Prejunctional opioid receptors in the pulmonary artery of the rabbit

The possible existence of prejunctional opioid receptors at postganglionic sympathetic vasoconstrictor axons was studied in superfused spiral strips of the main pulmonary artery of the rabbit. Ethylketocyclazocine 0.1 and 1 mumol/l and dynorphin-(1-13) 0.1 mumol/l but not morphine 0.01-1 mumol/l, (D-Ala2, NMePhe4, Gly-ol5)-enkephalin 0.001-1 mumol/l, Leu5-enkephalin 0.1-5 mumol/l and (D-Pen2, L-Pen5)-enkephalin 0.01-1 mumol/l reduced contractions of the artery strips elicited by electrical field stimulation at 0.5 or 1 Hz. The effect of ethylketocyclazocine was antagonized by naloxone. Ethylketocyclazocine 1 mumol/l did not change contractions elicited by exogenous noradrenaline (5-10 nmol/l). In artery strips preincubated with 3H-noradrenaline, ethylketocyclazocine 1 mumol/l diminished both the overflow of tritium and the contraction evoked by field stimulation at 1 Hz. All inhibitory effects were small, amounting maximally to about 20%. The results indicate that the postganglionic sympathetic axons of the artery possess prejunctional opioid receptors, activation of which leads to inhibition of the release of noradrenaline and, in consequence, inhibition of neurogenic vasoconstriction. It seems likely that the receptors are, at least predominantly, of the kappa-type.