G protein-independent activation of an inward Na+ current by muscarinic receptors in mouse pancreatic β-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 Ca2+ 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.