Comparison of the action of putative endogenous kappa-agonists, leumorphin and rimorphin in vitro.

Leumorphin and rimorphin (dynorphin B) were nearly equipotent in inhibiting the contraction of the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum and the rabbit vas deferens. Leumorphin had long duration of action in vitro after wash-out, whereas the duration of action of rimorphin was relatively shorter. These action of leumorphin and rimophin were antagonized less effectively by naloxone than by Mr2266, an antagonist relatively specific for the kappa-receptor. Rimorphin as well as leumorphin inhibits the contraction of the rabbit vas deferens which contains solely kappa-receptors. It is concluded that leumorphin and rimorphin have equipotent opioid activity and act at the kappa-receptor, like other opioid peptides derived from preproenkephalin B.