Antagonistic effects of the non-selective opioid receptor antagonist, naloxone, and the selective β2-adrenoceptor (β2-AR) antagonist, ICI 118,551, on NO-release induced by compounds 5, 10, 11 and 25. (a) NO release induced by 10 μM compound 5 (n = 5, 3 and 4 for vehicle, naloxone and ICI 118,551, respectively) in 7TM-MOR transfected cells was abolished by pretreatment with 10 μM naloxone but not by ICI 118,551. (b–d) NO release induced by 10 μM compounds (b) 10, (c) 11 or (d) 25 (n = 5, 4 and 4 for vehicle, naloxone and ICI 118,551 per transfected cell line per compound) was blocked by pretreatment with 10 μM ICI 118,551 in cells transfected with 6TM-MOR alone or together with β2-AR. Data expressed as mean ± SEM