br Acknowledgments This research was financially supported b
Acknowledgments This research was financially supported by National Natural Science Foundation of China, Nos. 31500926 and 81503282, Natural Science Fund of Guangdong Province, No 2014A030310023, Science and Technology Plan of Guangdong Province, Nos. 2014A020212309, 2013B010404044, 2017A010105011), and Education Project of Guangdong Province (2013KJCX0107, 2016KTSCX067). We thank Quliang Gu from Guangdong Pharmaceutical University and Yuhua Liu from South China Agricultural University for proofreading and providing critical comments on the manuscript.
Introduction Terpenes are isoprenoids formed by six units of isoprene (C5H8)6, derived from mevalonic salubrinal . Some pentacyclic triterpenes, such as oleanolic acid, are known to exhibit biological actions, especially in glucose homeostasis, reducing glycemia and acting as an antidiabetic agent, increasing serum insulin and glucose transporter-4 (GLUT4) translocation [2,3]. However, the mechanisms of action of most triterpenes in insulin secretion are not completely elucidated; specifically, betulinic acid (BA) has potent insulin-secreting and antihyperglycemic effects that are mediated by unknown mechanisms . The ionic channels are important targets of action for these compounds in the process of insulin secretion. The classical pathway of glucose-induced insulin secretion is mediated by the ATP-dependent potassium channels (KATP) and voltage-dependent calcium channels (VDCCs). The increase in calcium influx through the VDCCs leads to an abrupt increase in cytoplasmic calcium concentration, which in turn activates additional effector systems responsible for the exocytosis of insulin granules . Among these effectors are proteins kinases and other ionic channels, such as chloride channels [6,7]. The potassium, calcium and chloride channels are of great physiological and cellular importance, since they associate biochemical metabolism and cellular electrical activity . The various types of chloride channels present differential distributions and functions ; of these, the voltage-dependent chloride channels (ClC-3 channel) are suggested to have a critical role in vesicle acidification, due to the presence of a protonic type V electrogenic pump (H+-ATPase). This effect is important for the vesicular traffic caused by the proton electrochemical gradient  and essential for the process of insulin maturation and secretion in response to electrical changes and activation following the stimulus of a ligand to a specific receptor . Another important subgroup of the chloride channel family that can be associated with insulin secretion are the calcium-activated chloride channels (CaCCs), which are found at the plasma membrane of various tissues. The opening of these channels leads to depolarization of the cell membrane, followed by opening of VDCCs. CaCCs stimulators are still poorly understood, although it is known that these channels can be activated by increasing the intracellular calcium concentration in response to an agonist after the release of calcium from internal stores and by kinases such as protein kinase calcium/calmodulin-dependent (PKCaMKII) . Due to their cellular depolarization effects, these channels may be involved in vesicular exocytosis  and may participate in the process of insulin release. Diabetes mellitus and insulin resistance are disorders that require careful and controlled care considering the use of specific medications, such as the administration of exogenous insulin (type 1 diabetes) or oral hypoglycemic agents (type 2 diabetes and/or insulin resistance) that stimulate insulin secretion . However, as already described, chronic treatment with oral hypoglycemic agents has adverse effects, such as the chronic use of sulfonylureas, the most commonly administered drug, which can lead to eventual β-cell failure and reduced insulin secretion . Therefore, alternatives such as the use of new exogenous substances, such as compounds from medicinal plants and metabolites of natural compounds, have become an increasingly widespread practice. However, such treatments may be harmful to the patient due to the lack of scientific evidence of the action and mechanism of action of these substances. As BA is a potent insulin secretagogue and an insulinomimetic compound, the aim of the present study was to investigate the intracellular pathways involved in its effects on insulin secretion and on calcium influx in pancreatic islets.