Long lasting smooth muscle relaxation by a novel PACAP analogue in guinea-pig and primate airways in vitro

1. We compared the relaxant effect of pituitary adenylate cyclase activating peptide (PACAP) 1–27 with that of a newly developed PACAP 1–27 analogue, [Arg(15,20,21)Leu(17)]-PACAP-Gly-Lys-Arg-NH(2), in the guinea-pig trachea and primate bronchi in vitro (n=4–5). 2. In the guinea-pig trachea precontracted by a submaximally effective carbachol concentration (0.1 μM), cumulative administration of PACAP 1–27 and the β(2)-adrenoceptor agonist salbutamol (3 nM–3 μM) caused significant and concentration-dependent smooth muscle relaxation, with salbutamol being approximately one log-step more potent in this model. However, in primate bronchi precontracted by carbachol (0.1 μM), cumulative administration of PACAP 1–27 and salbutamol caused concentration-dependent smooth muscle relaxation with very similar potencies and maximum relaxant effects. 3. In the guinea-pig trachea, non-cumulative administration of the PACAP 1–27 analogue and the original PACAP 1–27 (0.3–3 μM) caused concentration-dependent relaxation with a very similar maximum relaxant effect and potency. However, the onset and offset of action was markedly slower for the PACAP 1–27 analogue than for the original PACAP 1–27 (>90% versus <10% of peak relaxation remaining 6 h after administration). Separate experiments confirmed that the PACAP 1–27 analogue also caused significant relaxation with slower onset and offset of action than did the original PACAP 1–27 in primate bronchi. 4. Peptidase inhibition by captopril (10 μM) and phosphoramidon (1 μM) significantly increased the maximum relaxant effect and duration of action of PACAP 1–27 but not of the PACAP 1–27 analogue, during the 3 h of observation in the guinea-pig trachea. 5. We conclude that [Arg(15,20,21)Leu(17)]-PACAP-Gly-Lys-Arg-NH(2) produces significant, concentration-dependent and sustained airway smooth muscle relaxation in vitro. The sustained relaxant effect is due, at least in part, to the PACAP 1–27 analogue being less susceptible to cleavage by peptidases than the original peptide PACAP 1–27