The calcium-sensing receptor (CaSR) exists in many tissues, and its expression has been identified in rat cardiac tissue. However, the physiological importance and pathophysiological involvement of CaSR in homeostatic regulation of cardiac function are unclear. To investigate the relation of CaSR and apoptosis in cardiomyocytes, we examined the role of the CaSR activator gadolinium chloride (GdCl3) in rat neonatal ventricular cardiomyocytes. Expression of the CaSR protein was observed by Western blot. The apoptotic ratio of rat neonatal ventricular cardiomyocytes was measured with flow cytometry and immunofluorescence techniques. A laser scan confocal microscope was used to detect the intracellular concentration of calcium ([Ca2+](i)) in rat neonatal ventricular cardiornyocytes using the acetoxymethyl ester of fluo-3 (fluo-3/(AM)) as a fluorescent dye. The results showed that GdCl3 increased the phosphorylation of extracellular signal-regulated protein kinase (ERK), c-Jun NH2-terminal protein kinases (JNK), and p38. GdCl3 also activated caspase 9 and increased apoptosis in myocyte by increasing [Ca2+](i). In conclusion, these results suggest that CaSR promotes cardiomyocyte apoptosis in rat neonatal ventricular cardiornyocytes through activation of mitogen-activated protein kinases and caspase 9 signaling pathways. (c) 2006 Elsevier Inc. All rights reserved.