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 [H-3]-choline to incorporate [H-3]-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 alpha(2)-adrenoceptor agonist UK14304 (0.1 and 1 mu M) inhibited the S-I efflux, in a concentration-dependent manner. The inhibition of S-I efflux produced by UK14304 (1 mu M) was antagonized by the selective alpha(2)-adrenoceptor antagonist idazoxan (0.3 mu M). Idazoxan (0.3 mu M) alone had no effect on the S-I efflux. 3 Angiotensin II (0.1 and 1 mu M) 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 mu M), angiotensin II (1 mu M) was also without effect on the S-I efflux. Similarly, in the presence of idazoxan (0.3 mu M), to block prejunctional alpha(2)-adrenoceptors, angiotensin II (0.1 and 1 mu M) did not alter the S-I efflux. When added alone, perindoprilat (10 mu M) did not alter the S-I efflux. 4 In epithelium-denuded preparations, bradykinin (0.01-1 mu M) inhibited the S-I efflux. In epithelium-intact preparations, there was also a tendency for bradykinin (0.1 and 1 mu M) 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 mu M) and phosphoramidon (1 mu M), respectively, bradykinin (1 mu M) 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 mu M) and phosphoramidon (1 mu M), was unaffected by the additional presence of the cyclo-oxygenase inhibitor indomethacin (10 mu M) and/or the nitric oxide synthase inhibitor N-G-nitro-L-arginine (100 mu M), in either epithelium-intact or epithelium-denuded preparations. 5 In conclusion, the findings of the present study suggest that airway parasympathetic nerves are endowed with alpha(2)-adrenoceptors which subserve inhibition of transmitter acetylcholine release. Under the present conditions, however, transmitter acetylcholine release is not subject to transneuronal modulation by noradrenaline released from adjacent sympathetic nerves in the airways. Moreover, angiotensin II and perindoprilat do not appear to modulate acetylcholine release from parasympathetic nerves of the airways. In contrast, bradykinin inhibits acetylcholine release from airway parasympathetic nerves but this action of bradykinin is limited by the activity of epithelial angiotensin-converting enzyme and/or neutral endopeptidase. The inhibitory action of bradykinin on cholinergic transmission in the airways does not appear to involve the liberation of prostaglandins or nitric oxide.