Comparison of the pre-junctional effect of opioids in two blood vessels of the rabbit.

Inhibition of contractile responses to adrenergic nerve stimulation by opioid peptides was compared using in vitro ring segments of rabbit ear and saphenous arteries transmurally stimulated with short (8 Hz, 1 s) and long (2 Hz, 25 s) stimulation trains. Both delta- ([Leu5]enkephalin or [Met5]enkephalin) and kappa-selective agonists (dynorphin-(1-13) or ethylketocyclazocine) inhibited nerve stimulation-evoked contraction; however, with delta-selective agonists, maximum inhibitory responses were significantly less in the saphenous than in the ear artery. If has been hypothesized that activation of pre-junctional alpha 2-adrenoceptors inhibits effects of opioid receptor activation. Thus the actions of opioid agonists were compared with and without yohimbine (10(-7) M). With yohimbine, inhibitory responses to opioids were not increased at any frequency tested (3-8 Hz) in either vessel. Thus delta and kappa opioid receptors inhibit adrenergic transmitter release in both blood vessels, with delta receptor effects being more prominent in the ear artery. Secondly, studies with alpha 2-adrenoceptor blockade suggest that there is no interaction between pre-junctional opioid and alpha 2-adrenoceptors.