Intermedin (IMD) is a novel member of the calcitonin/calcitonin gene-related peptide family. We aimed to explore whether the cardioprotective effect of IMD is mediated by inhibiting myocardial endoplasmic reticulum (sarcoplasmic reticulum) stress (ERS). In vitro, IMD1-53 (10(-9), 10(-8), and 10(-7) mol/l) directly inhibited the upregulation of ERS markers such as glucose-regulated protein 78, CCAAT/enhancer binding protein homologous protein, and caspase-12 induced by the ERS inducers tunicamycin (Tm, 10 mg/ml) or dithiothreitol (DTT, 2 mmol/l) in cardiac tissue. IMD1-53 also inhibited Tm- or DTT-induced upregulation of cleaved activating transcription factor 6 and 4. These inhibitory effects of IMD1-53 were abolished by the IMD receptor antagonist IMD17-47 (10(-6) mol/l) and phosphoinositide 3-kinase inhibitor LY294002 (10 mu mol/l). However, preincubation with PD98059 (20 mu mol/l), an extracellular signal-regulated protein kinase inhibitor, and H89 (10 mu mol/l), a protein kinase A inhibitor, could not block the ERS-inhibiting effects of IMD1-53. Furthermore, in an in vivo model of myocardium ischemia/reperfusion (I/R) in rats, administration of IMD1-53 (20 nmol/kg, intravenously) greatly attenuated ERS and ameliorated myocardium impairment induced by I/R. IMD1-53 could exert its cardioprotective effect by inhibiting myocardial ERS, which might be mediated by the phosphoinositide 3-kinase/Akt signaling pathway.