Stimulation by nicotine of enteric inhibitory nerves and release of vasoactive intestinal peptide in the taenia of the guinea-pig caecum

The stimulation by nicotine of intramural nerves and the role of ATP and vasoactive intestinal peptide (VIP) as inhibitory transmitters were studied in the isolated taenia of the guinea-pig caecum. Nicotine (4-32 microM) caused transient, concentration-dependent relaxations which were unaffected by atropine, prazosin or sotalol. Drugs with membrane-stabilizing activity, such as dl-propranolol (0.5 microM), d-propranolol (0.5 microM) or lidocaine (10 microM) antagonized the nicotine-induced relaxation without modifying the response to electrical field stimulation. Similar results were obtained with the cyclooxygenase inhibitor, indomethacin (2.8 microM). Nucleotide pyrophosphatase (0.5 U/ml), which hydrolyzes ATP to AMP, reversibly inhibited the response to nicotine but the response to field stimulation was not decreased. Nicotine evoked a calcium-dependent release of VIP, which was blocked by tetrodotoxin (1 microM), d-propranolol (0.5 microM) or, as previously shown, by apamin (0.2 microM). The finding that nicotine-induced relaxation was accompanied by the neuronal release of VIP is compatible with the possibility that VIP is an inhibitory transmitter but is not definitive evidence, since it could have been due to the stimulation of distinct populations of nerves by nicotine.