Membrane potential-dependent inactivation of voltage-gated ion channels in alpha-cells inhibits glucagon secretion from human islets.
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.
Ramracheya
Reshma
R
Oxford Centre for Diabetes Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, UK. matthias.braun@drl.ox.ac.uk
Ward
Caroline
C
Shigeto
Makoto
M
Walker
Jonathan N
JN
Amisten
Stefan
S
Zhang
Quan
Q
Johnson
Paul R
PR
Rorsman
Patrik
P
Braun
Matthias
M
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
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