Aims: 

Opioid peptides exert direct vascular actions (via opioid receptors located in the vascular wall), explained to a certain extent by the presynaptic inhibition of the neuro-effector junction and by the release of nitric oxide from the endothelium, but there are few studies regarding their action in resistance arteries. Based on our own studies upon analgesic properties, we have selected for testing the vascular effects the following peptides (10^-9-10^-6 M): leucin-enkephalin, methionin-enkephalin, endomorphine 1, endomorphine 2, morphiceptin, Phe-Gly-Gly and Phe-Gly (the C-terminal tri-peptide and di-peptide from nociceptin).

Methods: 

We used isometric myography to study the effect of these peptides in small mesenteric arteries (~2 mm wide and ~0.15 mm diameter rings) from adult male Wistar rats. In preparations precontracted by 0.01 mM phenylephrine we examined the relaxing effect of each peptide in three circumstances: de-endothelised rings, control intact rings and intact rings under inhibition of nitric-oxide-synthase by 0.01 mM L-NAME. The results are expressed as remnant active tension, % from the precontraction level (mean ± ESM; n = 4).

Results: 

We noticed, among others, the weak effect of methionin-enkephalin, the partial endothelium-dependence and NO-independence of the effect for the nociceptin fragments, the presence of the effect for small concentrations and the NO-dependence in the case of endomorphins.

Conclusion: 

All these represent novel findings and the possible physiological relevance is discussed in the context of few available data on the vascular action of enkephalin-related peptides.

*CNCSIS grant A/1222 **CNCSIS grant TD/421 ***CNCSIS grant interdisciplinary platform /68