Interaction between halothane and mu, delta and kappa opioid agonists on the isolated right atria of the rat.

We have examined the interaction between halothane and specific opioid agonists at mu (morphine and [D-ala2 N-mephe4, gly-ol5]-enkephalin (DAGO)), delta ([D-pen2,5]-enkephalin (DPDPE)) and kappa (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrroliknyl)cyclohexyl]- bencetamide methane sulphonate (U-50,488H)) receptors on the isolated right atria of the rat. All the opioid agonists tested decreased atrial rate. The maximal effects obtained with U-50,488H (75 (SE 3.3)%) were significantly (P < 0.001) greater than those obtained with morphine (12 (2.7)%), DAGO (8 (0.6)%) or DPDPE (11 (1.8)%). Halothane 1.5 v/v% did not modify the inhibitory effects induced by morphine, DAGO or DPDPE. However, U-50,488H had a potentiating effect in the presence of halothane 1.5 v/v% (P < 0.001). Naloxone 5 x 10(-7) and 1 x 10(-6) mol litre-1 antagonized the inhibitory effects of U-50,488H in the presence of halothane. We conclude that halothane increased the potency of a kappa agonist on isolated right atria and suggest that this effect was mediated by opioid receptors.