title: A new experimental model of ATP-sensitive K⁺ channel-independent insulinotropic action of glucose: a permissive role of cAMP for triggering of insulin release from rat pancreatic β-cells.
abstract: In pancreatic β-cells, glucose metabolism leads to closure of ATP sensitive K⁺ channels (K(ATP) channel) and Ca²⁺ influx, which is regarded as a required step for triggering of insulin release. Here, we demonstrate that glucose triggers rapid insulin release independent from its action on K(ATP) channels given the cellular cAMP is elevated. We measured insulin release from rat pancreatic islets by static and perifusion experiments. Changes in cytosolic free Ca²⁺ concentration ([Ca²⁺]i) were monitored using fura-2 loaded rat pancreatic β-cells. Glucose-induced insulin release was abolished when Ca²⁺ influx was inhibited by a combination of 250 μM diazoxide, an opener of K(ATP) channel, and 10 μM nifedipine, a blocker of L-type voltage-dependent Ca²⁺ channels. However, with both nifedipine and diazoxide, glucose induced a 5-fold increase in insulin release in the presence of 10 μM forskolin, an activator of adenylyl cyclase. In the presence of diazoxide, nifedipine, and forskolin, 22 mM glucose sharply increased the rate of insulin release within 2 min which peaked at 6 min: this was followed by a further gradual increase in insulin release. In contrast, it lowered [Ca(2+)]i with a nadir at 2-3 min followed by a gradual increase in [Ca²⁺]i. The glucose effect was mimicked by 20 mM α-ketoisocaproic acid, a mitochondrial fuel, and it was nullified by 2 mM sodium azide, an inhibitor of mitochondrial electron transport. Cerulenin, an inhibitor of protein acylation, decreased the glucose effect. In conclusion, a rise in [Ca²⁺]i through voltage-dependent Ca²⁺ channels is not mandatory for glucose-induced triggering of insulin release.
authors: Department of Internal Medicine, Division of Diabetes, Endocrinology and Metabolism, Shinshu University School of Medicine, Matsumoto 390-8621, Japan.
authors: Masahiro
authors: M
authors: Takei
authors: Katsuya
authors: K
authors: Dezaki
authors: Hiroaki
authors: H
authors: Ishii
authors: Shin-ichi
authors: S
authors: Nishio
authors: Yoshihiko
authors: Y
authors: Sato
authors: Satoru
authors: S
authors: Suzuki
authors: Toshihiko
authors: T
authors: Yada
authors: Mitsuhisa
authors: M
authors: Komatsu
mesh_terms: D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000819:agonists / Q000378:metabolism*; D020536:Enzyme Activators / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D007515:Islets of Langerhans / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D054086:KATP Channels / Q000819:agonists / Q000378:metabolism*; D007651:Keto Acids / Q000378:metabolism; D008297:Male; D049990:Membrane Transport Modulators / Q000494:pharmacology; D010085:Oxidative Phosphorylation / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar; D015290:Second Messenger Systems / Q000187:drug effects; D046509:Tissue Culture Techniques
chemical_list: D002121:Calcium Channel Blockers; D020536:Enzyme Activators; D004791:Enzyme Inhibitors; D007328:Insulin; D054086:KATP Channels; D007651:Keto Acids; D049990:Membrane Transport Modulators; C013082:alpha-ketoisocaproic acid; D000242:Cyclic AMP; D005947:Glucose