Release of [3H]Acetylcholine in Human Isolated Bronchi: Effect of Indomethacin on Muscarinic Autoinhibition

Receptor-mediated regulation of acetylcholine release in the airways, particularly in humans, remains unclear. In the present study, the tissue content of acetylcholine and release of [3H]acetylcholine were measured in freshly dissected human bronchi obtained at thoracotomy. Large (main and lobar bronchi) and small (segmental and subsegmental bronchi) airways contained considerable amounts of endogenous acetylcholine (300 +/- 50 pmol/100 mg wet weight), whereas significantly less was found in lung parenchyma (60 +/- 30 pmol/100 mg). Isolated small bronchi incubated in an organ bath with the precursor [3H]choline synthesized significant amounts of [3H]acetylcholine (26,000 +/- 4,000 dpm/100 mg). Subsequent transmural stimulation (four 20 s trains at 15 Hz) of radiolabeled bronchi caused an enhanced tritium outflow that was abolished by removal of extracellular calcium or by tetrodotoxin. HPLC analysis of the medium collected before, during, and after transmural stimulation showed that the electrically stimulated tritium outflow represented exclusively [3H]acetylcholine, whereas the outflow of [3H]choline and [3H]phosphorylcholine was not affected by electrical stimulation. Oxotremorine (0.1 and 1 mumol/L) inhibited evoked [3H]acetylcholine release in a concentration-related manner, whereas atropine (0.03 mumol/L) enhanced evoked [3H]acetylcholine release. Inactivation of cyclooxygenase activity by 3 mumol/L of indomethacin did not impair the inhibitory effect of 0.1 or 1 mumol/L of oxotremorine. In conclusion, the present experiments indicate a considerable cholinergic innervation of human large and small airways.