Abstract
1. The effects of several drugs, including antagonists of vasoactive intestinal peptide (VIP), and antisera to VIP or peptide histidine isoleucine (PHI), on relaxation responses of guinea-pig isolated trachea to electrical field stimulation (EFS) have been examined. 2. beta-Adrenoceptor blockade with propranolol only partially blocked the inhibitory response to EFS, but had no effect in tissues from animals pretreated with 6-hydroxydopamine or reserpine. 3. Neither adenosine deaminase, in the presence of dipyridamole, nor the potent adenosine antagonist NPC205 (1,3-n-dipropyl-8-(4-hydroxyphenyl)-xanthine) had any effect on the inhibitory response to EFS. 4. The VIP antagonists, [Ac-Tyr1, D-Phe2]-GRF(1-29)-NH2 and [4-Cl-D-Phe6, Leu17]-VIP had no effect on the inhibitory response to EFS. Moreover, they were without effect on responses to exogenous VIP or PHI. 5. Overnight incubation with VIP antisera markedly reduced the inhibitory response to EFS. PHI antisera had a similar, but smaller effect. 6. In the presence of a concentration of VIP that is maximal for its relaxant effect, inhibitory responses to electrical stimulation were greatly inhibited. 7. Naloxone and reactive blue 2 each had no effect on inhibitory responses indicating that endogenous opioids and adenosine 5'-triphosphate (ATP) respectively are not involved. 8. The results suggest that VIP and PHI, but not adenosine, contribute to non-adrenergic, noncholinergic inhibitory nerve responses of guinea-pig trachea. Moreover, the surprising lack of effect of both VIP antagonists on these responses, and in particular, on responses to exogenous VIP, suggests that the receptors mediating VIP-induced tracheal relaxation are different from those that mediate pancreatic secretion.