Protective role of epithelium in the guinea pig airway

Abstract

We developed an in vitro system to assess the role of the epithelium in regulating airway tone using the intact guinea pig trachea (J. Appl. Physiol. 64: 466-471, 1988). This method allows us to study the response of the airway when its inner epithelial surface or its outer serosal surface is stimulated independently. Using this system we evaluated how the presence of intact epithelium can affect pharmacological responsiveness. We first examined responses of tracheae with intact epithelium to histamine, acetylcholine, and hypertonic KCl when stimulated from the epithelial or serosal side. We then examined the effect of epithelial denudation on the responses to these agonists. With an intact epithelium, stimulation of the inner epithelial side always caused significantly smaller changes in diameter than stimulation of the outer serosal side. After mechanical denudation of the epithelium, these differences were almost completely abolished. In the absence of intact epithelium, the trachea was 35-fold more sensitive to histamine and 115-fold more sensitive to acetylcholine when these agents were applied to the inner epithelial side. In addition, the presence of an intact epithelium almost completely inhibited any response to epithelial side challenge with hypertonic KCl. These results indicate that the airway epithelial layer has a potent protective role in airway responses to luminal side stimuli, leading us to speculate that changes in airway reactivity measured in various conditions including asthma may result in part from changes in epithelial function.

Characterization of the histamine receptors in the guinea‐pig lung: evidence for relaxant histamine H3 receptors in the trachea

“…The fact that removal of epithelium increases the sensitivity of tracheal segments not only to several contracting stimuli, but also to dilator agents (isoprenaline and adenosine), has also been reported by Holroyde (1986). These results suggest that the epithelium represents a permeability barrier the removal of which facilitates the access of exogenous substances (both contractile and relaxant) (Munakata et al, 1989;Iriarte et al, 1991). However, the possibility of a simultaneously existing mechanism for release of an epithelium-dependent relaxing factor(s) in response to histamine cannot be ruled out (Farmer et al, 1986;Fedan et al, 1988;VanHoutte, 1988).…”

Delayed and persistent suppression of bronchoconstriction by trypsin in the airway lumen

“…At no time were there any contractions in response to K + , indicating that the epithelium had retained its impermeant property. This study and others have shown that while ASM contracts to high K + depolarising solution adventitially, K + is without effect when it is placed in the lumen of intact airways, unless the epithelium is first breached as illustrated in this study and elsewhere [19][20][21]. Further evidence that trypsin was largely restricted to the lumenal surface of the airway was: 1) the observation that trypsin produced a persistent effect after it had been washed from the bath; 2) the observation that lumenal application of another enzyme, thrombin, although strongly relaxing ASM, did not produce an effect lumenally; and 3) that low concentrations of trypsin would be achieved at the level of ASM; even if there was no diffusion barrier between lumenal and adventitial solutions the equilibrium concentration of trypsin in the solution bathing the adventitia would be some 200-fold lower than the concentration added to the airway lumen because the bath volume was ,200-fold greater than the lumen volume.…”

“…The possibility that a delayed effect of lumenal trypsin might be caused by the slow passage of trypsin across epithelialintercellular junctions before reaching the underlying ASM was considered. However, the epithelium is highly impermeant to ions and small molecules [19,20] which would prevent diffusion of this enzyme. For diffusion to occur, loss of epithelial tight junction proteins would be required before trypsin could reach ASM.…”

“…segments with normal intact epithelium, but does so when the epithelium is mechanically or chemically damaged ( fig. 6) [19][20][21]. In six of six experiments, high lumenal K + did not cause contraction either after washout of trypsin or for up to 120 min afterwards ( fig.…”

Endogenous nitric oxide modulation of potassium‐induced changes in guinea‐pig airway tone

“…In addition, arachidonic acid induces a relaxation in intact tracheae and a contraction in epitheliumdenuded tissues (Nijkamp & Folkerts, 1986). These findings led to the concept that intact epithelium may act as a protective barrier between constrictors and airway smooth muscle (Munakata et al, 1989;1990;Sparrow & Mitchell, 1991) or it may modulate the airway tone through the release of relaxant substances, which may include prostanoids and epitheliumderived relaxing factor(s). Recently, we provided pharmacological evidence that one of the epithelium-derived relaxing factors might be nitric oxide (NO) (Nijkamp et al, 1993).…”