Origin and modulation of ACh release from rat airway cholinergic nerves

Abstract

The release of acetylcholine (ACh) from airway parasympathetic nerves was studied in rat trachea. We established stimulus parameters, examined the role of extracellular Ca2+, and investigated the origin of the released ACh by use of vesamicol, an inhibitor of ACh uptake in synaptic vesicles. The role of muscarinic autoreceptors and prostanoids on ACh release was also studied. Tracheal rings were incubated in Krebs-Henseleit solution containing neostigmine and guanethidine with or without atropine. ACh release was measured by high-performance liquid chromatography with electrochemical detection. ACh release was dependent on frequency (0.5-16 Hz), voltage (10-25 V), and pulse duration (0.5-4 ms). At 4 Hz, one-fifth of electrical field stimulation-induced ACh release was extracellular Ca2+ independent and vesamicol resistant, indicating its nonvesicular origin. Three-fifths were Ca2+ dependent and vesamicol sensitive, indicating that it was newly synthesized, and one-fifth was Ca2+ dependent but vesamicol resistant, indicating its origin from prestored vesicles. At 16 Hz, two-fifths were nonvesicular and three-fifths were newly synthesized. Blockade of the muscarinic autoreceptor by atropine potentiated the release of ACh four- to fivefold. Neither of the cyclooxygenase inhibitors indomethacin or meclofenamate nor exogenous prostaglandin E2 affected ACh release, indicating that inhibitory prostanoids do not modulate ACh release.