In the present study, we determined whether endomorphin1 (EM1) and endomorphin2 (EM2), selective endogenous mu-opioid receptor (MOR) agonists, inhibited the response to EFS in rat isolated bronchus in a concentration- and frequency-dependent manner. EM1 (1 microM) produced significant inhibition at relatively low frequencies (< 5 Hz) (74.02 +/- 5.53%, 56.16 +/- 10.24% and 37.64 +/- 5.92% inhibition at 1, 2 and 4 Hz, respectively, p < 0.05 versus control), but no significant inhibition at 8, 16, 32 and 64 Hz (17.15 +/- 9.4%, 14.51 +/- 4.23%, 9.11 +/- 2.38% and 5.93 +/- 3.5%, respectively, p > 0.05 versus control). Similar modulations were observed in response to EM2 (1 microM). It is therefore considered that the inhibition effects of EM1 and EM2 may take place at frequencies under physiological conditions. Furthermore, EM1 and EM2 (0.01-10 microM) induced inhibition of cholinergic constriction in a dose-dependent manner at 1, 2 and 4 Hz. The inhibitory effect on EFS was blocked by the opioid receptor antagonist naloxone (10 microM), indicating that opioid receptors were involved. Neither EM1 nor EM2 (1 microM) had an effect on the contractile response to exogenous acetylcholine, indicating a prejunctional effect. All the results indicate that EM1 and EM2 are potent inhibitors of EFS-induced cholinergic bronchoconstriction. These also imply that EM1 and EM2 may modulate cholinergic bronchoconstriction under physiological conditions and that these tetrapeptides could have therapeutic potential in the treatment of airway diseases.