title: Membrane potential-dependent inactivation of voltage-gated ion channels in alpha-cells inhibits glucagon secretion from human islets.
abstract: OBJECTIVE
  To document the properties of the voltage-gated ion channels in human pancreatic alpha-cells and their role in glucagon release.
  
  
  METHODS
  Glucagon release was measured from intact islets. [Ca(2+)](i) was recorded in cells showing spontaneous activity at 1 mmol/l glucose. Membrane currents and potential were measured by whole-cell patch-clamping in isolated alpha-cells identified by immunocytochemistry.
  
  
  RESULTS
  Glucose inhibited glucagon secretion from human islets; maximal inhibition was observed at 6 mmol/l glucose. Glucagon secretion at 1 mmol/l glucose was inhibited by insulin but not by ZnCl(2). Glucose remained inhibitory in the presence of ZnCl(2) and after blockade of type-2 somatostatin receptors. Human alpha-cells are electrically active at 1 mmol/l glucose. Inhibition of K(ATP)-channels with tolbutamide depolarized alpha-cells by 10 mV and reduced the action potential amplitude. Human alpha-cells contain heteropodatoxin-sensitive A-type K(+)-channels, stromatoxin-sensitive delayed rectifying K(+)-channels, tetrodotoxin-sensitive Na(+)-currents, and low-threshold T-type, isradipine-sensitive L-type, and omega-agatoxin-sensitive P/Q-type Ca(2+)-channels. Glucagon secretion at 1 mmol/l glucose was inhibited by 40-70% by tetrodotoxin, heteropodatoxin-2, stromatoxin, omega-agatoxin, and isradipine. The [Ca(2+)](i) oscillations depend principally on Ca(2+)-influx via L-type Ca(2+)-channels. Capacitance measurements revealed a rapid (<50 ms) component of exocytosis. Exocytosis was negligible at voltages below -20 mV and peaked at 0 mV. Blocking P/Q-type Ca(2+)-currents abolished depolarization-evoked exocytosis.
  
  
  CONCLUSIONS
  Human alpha-cells are electrically excitable, and blockade of any ion channel involved in action potential depolarization or repolarization results in inhibition of glucagon secretion. We propose that voltage-dependent inactivation of these channels underlies the inhibition of glucagon secretion by tolbutamide and glucose.
authors.affiliation: Oxford Centre for Diabetes Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, UK. matthias.braun@drl.ox.ac.uk
authors.forename: Reshma
authors.initials: R
authors.lastname: Ramracheya
authors.forename: Caroline
authors.initials: C
authors.lastname: Ward
authors.forename: Makoto
authors.initials: M
authors.lastname: Shigeto
authors.forename: Jonathan N
authors.initials: JN
authors.lastname: Walker
authors.forename: Stefan
authors.initials: S
authors.lastname: Amisten
authors.forename: Quan
authors.initials: Q
authors.lastname: Zhang
authors.forename: Paul R
authors.initials: PR
authors.lastname: Johnson
authors.forename: Patrik
authors.initials: P
authors.lastname: Rorsman
authors.forename: Matthias
authors.initials: M
authors.lastname: Braun
mesh_terms: D002118:Calcium / Q000502:physiology*; D018929:Cell Culture Techniques; D002712:Chlorides / Q000494:pharmacology; D005934:Glucagon / Q000009:adverse effects / Q000378:metabolism*; D050416:Glucagon-Secreting Cells / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D005947:Glucose / Q000494:pharmacology*; D006801:Humans; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D015640:Ion Channel Gating / Q000502:physiology*; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology*; D015221:Potassium Channels / Q000502:physiology; D013004:Somatostatin / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology; D014044:Tolbutamide / Q000494:pharmacology; D017967:Zinc Compounds / Q000494:pharmacology
chemical_list: D002712:Chlorides; D007328:Insulin; D015221:Potassium Channels; D017967:Zinc Compounds; D013779:Tetrodotoxin; D013004:Somatostatin; C016837:zinc chloride; D005934:Glucagon; D014044:Tolbutamide; D005947:Glucose; D002118:Calcium