Voltage-dependent K⁺ channels are positive regulators of alpha cell action potential generation and glucagon secretion in mice and humans

• Published in: Diabetologia 2010-09, Vol.53 (9), p.1917-1926
• Main Authors: Spigelman, A. F, Dai, X, MacDonald, P. E
• Format: Article
• Language: English
• Subjects: 4-Aminopyridine - pharmacology; Action potential; Action Potentials - physiology; Alpha cell; Animals; Biological and medical sciences; Diabetes. Impaired glucose tolerance; Electrophysiology; Endocrine pancreas. Apud cells (diseases); Endocrinopathies; Enzyme-Linked Immunosorbent Assay; Etiopathogenesis. Screening. Investigations. Target tissue resistance; glucagon; Glucagon - secretion; Glucagon-Secreting Cells - drug effects; Glucagon-Secreting Cells - metabolism; Human Physiology; Humans; In Vitro Techniques; Internal Medicine; ion channels; Islets of Langerhans - metabolism; Male; Medical sciences; Medicine; Medicine & Public Health; Metabolic Diseases; Mice; Mice, Inbred C57BL; Potassium Channels, Voltage-Gated - antagonists & inhibitors; Potassium Channels, Voltage-Gated - metabolism; Tetraethylammonium; Tetraethylammonium - pharmacology; Voltage-dependent K⁺ channels
• ISSN: 0012-186X; 1432-0428
• DOI: 10.1007/s00125-010-1759-z

Aims/hypothesis 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/interpretation 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.