Classification of presynaptic opioid receptors in the rabbit ear artery by competitive antagonists.

Electrical field stimulation (5 impulses at 5 Hz every min) evoked vasoconstriction in the rabbit isolated ear artery. The vasoconstriction was depressed by [Leu5]enkephalin (IC50 = 47 nM) and ethylketocyclazocine (IC50 = 80 nM). Naloxone 300 nM antagonized the effect of both agonists with a similar potency; the respective KB values were 41 and 64 nM. ICI 174864 1000 nM antagonized only the effect of [Leu5]enkephalin (KB = 126 nM) but not that of ethylketocyclazocine. In contrast, MR 2266 100 nM was a more potent antagonist of ethylketocyclazocine (KB = 8 nM) than of [Leu5]enkephalin (KB = 45 nM). The results suggest that the rabbit ear artery contains both presynaptic delta- and kappa-receptors.