Abstract
Basal and stimulated outflow of radioactive acetylcholine, phosphorylcholine and choline from rat and guinea-pig isolated tracheae were measured by reverse phase HPLC followed by liquid-scintillation-spectrometry. Tracheae were stimulated either by an electrical field (transmural stimulation) or by a local stimulation of the innervating parasympathetic nerves (preganglionic stimulation). Epithelium was removed in most experiments, as the epithelium inhibits acetylcholine release. The basal tritium efflux (1,600 dpm/3 min) from rat isolated tracheae incubated with [3H]choline consisted of 56% [3H]phosphorylcholine and 38% [3H]choline. Preganglionic stimulation (15 Hz, 1,200 pulses) caused a 2-fold increase in tritium outflow that was abolished by the removal of extracellular calcium or by the addition of tetrodotoxin. The stimulated outflow of tritium induced by preganglionic nerve stimulation was caused by an exclusive release of [3H]acetylcholine, whereas the efflux of [3H]phosphorylcholine and [3H]choline remained unaffected by this stimulation mode. Transmural stimulation of the rat or guinea-pig trachea, however, caused, in addition to the release of [3H]acetylcholine, the outflow of [3H]phosphorylcholine. Hexamethonium (300 mumol/l) or tubocurarine (100 mumol/l) inhibited (80%) the increase in tritium outflow evoked by preganglionic stimulation, but did not affect tritium outflow evoked by transmural stimulation. Oxotremorine reduced [3H]acetylcholine release evoked by both stimulation modes, but oxotremorine was less potent with transmural stimulation. Scopolamine (0.3 mumol/l) enhanced (120%) the release of [3H]acetylcholine evoked by preganglionic nerve stimulation indicating the blockade of an endogenous negative muscarinic feedback mechanism. Epithelium-dependent inhibition of [3H]acetylcholine release was evident with both preganglionic and transmural stimulation. The present experiments demonstrate that release of [3H]acetylcholine evoked from the isolated trachea by stimulation of the preganglionic trunk of the parasympathetic cholinergic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)
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