65 Original Paper Cell Physiol Biochem 2007;20:65-74 Accepted: February 08, 2007 Cellular Physiology Cellular Physiology Cellular Physiology Cellular Physiology Cellular Physiology and Biochemistr and Biochemistr and Biochemistr and Biochemistr and Biochemistry Copyright © 2007 S. Karger AG, Basel Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com © 2007 S. Karger AG, Basel 1015-8987/07/0204-0065$23.50/0 Accessible online at: www.karger.com/cpb The K ATP Channel is Critical for Calcium Sequestration into Non-ER Compartments in Mouse Pancreatic Beta Cells Martina Düfer 1 , Dirk Haspel 1 , Peter Krippeit-Drews 1 , Mandy Kelm 1 , Felicia Ranta 4 , Roland Nitschke 5 , Susanne Ullrich 4 , Lydia Aguilar- Bryan 2 , Joseph Bryan 3 and Gisela Drews 1 1 Institute of Pharmacy, Department of Pharmacology, University of Tübingen, 2 Departments of Medicine and 3 Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas, 4 Institute of Physiology, University of Tübingen, 5 Institute of Biology I, Life Imaging Center, University of Freiburg Prof. Dr. Gisela Drews Institute of Pharmacy, Department of Pharmacology, University of Tübingen Auf der Morgenstelle 8, D-72076 Tübingen (Germany) Tel. +49-7071-2977559, Fax +49-7071-295382 E-Mail gisela.drews@uni-tuebingen.de Key Words Cyclopiazonic acid  Cytosolic calcium concentration  FCCP  K ATP channels  Mitochondrial Ca 2+ storage  Ratiometric pericam  Sulfonylureas  Sur1KO mice Abstract K ATP channel activity influences beta cell Ca 2+ homeostasis by regulating Ca 2+ influx through L-type Ca 2+ channels. The present paper demonstrates that loss of K ATP channel activity due to pharmacologic or genetic ablation affects Ca 2+ storage in intracellular organelles. ATP depletion, by the mitochondrial inhibitor FCCP, led to Ca 2+ release from the endoplasmic reticulum (ER) of wildtype beta cells. Blockade of ER Ca 2+ ATPases by cyclopiazonic acid abolished the FCCP-induced Ca 2+ transient. In beta cells treated with K ATP channel inhibitors FCCP elicited a significantly larger Ca 2+ transient. Cyclopiazonic acid did not abolish this Ca 2+ transient suggesting that non- ER compartments are recruited as additional Ca 2+ stores in beta cells lacking K ATP channel activity. Genetic ablation of K ATP channels in SUR1KO mice produced identical results. In INS-1 cells transfected with a mitochondrial-targeted Ca 2+ -sensitive fluorescence dye (ratiometric pericam) the increase in mitochondrial Ca 2+ evoked by tolbutamide was 5- fold larger compared to 15 mM glucose. These data show that genetic or pharmacologic ablation of K ATP channel activity conveys Ca 2+ release from a non-ER store. Based on the sensitivity to FCCP and the property of tolbutamide to increase mitochondrial Ca 2+ it is suggested that mitochondria are the recruited store. The change in Ca 2+ sequestration in beta cells treated with insulinotropic antidiabetics may have implications for beta cell survival and the therapeutic use of these drugs. Introduction An increase in the cytosolic Ca 2+ concentration ([Ca 2+ ] c ) is essential for stimulus-secretion coupling in pancreatic beta cells because Ca 2+ constitutes the trigger signal for the exocytosis of insulin. Glucose, the most important natural beta cell fuel, induces closure of K ATP channels via changes in the ATP/ADP ratio which lead