Voltage-gated ion channels in human pancreatic beta-cells: electrophysiological characterization and role in insulin secretion.
OBJECTIVE
To characterize the voltage-gated ion channels in human beta-cells from nondiabetic donors and their role in glucose-stimulated insulin release.


METHODS
Insulin release was measured from intact islets. Whole-cell patch-clamp experiments and measurements of cell capacitance were performed on isolated beta-cells. The ion channel complement was determined by quantitative PCR.


RESULTS
Human beta-cells express two types of voltage-gated K(+) currents that flow through delayed rectifying (K(V)2.1/2.2) and large-conductance Ca(2+)-activated K(+) (BK) channels. Blockade of BK channels (using iberiotoxin) increased action potential amplitude and enhanced insulin secretion by 70%, whereas inhibition of K(V)2.1/2.2 (with stromatoxin) was without stimulatory effect on electrical activity and secretion. Voltage-gated tetrodotoxin (TTX)-sensitive Na(+) currents (Na(V)1.6/1.7) contribute to the upstroke of action potentials. Inhibition of Na(+) currents with TTX reduced glucose-stimulated (6-20 mmol/l) insulin secretion by 55-70%. Human beta-cells are equipped with L- (Ca(V)1.3), P/Q- (Ca(V)2.1), and T- (Ca(V)3.2), but not N- or R-type Ca(2+) channels. Blockade of L-type channels abolished glucose-stimulated insulin release, while inhibition of T- and P/Q-type Ca(2+) channels reduced glucose-induced (6 mmol/l) secretion by 60-70%. Membrane potential recordings suggest that L- and T-type Ca(2+) channels participate in action potential generation. Blockade of P/Q-type Ca(2+) channels suppressed exocytosis (measured as an increase in cell capacitance) by >80%, whereas inhibition of L-type Ca(2+) channels only had a minor effect.


CONCLUSIONS
Voltage-gated T-type and L-type Ca(2+) channels as well as Na(+) channels participate in glucose-stimulated electrical activity and insulin secretion. Ca(2+)-activated BK channels are required for rapid membrane repolarization. Exocytosis of insulin-containing granules is principally triggered by Ca(2+) influx through P/Q-type Ca(2+) channels.
Braun
Matthias
M
Oxford Centre for Diabetes Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, UK. matthias.braun@drl.ox.ac.uk
Ramracheya
Reshma
R
Bengtsson
Martin
M
Zhang
Quan
Q
Karanauskaite
Jovita
J
Partridge
Chris
C
Johnson
Paul R
PR
Rorsman
Patrik
P
D002478:Cells, Cultured; D003035:Cobalt / Q000494:pharmacology; D004594:Electrophysiology; D020869:Gene Expression Profiling; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D007515:Islets of Langerhans / Q000378:metabolism / Q000502:physiology; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000235:genetics / Q000502:physiology*; D010455:Peptides / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012604:Scorpion Venoms / Q000494:pharmacology; D020867:omega-Agatoxin IVA / Q000494:pharmacology; D020866:omega-Conotoxin GVIA / Q000494:pharmacology
D007328:Insulin; D051036:Large-Conductance Calcium-Activated Potassium Channels; D010455:Peptides; D012604:Scorpion Venoms; D020867:omega-Agatoxin IVA; D003035:Cobalt; C064719:iberiotoxin; D020866:omega-Conotoxin GVIA