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

Abstract

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). 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. 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. Guinea‐pig ileum challenged with ACh was used as a bioassay system for cholinesterase activity. Scrapings of tracheal epithelium did not hydrolyse ACh. 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. Neither tetrodotoxin (3 μm) nor hexamethonium (500 μm) modified the ability of epithelium removal to potentiate ACh. 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. 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. 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.

Bronchial Hyperreactivity in the Peripheral Airways.

“…However, pathophysiological roles of cholinesterase on airway smooth muscle responsiveness remain unclear. Active staining showed that both acetylcholinesterase and butyrylcholinesterase activities were found in the smooth muscle and ganglia of the rat trachea (Small et al 1990). Chemical oxidant Nchlorosuccinamide (NCS) inhibited both enzyme activities from rat tracheal homogenates in a concentration-dependent fashion .…”

Enzymatic Modulation of Bronchoconstriction, Gland Secretion, Plasma Extravasation and Cough.

“…Although viral respiratory infection decreased epithelium-associated enzymes such as NEP and histamine N-methyltransferase (Dusser et al 1989a; Nakazawa et al, in press), ACNE activity was not influenced by viral respiratory infection (Nakazawa et al in press). Because the airway epithelium does not represent a significant source of ACNE activity (Small et al 1990;Ohrui et al 1991) and AChE only exists in the smooth muscle and ganglia, the location far from the luminal surface of the airway may explain the resistance of this enzyme to noxious stimuli reached from the luminal side. Therefore, pathophysiological significance of AChE may be less in airway diseases.…”

“…Studies of cholinesterase activity and of histochemical localization indicate that both ACNE and butyrylcholinesterase (BuChE; EC 3.1.1.8) are present in the smooth muscle and ganglia (Small et al 1990; Ohrui et al 1991). Although both ACNE and BuChE can hydrolyze acetylcholine and are inhibited by physostigmine with different IC50 values (Atack et al 1989), ACNE is a much more efficient enzyme than BuChE.…”

An ubiquitous modulating function of rabbit tracheal epithelium: degradation of tachykinins

“…Small et al (1990) showed that epithelium could modulate smooth muscle activity by both release of epithelium-derived relaxing factors (EpDRF), and by acting as a barrier to drug diffusion in guinea-pig trachea. We cannot evaluate the barrier function because most of the muscle surface in both of our preparations was exposed to the test solution.…”