Beta-endorphin-sensitive opioid receptors in the rat tail artery

Isolated tail arteries of rats were perfused and field-stimulated every 2 min with 2 pulses at 1 Hz. Different opioid peptides depressed the contractile responses to stimulation; their concentration-response curves showed a maximum at about 40% inhibition. The rank order of potency of the peptides was beta-endorphin (IC50 = 97 nmol/l) approximately equal to BAM-22P greater than FK-33824 greater than DAGO greater than [D-Ala2,D-Leu5]-enkephalin greater than or equal to metorphamide greater than dynorphin A-(1-13) approximately equal to [Met5]enkephalin. All these substances have in common a certain activity at opioid mu-receptors, although the enkephalins are preferential delta-, and the dynorphins preferential kappa-agonists. However, the selective delta-agonist [D-Pen2,L-Pen5]enkephalin was ineffective at up to 10 mumol/l, and the kappa-agonists ethylketocyclazocine and U-50488 acted only at concentrations higher than 3 mumol/l. Whereas the effects of beta-endorphin, DAGO and [D-Ala2,D-Leu5]enkephalin could be reduced by the mu-preferential antagonist naloxone, the effects of ethylketocyclazocine and U-50488 were not changed. The delta-selective antagonist ICI 174864 did not influence the action of [D-Ala2,D-Leu5]enkephalin. Naloxone in a concentration (1 mumol/l) which nearly abolished the effect of DAGO 3 mumol/l, slightly enhanced responses to stimulation. Neither beta-endorphin nor DAGO influenced vasoconstriction evoked by the application of noradrenaline or adenosine triphosphate; U-50488 reduced it. In arteries preincubated with [3H]noradrenaline DAGO depressed, whereas naloxone enhanced the tritium overflow and vasoconstriction evoked by field stimulation (0.4 Hz, 24 pulses every 14 min). In addition, naloxone antagonized the effect of DAGO.(ABSTRACT TRUNCATED AT 250 WORDS)