Intracellular Ca2+ modulation of ATP-sensitive K+ channel activity in acetylcholine-induced activation of rat pancreatic β-cells

We investigated the mechanism by which acetylcholine (ACh) regulates insulin secretion from rat pancreatic beta-cells. In an extracellular solution with 5.5 mM glucose, ACh increased the rate of insulin secretion from rat islets. In islets treated with bisindolylmaleimide (BIM), a PKC inhibitor, ACh still increased insulin secretion, but the increment was lower than that without BIM. In the presence of nifedipine, an L-type Ca2+ channel blocker, on the other hand, ACh did not increase insulin secretion. In isolated rat pancreatic beta-cells, ACh caused depolarization followed by action potentials. This ACh effect was observed even in cells treated with BIM. In the presence of nifedipine, ACh caused only depolarization. These ACh effects were prevented by atropine. In the perforated whole-cell configuration, ramp pulses from -90 to -50 mV induced membrane currents mostly through ATP-sensitive K+ channels (K-ATP). These currents were reduced in size by ACh in cells either treated or untreated with BIM; whereas the loading of cells with U-73122 (a phospholipase C inhibitor) or BAPTA/AM (a Ca2+ chelator) abolished the ACh effect. In the standard whole-cell configuration, ACh reduced the currents through K-ATP with 0.5 mM EGTA, but not with 10 mM EGTA, in the pipette solution. Intracellular application of GDPbetaS or heparin also inhibited the ACh effect. In the inside-out single-channel recordings, elevation of the Ca2+ concentration inside the membrane from 10 nM-10 muM decreased K-ATP activity only in the presence of ATP. The affinity of ATP to K-ATP became 4.5 times higher with the higher concentration of Ca2+. These results suggest that Ca2+ from ACh receptor signaling modulates the sensitivity of K-ATP to ATP. A positive-feedback mechanism of intracellular Ca2+-dependent Ca2+ influx was also demonstrated.