Effect of a novel PACAP-27 analogue on muscarinic airway responsiveness in guinea-pigs in vivo.

A recent study showed that the novel pituitary adenylate cyclase-activating peptide (PACAP)-27 analogue [Arg15,20,21,Leu17]-PACAP-27-Gly-Lys-Arg-NH2 causes sustained airway smooth muscle relaxation in vitro. This study examined whether this analogue also has bronchoprotective effects, by inhibiting muscarinic airway responsiveness in vivo. Total lung resistance was measured in anaesthetized, tracheostomized and ventilated guinea-pigs. Increasing doses of acetylcholine were given i.v. once before and thereafter repeatedly each hour after intratracheal instillation of either the PACAP-27 analogue or the clinical beta2-agonist bronchodilator salbutamol. Mean arterial blood pressure (MAP) was monitored to detect cardiovascular side-effects. Both the PACAP-27 analogue and salbutamol significantly attenuated the airway responsiveness to acetylcholine. The total inhibitory effect of the PACAP-27 analogue (350 nmol) corresponded to that of salbutamol (35 nmol). The inhibitory effect of salbutamol (35 nmol) peaked during the second hour and disappeared prior to 5 h after administration. In contrast, the corresponding effect of the analogue (350 nmol) gradually increased and peaked during the fifth hour after administration, whereas it did not fade during the observation period. Both the PACAP-27 analogue (350 nmol) and salbutamol (35 nmol) produced a transient decrease in MAP within 6 min after administration. In conclusion, the novel pituitary adenylate cyclase-activating peptide-27 analogue has bronchoprotective properties, by decreasing muscarinic airway responsiveness in guinea pigs in vivo. The time course of its effect is compatible with a more sustained duration of action compared with salbutamol.