Interaction of the renin–angiotensin system, bradykinin and sympathetic nerves with cholinergic transmission in the rat isolated trachea

1 The present study was undertaken to investigate the interaction of the renin–angiotensin system (RAS), bradykinin and the sympathetic nervous system with cholinergic transmission in the rat airways. Experiments were performed on epithelium‐intact and epithelium‐denuded preparations of rat isolated trachea which had been incubated with [3H]‐choline to incorporate [3H]‐acetylcholine into the cholinergic transmitter stores. Tracheal preparations were subjected to electrical field stimulation (trains of 1 ms pulses, 5 Hz, 15 V) and the stimulation‐induced (S‐I) efflux taken as an index of transmitter acetylcholine release. 2 In both epithelium‐intact and epithelium‐denuded tracheal preparations, the α2‐adrenoceptor agonist UK14304 (0.1 and 1 μM) inhibited the S‐I efflux, in a concentration‐dependent manner. The inhibition of S‐I efflux produced by UK14304 (1 μM) was antagonized by the selective α2‐adrenoceptor antagonist idazoxan (0.3 μM). Idazoxan (0.3 μM) alone had no effect on the S‐I efflux. 3 Angiotensin II (0.1 and 1 μM) was without effect on the S‐I efflux in either epithelium‐intact or epithelium‐denuded tracheal preparations. When angiotensin‐converting enzyme was inhibited by perindoprilat (10 μM), angiotensin II (1 μM) was also without effect on the S‐I efflux. Similarly, in the presence of idazoxan (0.3 μM), to block prejunctional α2‐adrenoceptors, angiotensin II (0.1 and 1 μM) did not alter the S‐I efflux. When added alone, perindoprilat (10 μM) did not alter the S‐I efflux. 4 In epithelium‐denuded preparations, bradykinin (0.01–1 μM) inhibited the S‐I efflux. In epithelium‐intact preparations, there was also a tendency for bradykinin (0.1 and 1 μM) to inhibit the S‐I efflux but this was not statistically significant. However, when angiotensin‐converting enzyme and neutral endopeptidase were inhibited by perindoprilat (10 μM) and phosphoramidon (1 μM), respectively, bradykinin (1 μM) significantly inhibited the S‐I efflux in epithelium‐intact preparations as well as in epithelium‐denuded preparations. The inhibition of the S‐I efflux produced by bradykinin, in the combined presence of perindoprilat (10 μM) and phosphoramidon (1 μM), was unaffected by the additional presence of the cyclo‐oxygenase inhibitor indomethacin (10 μM) and/or the nitric oxide synthase inhibitor NG‐nitro‐L‐arginine (100 μM), in either epithelium‐intact or epithelium‐denuded preparations. 5 In conclusion, the findings of the present study suggest that airway parasympatheti