing at low glucose is required for glucagon secretion, we hypothesised that voltage-dependent K(+) (Kv) currents limit glucagon secretion under these conditions, similarly to their role in insulin secretion.


METHODS
Kv currents and action potential firing of mouse and human alpha cells, identified by immunostaining, were examined by whole-cell patch-clamp. Glucagon secretion from mouse and human islets was measured by ELISA.


RESULTS
Kv current density was 35% larger in alpha than in beta cells. Alpha cell Kv channels were sensitive to block by tetraethylammonium (TEA) and 4-aminopyridine. Surprisingly, Kv channel inhibition reduced glucagon release to the same extent as glucose. Robust action potential firing was observed in beta cells when ATP-sensitive K(+) channels were closed, but in alpha cells a negative current (-8 pA) injection was required for action potential firing. TEA (0.5 mmol/l) impaired alpha cell action potential firing, which could be restored by further hyperpolarising current injection (-16 pA). Kv currents were more sensitive to the Kv2 inhibitor stromatoxin (100 nmol/l) in mouse (80%) than in human (45%) alpha cells. Finally, the maxi-K (BK) channel inhibitor iberiotoxin (100 nmol/l) blocked 55% of the current in human alpha cells and inhibited glucagon release from human islets.


CONCLUSIONS
Kv currents in alpha cells are positive regulators of glucagon secretion. These currents, mediated by Kv2 and BK channels, limit membrane depolarisation, and prevent inactivation of alpha cell action potentials and suppression of glucagon release.
Spigelman
A F
AF
Alberta Diabetes Institute, University of Alberta, Edmonton, AB, Canada.
Dai
X
X
MacDonald
P E
PE
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D004594:Electrophysiology; D004797:Enzyme-Linked Immunosorbent Assay; D005934:Glucagon / Q000378:metabolism*; D050416:Glucagon-Secreting Cells / Q000187:drug effects* / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007515:Islets of Langerhans / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D024642:Potassium Channels, Voltage-Gated / Q000037:antagonists & inhibitors / Q000378:metabolism*; D019789:Tetraethylammonium / Q000494:pharmacology
D024642:Potassium Channels, Voltage-Gated; D019789:Tetraethylammonium; D005934:Glucagon; D015761:4-Aminopyridine
J Physiol. 2005 Aug 15;567(Pt 1):159-75
15932888
Diabetes. 2004 Dec;53 Suppl 3:S181-9
15561909
Diabetes. 2004 Mar;53(3):597-607
14988243
J Physiol. 2000 Nov 1;528(Pt 3):497-507
11060127
Endocr Rev. 2007 Feb;28(1):84-116
17261637
J Physiol. 2000 Nov 1;528(Pt 3):509-20
11060128
Diabetes. 2008 Jun;57(6):1618-28
18390794
PLoS Biol. 2007 Jun;5(6):e143
17503968
Endocrinology. 2006 Oct;147(10):4655-63
16857746
Endocrinology. 2007 May;148(5):2157-67
17303668
Pharmacol Rev. 2005 Dec;57(4):473-508
16382104
Mol Endocrinol. 2001 Aug;15(8):1423-35
11463864
Endocrinology. 2005 Nov;146(11):4861-70
16081632
J Physiol. 1970 Sep;210(2):255-64
5501259
J Biol Chem. 1996 Dec 13;271(50):32241-6
8943282
J Biol Chem. 2009 Oct 30;284(44):30441-52
19690348
Cell Metab. 2007 Sep;6(3):229-35
17767909
Endocrinology. 2005 Nov;146(11):4766-75
16109783
Endocr Rev. 2007 May;28(3):253-83
17409288
J Gen Physiol. 1988 Feb;91(2):223-42
2453603
Diabetes. 2009 Feb;58(2):412-21
19008345
Diabetes. 2000 Sep;49(9):1500-10
10969834
J Biol Chem. 2002 Nov 22;277(47):44938-45
12270920
Diabetologia. 2003 Aug;46(8):1046-62
12830383