G protein-independent activation of an inward Na(+) current by muscarinic receptors in mouse pancreatic beta-cells.
Depolarization of pancreatic beta-cells is critical for stimulation of insulin secretion by acetylcholine but remains unexplained. Using voltage-clamped beta-cells, we identified a small inward current produced by acetylcholine, which was suppressed by atropine or external Na(+) omission, but was not mimicked by nicotine, and was insensitive to nicotinic antagonists, tetrodotoxin, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DiDS), thapsigargin pretreatment, and external Ca(2+) and K(+) removal. This suggests that muscarinic receptor stimulation activates voltage-insensitive Na(+) channels distinct from store-operated channels. No outward Na(+) current was produced by acetylcholine when the electrochemical Na(+) gradient was reversed, indicating that the channels are inward rectifiers. No outward K(+) current occurred either, and the reversal potential of the current activated by acetylcholine in the presence of Na(+) and K(+) was close to that expected for a Na(+)-selective membrane, suggesting that the channels opened by acetylcholine are specific for Na(+). Overnight pretreatment with pertussis toxin or the addition of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) or guanosine-5'-O-(2-thiodiphosphate) (GDP-beta-S) instead of GTP to the pipette solution did not alter this current, excluding involvement of G proteins. Injection of a current of a similar amplitude to that induced by acetylcholine elicited electrical activity in beta-cells perifused with a subthreshold glucose concentration. These results demonstrate that muscarinic receptor activation in pancreatic beta-cells triggers, by a G protein-independent mechanism, a selective Na(+) current that explains the plasma membrane depolarization.
Rolland
Jean-François
JF
Unité d'Endocrinologie et Métabolisme, University of Louvain, Faculty of Medicine, UCL 55.30, Avenue Hippocrate 55, B-1200 Brussels, Belgium.
Henquin
Jean-Claude
JC
Gilon
Patrick
P
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D018408:Patch-Clamp Techniques; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism*
D011976:Receptors, Muscarinic; D015222:Sodium Channels; D012964:Sodium; D019204:GTP-Binding Proteins; D000109:Acetylcholine; D002118:Calcium