The effects of epithelium removal on the actions of cholinomimetic drugs in opened segments and perfused tubular preparations of guinea-pig trachea

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Abstract
1 Isolated segments of guinea-pig trachea or perfused tracheal tubes were arranged for the recording of trachealis tension changes in Krebs solution containing indomethacin (2.8 μm). 2 In opened tracheal segments, epithelium removal caused modest (2–3 fold) potentiation of the effects of acetylcholine (ACh) and methacholine (MeCh) but failed to potentiate carbachol (CCh), bethanechol (BeCh), oxotremorine or KCl. 3 Pretreatment with ecothiopate potentiated effects of ACh and MeCh but not of CCh or BeCh. Removal of epithelium in ecothiopate-treated tissue potentiated effects of ACh and MeCh but not of CCh or BeCh. 4 Guinea-pig ileum challenged with ACh was used as a bioassay system for cholinesterase activity. Scrapings of tracheal epithelium did not hydrolyse ACh. 5 Histochemical staining revealed no fibres positive for acetylcholinesterase or pseudocholinesterase in the tracheal epithelium. However, the underlying tissues contained acetylcholinesterase-positive nerve fibres and the trachealis muscle itself stained positively for pseudocholinesterase activity. 6 Neither tetrodotoxin (3 μm) nor hexamethonium (500 μm) modified the ability of epithelium removal to potentiate ACh. 7 In perfused tracheal tubes where spasmogens were added to the luminal perfusate, epithelium removal potentiated effects of ACh (31 fold), CCh (10 fold), oxotremorine (2 fold) and KCl. 8 In perfused tracheal tubes where spasmogens were added to the Krebs solution superfusing the adventitial surface of the tissue, epithelium removal significantly reduced the potency of CCh, oxotremorine and KCl. 9 It is concluded that the selectivity and magnitude of the potentiation of cholinomimetics caused by epithelium removal depends on the route by which the cholinomimetic agent gains access to the trachealis muscle. The potentiation of acetic acid esters of choline seen in opened tracheal segments does not reflect the loss of epithelial cholinesterase activity and does not depend on the activity of nervous reflex arcs in the tracheal wall. The reduced potency of adventitially-applied cholinomimetics and KCl seen in epithelium-denuded tissue strongly suggests that the epithelium can moderate trachealis sensitivity to cholinomimetic agents not only by releasing epithelium-derived relaxing factor but also by acting as a barrier to drug diffusion.