ORIGIN AND MODULATION OF ACH RELEASE FROM RAT AIRWAY CHOLINERGIC NERVES

The release of acetylcholine (ACh) from airway parasympathetic nerves was studied in rat trachea. We established stimulus parameters, examin ed the role of extracellular Ca2+, and investigated the origin of the released ACh by use of vesamicol, an inhibitor of ACh uptake in synapt ic vesicles. The role of muscarinic autoreceptors and prostanoids on A Ch release was also studied. Tracheal rings were incubated in Krebs-He nseleit solution containing neostigmine and guanethidine with or witho ut atropine. ACh release was measured by high-performance liquid chrom atography with electrochemical detection. ACh release was dependent on frequency (0.5-16 Hz), voltage (10-25 V), and pulse duration (0.5-4 m s). At 4 Hz, one-fifth of electrical field stimulation-induced ACh rel ease was extracellular Ca2+ independent and vesamicol resistant, indic ating its nonvesicular origin. Three-fifths were Ca2+ dependent and ve samicol sensitive, indicating that it was newly synthesized, and one-f ifth was Ca2+ dependent but vesamicol resistant, indicating its origin from prestored vesicles. At 16 Hz, two-fifths were nonvesicular and t hree-fifths were newly synthesized. Blockade of the muscarinic autorec eptor by atropine potentiated the release of ACh four- to fivefold. Ne ither of the cyclooxygenase inhibitors indomethacin or meclofenamate n or exogenous prostaglandin E(2) affected ACh release, indicating that inhibitory prostanoids do not modulate ACh release.