Kiyonori Ito, 1 Katsuya Dezaki, 2 Masashi Yoshida, 1 Hodaka Yamada, 1 Rina Miura, 2 Rauza Sukma Rita, 2 Susumu Ookawara, 1 Kaoru Tabei, 1,3 Masanobu Kawakami, 1,4 Kazuo Hara, 1 Yoshiyuki Morishita, 1 Toshihiko Yada, 2 and Masafumi Kakei 1,5 Endogenous a2A-Adrenoceptor– Operated Sympathoadrenergic Tones Attenuate Insulin Secretion via cAMP/TRPM2 Signaling Diabetes 2017;66:699–709 | DOI: 10.2337/db16-1166 In pancreatic b-cells, pharmacological concentrations of catecholamines, including adrenaline, have been used to inhibit insulin release and explore the multiple mechanisms involved. However, the signi ficance of these signaling path- ways for physiological adrenergic functions in b-cells is largely unknown. In the process of glucose-induced insulin secretion, opening of background current through nonse- lective cation channels (NSCCs) might facilitate membrane depolarization by closure of the ATP-sensitive K + channels. Here, we examined whether physiological insulinostatic adrenaline action is mediated via the transient receptor po- tential melastatin 2 (TRPM2) channel, a type of NSCC, in b-cells. Results showed that physiological concentrations of adrenaline strongly suppressed glucose-induced and incretin-potentiated cAMP production and insulin secretion and inhibited NSCCs current and membrane excitability via the a2A-adrenoceptor in wild-type mice; however, insulin secretion was not attenuated in TRPM2-knockout (KO) mice. Administration of yohimbine, an a2-adrenoceptor antagonist, failed to affect glucose tolerance in TRPM2-KO mice, in contrast to an improved glucose tolerance in wild- type mice receiving the antagonist. The current study dem- onstrated that a physiological concentration of adrenaline attenuates insulin release via coupling of a2A-adrenoceptor to cAMP/TRPM2 signaling, thereby providing a potential therapeutic tool to treat patients with type 2 diabetes. Adrenaline secreted from the chromaffin cells of the adrenal medulla and noradrenaline released from sympathetic nerve endings inhibit insulin release from the pancreatic islets and elevate blood glucose levels. In the islet b-cells, catechol- amines are reported to inhibit insulin release via multiple mechanisms: activation of a2-adrenoceptors coupled to per- tussis toxin–sensitive Gi/Go proteins resulting in inhibition of adenylyl cyclase (1), activation of hyperpolarizing K + cur- rents (2,3), inhibition of Ca 2+ currents (4,5), and direct inhibition of exocytosis at a distal step (6–8). Nevertheless, the significance of these signaling pathways for physiolog- ical adrenergic functions in b-cells is largely unknown, be- cause pharmacologically higher concentrations (mmol/L order) of catecholamines have been used for the analysis of these signaling mechanisms. During the process of glucose-stimulated insulin se- cretion in b-cells, opening of the background inward current through nonselective cation channels (NSCCs) might facilitate depolarization after glucose metabolism– induced closure of the ATP-sensitive K + (K ATP ) channels (9–13). We previously reported that the transient recep- tor potential melastatin 2 (TRPM2) channel, a type of NSCC, in b-cells plays an essential role in glucose-induced and incretin-potentiated insulin secretion (9). Glucose metabolism and glucagon-like peptide 1 (GLP-1) receptor stimulation both increase the activity of TRPM2 chan- nels via cAMP signaling (9). Thus, Gi/Go-mediated in- hibition of cAMP production is expected to attenuate the channel activity. However, whether the modulation 1 First Department of Integrated Medicine, Saitama Medical Center, Jichi Medical University, Saitama, Japan 2 Division of Integrative Physiology, Department of Physiology, Jichi Medical Uni- versity, Tochigi, Japan 3 Minamiuonuma City Hospital, Niigata, Japan 4 Nerima Hikarigaoka Hospital, Tokyo, Japan 5 Saitama Citizens Medical Center, Saitama, Japan Corresponding authors: Katsuya Dezaki, dezaki@jichi.ac.jp, and Masafumi Kakei, mkakei@jichi.ac.jp. Received 25 September 2016 and accepted 21 December 2016. This article contains Supplementary Data online at http://diabetes .diabetesjournals.org/lookup/suppl/doi:10.2337/db16-1166/-/DC1. K.I., K.D., and M.Y. contributed equally to this work. R.S.R. is currently affiliated with the Department of Biochemistry, Faculty of Medicine, Andalas University, Padang, Indonesia. © 2017 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. More information is available at http://www.diabetesjournals .org/content/license. Diabetes Volume 66, March 2017 699 ISLET STUDIES Downloaded from http://diabetesjournals.org/diabetes/article-pdf/66/3/699/535702/db161166.pdf by guest on 22 April 2023