INHIBITION OF ATP-SENSITIVE K+ CHANNEL BY A NON-SULFONYLUREA COMPOUND KAD-2229 IN A PANCREATIC BETA-CELL LINE, MIN-6 CELL

We studied the mechanism of action of KAD-1229, a non-sulfonylurea compound shown to stimulate insulin secretion, in a glucose responsive insulinoma cell line, MIN 6 cells. In microsomal fraction of MIN 6 cells, KAD-1229 displaced binding of [H-3]glibenclamide in a concentration-dependent manner. The dissociation constant and the maximum binding capacity were 0.61 nM and 8.70 pmol/mg.protein, respectively. In inside out configuration of patch-clamp technique, KAD-1229 attenuated the opening of ATP-sensitive K+ channels. The effect of KAD-1229 was detected at 10(-8) M, and 10(-5) M KAD-1229 almost completely blocked the activity of ATP-sensitive Kf channel. When membrane potential was monitored by a perforated mode of patch clamp, KAD-1229 induced depolarization of plasma membrane, which was followed by a burst of action potentials. These action potentials were blocked by cobalt. In a fura-2-loaded single MIN 6 cell, KAD evoked an elevation of intracellular free Ca2+ concentration, [Ca2+](i). The KAD-1229-mediated elevation of [Ca2+](i) was attenuated by either removal of extracellular Ca2+ or an addition of nifedipine. Finally, KAD-1229 augmented insulin secretion in MIN 6 cells in a concentration-dependent manner. KAD-1229 also enhanced the effect of glucose and nifedipine inhibited the action of KAD-1229 on insulin secretion. These results indicate that KAD-1229 stimulates insulin secretion by stimulating Ca2+ influx and that, despite the lack of sulfonylurea structure, KAD-1229 binds to sulfonylurea receptors and inhibits the activity of ATP-sensitive K+ channel in MIN 6 cells.