Fig. 1. Effect of fentanyl on phenylephrine concentration–response curve in endothelium–denuded pulmonary arterial rings. Fentanyl caused a rightward shift in the phenylephrine concentration–response curve compared with control. Values represent mean ± SEM in all figures. n = 6. * Statistically significant increase ( P < 0.05) in the phenylephrine ED50values after fentanyl treatment compared with control .