In guinea‑pig trachea, the peptide VIP makes the airway muscles relax. Two synthetic versions of a related peptide (one based on GRF‑1‑29) can block this relaxation, but only at relatively high concentrations. The study shows how these compounds interfere with VIP’s effect in a lab setting.

1. The tracheal pouch, a surgical preparation designed to demonstrate non-adrenergic non-cholinergic inhibition, was prepared in chloralose/urethane-anaesthetized and positively ventilated guinea-pigs. The animals were given atropine and propranolol intraperitoneally to block cholinergic and adrenergic divisions of the autonomic innervation. The cervical vagi and sympathetic trunks were isolated bilaterally and cut cranially. 2. The relaxation responses of the pouch to graded concentrations of VIP (10(-11) M to 10(-6) M) were determined. Two consecutive dose-response curves at 20 min apart were determined in the control group. The VIP dose-response curves in the control group were reproducible and failed to show statistical significance upon paired Student's t-test. 3. [Ac-Tyr1,D-Phe2]-GRF(1-29)-amide, a VIP antagonist hereby referred to as antagonist-1 was tested for its ability to inhibit the VIP-induced pouch relaxation. Separate groups of animals were used for each concentration (10(-8) M, 10(-6) M or 10(-5) M) of the antagonist. VIP dose-response curves were determined before and after the pouch was incubated with the antagonist for 10 min. The second curve was determined after rinsing the pouch gently with saline and allowing 5 min for the pouch to stabilize. Statistical analysis showed that only 10(-5) M of the antagonist significantly blocked the VIP-induced tracheal pouch relaxation. 4. [4-C1-D-Phe6,Leu17]-VIP, another VIP antagonist hereby referred to as antagonist-2 was tested similarly for its ability to block the VIP-induced pouch relaxation. The significant blockade of the VIP-induced pouch relaxation was obtained with this antagonist at a concentration of 10(-6) M.(ABSTRACT TRUNCATED AT 250 WORDS)