The novel insulinotropic mechanism of pimobendan:: Direct enhancement of the exocytotic process of insulin secretory granules by increased Ca2+ sensitivity in β-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 mu M 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 mu M) significantly augmented insulin release at 0.32 mu M Ca2+, and a lower threshold for Ca2+-induced insulin release was apparent in pimobendan-treated islets. Moreover, 1 mu M 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.