Background Opioid preconditioning (PC) reduces anoxiaJreoxygenation (NR) injury to various cells. However, it remains unclear whether opioid-induced delayed PC would show anti-apoptotic effects on pulmonary artery endothelial cells (PAECs) suffering from A/R injury. The present study was conducted to elucidate this issue and to investigate the potential mechanism of opioid-induced delayed PC.Methods Cultured porcine PAECs underwent 16-hour anoxia followed by 1-hour reoxygenation 24 hours after pretreatment with saline (NaCI; 0.9%) or morphine (1 μmol/L). To determine the underlying mechanism, a non-selective KATP channel inhibitor glibenclamide (Glib; 10 μmol/L), a nitric oxide (NO) synthase blocker NG-nitro-L-arginine methyl ester (L-NAME; 100 μmol/L), and an opioid receptor antagonist naloxone (Nal; 10 pmol/L) were given 30 minutes before the A/R load. The percentage of apoptotic cells was assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining, eNOS mRNA level was measured by real-time polymerase chain reaction (PCR). NO content of PAECs supatants was measured with the Griess reagent.Results Compared to the A/R PAECs, morphine-induced delayed PC significantly reduced PAECs apoptosis ((18.1±1.9)% vs (5.5±0.3)%; P <0.05), increased NO release ((11.4±1.3) μmol/L vs (20.5±2.1) μmol/L, P <0.05), and up-regulated eNOS gene expression nearly 9 times (P<0.05). The anti-apoptosis effect of morphine was abolished by pretreatment with Glib, L-NAME and Nal, but the three agent-selves did not aggravate the A/R injury. Furthermore, L-NAME and Nal offset the enhanced release of NO caused by pretreatment with morphine.Conclusions Morphine-induced delayed PC prevents A/R injury of PAECs. This effect may be mediated by activation of KATP channel via opioid receptor and NO signaling pathways.