The novel insulinotropic mechanism of pimobendan: direct enhancement of the exocytotic process of insulin secretory granules by increased Ca2+ sensitivity in beta-cells.
Pimobendan is a new class of inotropic drug that augments Ca2+ sensitivity and inhibits phosphodiesterase (PDE) activity in cardiomyocytes. To examine the insulinotropic effect of pimobendan in pancreatic beta-cells, which have an intracellular signaling mechanism similar to that of cardiomyocytes, we measured insulin release from rat isolated islets of Langerhans. Pimobendan augmented glucose-induced insulin release in a dose-dependent manner, but did not increase cAMP content in pancreatic islets, indicating that the PDE inhibitory effects may not be important in beta-cells. This agent increased the intracellular Ca2+ concentration ([Ca2+]i) in the presence of 30 mM K+, 16.7 mM glucose, and 200 microM diazoxide, but failed to enhance the 30 mM K+-evoked [Ca2+]i rise in the presence of 3.3 mM glucose. Insulin release evoked by 30 mM K+ in 3.3 mM glucose was augmented. Then, the direct effects of pimobendan on the Ca2+-sensitive exocytotic apparatus were examined using electrically permeabilized islets in which [Ca2+]i can be manipulated. Pimobendan (50 microM) significantly augmented insulin release at 0.32 microM Ca2+, and a lower threshold for Ca2+-induced insulin release was apparent in pimobendan-treated islets. Moreover, 1 microM KN93 (Ca2+/calmodulin-dependent protein kinase II inhibitor) significantly suppressed this augmentation. Pimobendan, therefore, enhances insulin release by directly sensitizing the intracellular Ca2+-sensitive exocytotic mechanism distal to the [Ca2+]i rise. In addition, Ca2+/calmodulin-dependent protein kinase II activation may at least in part be involved in this Ca2+ sensitization for exocytosis of insulin secretory granules.
Fujimoto
S
S
Department of Metabolism and Clinical Nutrition, Kyoto University Faculty of Medicine, Japan. fujimoto@metab.kuhp.kyotou.ac.jp
Ishida
H
H
Kato
S
S
Okamoto
Y
Y
Tsuji
K
K
Mizuno
N
N
Ueda
S
S
Mukai
E
E
Seino
Y
Y
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D001596:Benzylamines / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002316:Cardiotonic Agents / Q000494:pharmacology*; D000242:Cyclic AMP / Q000032:analysis; D003594:Cytoplasmic Granules / Q000187:drug effects* / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects*; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D009568:Nitrendipine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology; D011724:Pyridazines / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D013449:Sulfonamides / Q000494:pharmacology; D000077191:Wortmannin; D000096926:Benzenesulfonamides
D000730:Androstadienes; D001596:Benzylamines; D002118:Calcium; D002316:Cardiotonic Agents; D000242:Cyclic AMP; D005947:Glucose; D007328:Insulin; D009568:Nitrendipine; D011188:Potassium; D011493:Protein Kinase C; D011724:Pyridazines; D013449:Sulfonamides; D000077191:Wortmannin; C072105:KN 93; D000096926:Benzenesulfonamides; C041648:pimobendan