Acetylcholine activates intracellular movement of insulin granules in pancreatic beta-cells via inositol trisphosphate-dependent [correction of triphosphate-dependent] mobilization of intracellular Ca2+.
Intracellular movement of secretory granules is a proximal stage in the secretory cascade that ends in the release product from cells. We investigated mechanisms underlying the control of this movement by acetylcholine using an insulinoma cell line, MIN6, in which acetylcholine increases both insulin secretion and granule movement. The peak activation of movement was observed 3 min after an acetylcholine challenge. The effects were nullified by the muscarinic inhibitor atropine, phospholipase C (PLC) inhibitors (D 609 and compound 48/80), and pretreatment with the Ca2+ pump inhibitor, thapsigargin. Inhibitors of Ca2+-dependent phospholipase A2 (arachidonyl trifluoromethyl ketone and methyl arachidonyl fluorophosphate) also partially inhibited the movement caused by acetylcholine, but downregulation of protein kinase C by overnight incubation with the phorbol ester 12-o-tetradecanoylphorbol-13-acetate failed to exert any influence. Acetylcholine stimulation of granule movement was not reproduced by membrane depolarization with high K+. Phosphorylation of the endogenous myosin light chain in MIN6 cells was increased by addition of acetylcholine and decreased by the Ca2+ chelator BAPTA (1,2-bis[2-aminophenoxy]ethane-N,N,N',N'-tetraacetic acid). The calmodulin inhibitor W-7 and the myosin light-chain kinase inhibitor ML-9 decreased the motile events in the beta-cells under both nonstimulated and acetylcholine-stimulated conditions. These findings led us to conclude that inositol trisphosphate [corrected] causes Ca2+ mobilization by muscarinic activation of PLC, leading to intracellular translocation of insulin granules to the ready-releasable pool in pancreatic beta-cells via Ca2+/calmodulin-dependent phosphorylation of myosin light chains.