Opioid receptor types on adrenergic nerve terminals of rabbit ear artery

Abstract

Methionine enkephalin, leucine enkephalin, [D-Ala2, D-Leu5] enkephalin, alpha-neoendorphin, beta-endorphin, dynorphin (1-13) and ethylketocyclazocine inhibited the contractions of rabbit ear artery ring segments elicited by transmural nerve stimulation at 8 Hz. Ethylketocyclazocine, dynorphin (1-13) and leucine enkephalin produced partial inhibition, their apparent intrinsic activities (alpha) being 0.57, 0.75 and 0.66, respectively. Morphine and normorphine, which are agonists at mu-receptors, did not inhibit the response of the artery. Naloxone antagonized the actions of opioids and ethylketocyclazocine, and was more effective against methionine enkephalin, leucine enkephalin and [D-Ala2, D-Leu5] enkephalin than against alpha-neoendorphin, ethylketocyclazocine and dynorphin (1-13). The pA2 values of naloxone against so-called delta-agonists were approx. 8.5, and against so-called kappa-agonists were approx. 7.7. The supposed kappa-antagonist, Mr2266, was more effective than naloxone in antagonizing the actions of alpha-neoendorphin, and the kappa-agonists dynorphin (1-13) and ethylketocyclazocine. The pA2 values of Mr2266 against kappa-agonists were 8.5-9.0, and against delta-agonists were 7.8 or less. The opioid peptides and opioids tested did not cause dilatation of the artery previously contracted with histamine. These results suggest that the opioid peptides and ethylketocyclazocine acted on opioid receptors at adrenergic nerve terminals in the ear artery. The opioid receptors appear to be of the delta- and kappa-types, not the mu-type.