A novel enhancer of insulinotrophic action by high glucose (JTT-608) stimulates insulin secretion from pancreatic beta-cells via a new cellular mechanism

Insulin secretion from MIN6 cells (a pancreatic beta -cell line) induced by high glucose (greater than 16.8 mM) was potentiated by a novel hypoglycemi c agent [trans-4-(4-methylcyclohexyl)-4-oxobutyric acid (JTT-608)] (but not glibenclamide, a sulfonylurea). The extracellular Ca2+-free condition, a L -type Ca2+ channel blocker (nifedipine) and an ATP-sensitive Ki channel ope ner, diazoxide, completely inhibited increases in cytosolic free Ca2+ ([Ca2 +](i)) and insulin secretion evoked by JTT-608 in the presence of extracell ular Ca2+. An electrophysiological study using single-barreled microelectro de techniques demonstrated that membrane potential (V-m) and input resistan ce of the cell membrane (R-i) are depolarized acid increased by JTT-608, re spectively. The apparent transference number for Kt was also significantly decreased after the addition of JTT-608. These effects immediately occurred after addition of JTT-608 and very rapidly disappeared after removal of JT T-608, which has not been observed in sulfonylureas. Also, these effects of JTT-608 were diminished, but not completely by diazoxide, JTT-608 did not affect the specific binding of [H-3]glibenclamide to the sulfonylurea recep tor. These findings suggest that JTT-608 mainly inhibits ATP-sensitive K+ c hannel activity via a binding site distinct from the sulfonylurea receptor and then depolarizes V-m to open voltage-dependent L-type Ca2+ channels. Su bsequently, these events stimulate Ca2+ entry to increase [Ca2+](i) and ind uce insulin secretion from MIN6 cells. Therefore, JTT-608 is a unique hypog lycemic agent that enhances high glucose-induced insulin secretion. The pre sent findings indicate that JTT-608 is a more useful new class of therapeut ic drug for patients with non-insulin-dependent diabetes mellitus, compared with sulfonylurea derivatives.