Calcium‐Sensing Receptors Induce Apoptosis in Rat Cardiomyocytes via the Endo(sarco)plasmic Reticulum Pathway during Hypoxia/Reoxygenation

: The calcium‐sensing receptor (CaR) is a G protein‐coupled receptor. The CaR stimulation elicits phospholipase C‐mediated inositol triphosphate formation, leading to an elevation in the level of intracellular calcium released from endoplasmic reticulum (ER). Depletion of ER Ca2+ leads to ER stress, which is thought to induce apoptosis. Intracellular calcium overload‐induced apoptosis in cardiac myocytes during hypoxia–reoxygenation (H/Re) has been demonstrated. However, the links between CaR, ER stress and apoptosis during H/Re are unclear. This study hypothesized that the CaR could induce apoptosis in neonatal rat cardiomyocytes during H/Re via the ER stress pathway. Neonatal rat cardiomyocytes were subjected to 3 hr of hypoxia, followed by 6 hr of reoxygenation. CaR expression was elevated and the number of apoptotic cells was significantly increased, as shown by transferase‐mediated dUTP nick end‐labelling, with exposure to CaCl2, a CaR activator, during H/Re. The intracellular calcium concentration was significantly elevated and the Ca2+ concentration in the ER was dramatically decreased during H/Re with CaCl2; both intracellular and ER calcium concentrations were detected by laser confocal microscopy. Expression of GRP78 (glucose‐regulated protein 78), the cleavage products of ATF6 (activating transcription factor 6), phospho‐PERK [pancreatic ER kinase (PKR)‐like ER kinase], the activated fragments of caspase‐12, and phospho‐JNK (c‐Jun NH2‐terminal kinase) were increased following exposure to CaCl2 during H/Re. Our results confirmed that the activated CaR can induce cardiomyocyte apoptosis via ER stress‐associated apoptotic pathways during H/Re.