The Novel Insulinotropic Mechanism of Pimobendan: Direct Enhancement of the Exocytotic Process of Insulin Secretory Granules by Increased Ca2+ Sensitivity in -Cells

Abstract

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 β-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 inβ -cells. This agent increased the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the presence of 30 mm K⁺, 16.7 mm glucose, and 200μ 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 (50μ m) significantly augmented insulin release at 0.32μ m Ca²⁺, and a lower threshold for Ca²⁺-induced insulin release was apparent in pimobendan-treated islets. Moreover, 1 μm 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.