A new hypoglycemic agent, JTT-608, evokes protein kinase A-mediated Ca(2+) signaling in rat islet beta-cells: strict regulation by glucose, link to insulin release, and cooperation with glucagon-like peptide-1(7-36)amide and pituitary adenylate cyclase-activating polypeptide.
A new nonsulfonylurea oral hypoglycemic agent, JTT-608, has been reported to stimulate insulin release at elevated, but not low, glucose concentrations and consequently not to induce hypoglycemia in rats. Accordingly, this drug is potentially a safer antidiabetic agent than sulfonylureas. To explore the mechanisms underlying this glucose-dependent insulinotropism, the present study investigated the effects of JTT-608 on cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and protein kinase A (PKA) activity in rat islet beta-cells by microfluorometry using, respectively, fura-2 and a fluorescence PKA substrate, DR II. In the presence of glucose at normal and elevated concentrations (5.0-16.7 mM) JTT-608 (30-1000 microM) concentration dependently increased [Ca(2+)](i) in up to 88% of single beta-cells, whereas at lower glucose concentrations (2.8 and 4.2 mM) it had little effect. The [Ca(2+)](i) responses were inhibited under Ca(2+)-free conditions and by nitrendipine, an L-type Ca(2+) channel blocker. JTT-608 rapidly activated PKA and a PKA inhibitor, H89, inhibited [Ca(2+)](i) responses to JTT-608. JTT-608 also stimulated insulin release from rat islets in a glucose- and Ca(2+)-dependent manner. The glucose-unresponsive beta-cells, which failed to respond to 8.3 mM glucose with increases in [Ca(2+)](i), were frequently recruited to [Ca(2+)](i) increases by JTT-608. JTT-608 also induced oscillations of [Ca(2+)](i). Glucagon-like peptide-1(7-36)amide (GLP-1), pituitary adenylate cyclase-activating polypeptide (PACAP), and acetylcholine (ACh) enhanced the action of JTT-608 on [Ca(2+)](i). In conclusion, JTT-608 evokes PKA-mediated Ca(2+) influx and Ca(2+) signaling in rat islet beta-cells in a glucose-regulated manner, which may account for its glucose-dependent insulinotropism. JTT-608 and neurohormones may cooperatively activate islet beta-cells under physiological conditions.
Hashiguchi
S
S
Department of Physiology, Faculty of Medicine, Kagoshima University, Kagoshima, Japan.
Yada
T
T
Arima
T
T
D000109:Acetylcholine / Q000502:physiology; D000818:Animals; D002087:Butyrates / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects*; D002469:Cell Separation; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D003510:Cyclohexanes / Q000494:pharmacology*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005934:Glucagon; D052216:Glucagon-Like Peptide 1; D004763:Glucagon-Like Peptides; D005947:Glucose / Q000502:physiology*; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D009479:Neuropeptides / Q000494:pharmacology*; D010446:Peptide Fragments / Q000494:pharmacology*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D051381:Rats; D017208:Rats, Wistar
C495403:Adcyap1 protein, rat; D002087:Butyrates; D002121:Calcium Channel Blockers; D003510:Cyclohexanes; D004791:Enzyme Inhibitors; D007004:Hypoglycemic Agents; D007328:Insulin; D009479:Neuropeptides; D010446:Peptide Fragments; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; C055453:glucagon-like peptide 1 (7-36)amide; D004763:Glucagon-Like Peptides; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D017868:Cyclic AMP-Dependent Protein Kinases; D005947:Glucose; D000109:Acetylcholine; C116719:4-(4-methylcyclohexyl)-4-oxobutyric acid