INTERACTION BETWEEN HALOTHANE AND MU-OPIOID, DELTA-OPIOID 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-pyrrolikynyl)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.