Abstract
Insulin secretion from MIN6 cells (a pancreatic β-cell line) induced by high glucose (greater than 16.8 mM) was potentiated by a novel hypoglycemic 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 K+ channel opener, diazoxide, completely inhibited increases in cytosolic free Ca2+ ([Ca2+]i) and insulin secretion evoked by JTT-608 in the presence of extracellular Ca2+. An electrophysiological study using single-barreled microelectrode techniques demonstrated that membrane potential (Vm) and input resistance of the cell membrane (Ri) are depolarized and increased by JTT-608, respectively. The apparent transference number for K+ 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 JTT-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 [3H]glibenclamide to the sulfonylurea receptor. These findings suggest that JTT-608 mainly inhibits ATP-sensitive K+ channel activity via a binding site distinct from the sulfonylurea receptor and then depolarizesVm to open voltage-dependent L-type Ca2+ channels. Subsequently, these events stimulate Ca2+ entry to increase [Ca2+]i and induce insulin secretion from MIN6 cells. Therefore, JTT-608 is a unique hypoglycemic agent that enhances high glucose-induced insulin secretion. The present findings indicate that JTT-608 is a more useful new class of therapeutic drug for patients with non-insulin-dependent diabetes mellitus, compared with sulfonylurea derivatives.
Footnotes
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Send reprint requests to: Dr. Koji Okada, Division of Endocrinology and Metabolism, Department of Medicine, Jichi Medical School, 3311-1 Minamikawachi Tochigi 329-0498, Japan.
- Abbreviations:
- NIDDM
- non-insulin-dependent diabetes mellitus
- JTT-608
- trans-4-(4-methylcyclohexyl)-4-oxobutyric acid
- DX
- diazoxide
- Vm
- membrane potential
- Ri
- input resistance of the cell membrane
- tk
- apparent transference number for K+
- KRBB
- Krebs-Ringer bicarbonate buffer
- BSA
- bovine serum albumin
- Received August 28, 2000.
- Accepted March 1, 2001.
- The American Society for Pharmacology and Experimental Therapeutics