Acetylcholine release from canine isolated airway is not modulated by norepinephrine

We measured acetylcholine (ACh) release from canine isolated tracheal smooth muscle (TSM) and bronchial spirals using a radioenzymic assay technique. Tissue was incubated in physiological salt solution containing physostigmine (3.10(-5) M), atropine (10(-7) M), and choline (5.10(-6) M), and bath fluid was collected every 15 min for assay. There was a resting release of ACh of 209 +/- 44 pmol/g tissue (mean +/- SE) from 53 to 77 specimens of TSM. Electrical field stimulation (ES) increased ACh release, which was blocked by tetrodotoxin (10(-6) g/ml), confirming the neural origin of ACh. The ACh output during ES (2-ms pulses) at 10 Hz increased linearly from 188 +/- 50 pmol/g tissue (mean +/- SE) for a 1-min volley, to 323 +/- 57 for three volleys, and 544 +/- 128 for five volleys. The ACh output/pulse was constant during ES at 20, 15, 10, and 5 Hz, but it was significantly higher at 2 than at 5 Hz (P less than 0.005). Incubation of TSM with norepinephrine (NE, 10(-5) M) did not affect ACh output either at 2 or 10 Hz. Likewise, ACh output from bronchial spirals during ES and 2 Hz was unaffected by NE. In contrast, NE treatment of isolated guinea pig ileum reduced the ACh released by ES at 2 Hz to 40 +/- 7% (P less than 0.001) of the control ACh output. It is concluded that evoked release of ACh (output/pulse) from cholinergic nerves in canine airway is frequency dependent, as in guinea pig ileum, but that, unlike guinea pig ileum, NE does not modulate its release.