Role of epithelium in agonist-induced contractile responses of guinea-pig trachealis: influence of the surface through which drug enters the tissue

Loop diuretics and in vitro relaxation of human fetal and newborn mouse airways

This study was designed to test the hypotheses that furosemide directly causes relaxation in human fetal airway and that delivery of loop diuretics to either the adventitial or epithelial surface of newborn mouse airway results in equivalent relaxation. Isometric tension changes were measured in human fetal (11-16 wk) trachea and mainstem bronchus rings exposed to furosemide (300 microM) or saline after acetylcholine or leukotriene D(4) constriction. Significant decreases in isometric tension to furosemide were demonstrated after constriction with acetylcholine or leukotriene D(4). To examine the site of effect and mimic aerosolized and systemic administration, furosemide (3-300 microM) and bumetanide (0.3-30 microM) were applied separately to epithelial and adventitial surfaces of newborn mouse airways. No differences in airway diameter changes to epithelial or adventitial furosemide or bumetanide were observed, but a 10-fold difference in potency was found. In summary, human fetal airway relaxed to furosemide when constricted with either neurotransmitter or inflammatory mediator in vitro. Further, no differences in relaxation to equimolar epithelial and adventitial furosemide were observed in isolated newborn mouse airway. Taken together, this provides evidence that furosemide has a direct, nonepithelial-dependent effect on airway smooth muscle tone.

Effects of phorbol 12,13-diacetate on human isolated bronchus

Protein kinase C appears to be involved in the regulation of airway contractility. Phorbol 12,13-diacetate (PDA; 0.01-10 microM), a protein kinase C activator, produced a transient relaxation followed by a sustained contraction of human isolated bronchus. Different protein kinase C inhibitors (calphostin C, staurosporine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine) (H-7), nifedipine (NIF; 1 microM) or incubation with Ca(2+)-free medium, inhibited the spasmogenic response to phorbol, while ouabain (10 microM) suppressed only the initial relaxation. These results indicate that the initial relaxation, in response to PDA, is related to the activation of Na(+)/K(+)-ATPase, while the ensuing contraction depends on extracellular Ca(2+) entry.Incubation with PDA (1-5 microM) depressed the maximal relaxation to theophylline and caffeine obtained at 37 degrees C but augmented the spasmogenic responses to methylxanthines (10 mM) obtained in cooled preparations. These effects do not result apparently from increased extracellular entry of Ca(2+), but instead, from facilitation of the release of Ca(2+) from intracellular stores.

Epithelium smooth muscle regulation and interactions

In the 1980s, studies in the vascular field revealed that the endothelium was not simply a metabolic and physical barrier, but liberated substances that could modulate the function of underlying vascular smooth muscle. Investigators in the respiratory field also found that the airway epithelium was more than a physical barrier to airborne insults. The epithelium is composed of at least eight different cell types that have a range of functions, including ciliary motility and mucous secretion, and contain enzymes for liberating arachidonic acid metabolites and peptides. The epithelium also contains degradative enzymes for a number of peptides and biological amines. It was also recognized that the epithelium released substances that, like their vascular counterparts, could regulate the function of a number of cell types, including nerves and airway smooth muscle. These studies document the importance the epithelium plays in the regulation of human airway smooth muscle.

Bradykinin-induced responses in a coaxial bioassay system composed of rat anococcygeus muscle and guinea pig trachea

1. Epithelium-dependent effects of bradykinin (BK) were investigated in a coaxial bioassay system which consisted of guinea pig trachea as donor organ and rat anococcygeus muscle as test tissue. 2. BK (10(-9) to 10(-5) M) produced concentration-dependent relaxations on the phenylephrine (3 x 10(-6) M)-precontracted rat anococcygeus muscle mounted alone. Relaxations decreased significantly when muscle was mounted in epithelium-intact trachea. There was also a significant difference between the relaxations obtained in the muscle within epithelium-intact and epithelium-denuded trachea (at 10(-7) to 10(-5) M concentrations). 3. Capsaicin (10(-5) M) pretreatment did not change BK (10(-9) to 10(-5) M)-induced relaxations in each preparation compared with vehicle pretreatment. Indomethacin (10(-6) M) in combination with thiorphan (10(-5) M) and atropine (10(-6) M) did not affect the BK-induced relaxations of the muscle within capsaicin-pretreated epithelium-intact or denuded trachea. 4. CGS 8515 (a specific 5-lipoxygenase inhibitor, 10(-6) M) did not change BK (10(-5) M)-induced relaxation on the muscle alone, and caused an increase of BK-induced relaxation on the muscle within epithelium-intact trachea compared with that obtained without CGS 8515. 5. Results showed that epithelial or nonepithelial factors were capable of modulating the responsiveness of rat anococcygeus muscle to BK. The decreased relaxation by BK in anococcygeus muscle did not occur by the release of cyclooxygenase products or tachykinins from tracheal epithelium, but it may have occurred by the contractile action of lipoxygenase product secreted by nonepithelial sources. In addition, BK might stimulate the secretion of an epithelium-derived inhibitory factor from the trachea.

The spasmogenic effects of vanadate in human isolated bronchus

1. Inhalation of vanadium compounds, particularly vanadate, is a cause of occupational bronchial asthma. We have now studied the action of vanadate on human isolated bronchus. Vanadate (0.1 microM-3 mM) produced concentration-dependent, well-sustained contraction. Its -logEC50 was 3.74 +/- 0.05 (mean +/- s.e.mean) and its maximal effect was equivalent to 97.5 +/- 4.2% of the response to acetylcholine (ACh, 1 mM). 2. Vanadate (200 microM)-induced contraction of human bronchus was epithelium-independent and was not inhibited by indomethacin (2.8 microM), zileuton (10 microM), a mixture of atropine, mepyramine and phentolamine (each at 1 microM), or by mast cell degranulation with compound 48/80. 3. Vanadate (200 microM)-induced contraction was unaltered by tissue exposure to verapamil or nifedipine (each 1 microM) or to a Ca2+-free, EGTA (0.1 mM)-containing physiological salt solution (PSS). However, tissue incubation with ryanodine (10 microM) in Ca2+-free, EGTA (0.1 mM)-containing PSS reduced vanadate-induced contraction. A series of vanadate challenges was made in tissues exposed to Ca2+-free EGTA (0.1 mM)-containing PSS with the object of depleting intracellular Ca2+ stores. In such tissues cyclopiazonic acid (CPA; 10 microM) prevented Ca2+-induced recovery of vanadate-induced contraction. 4. Tissue incubation in K+-rich (80 mM) PSS, K+-free PSS, or PSS containing ouabain (10 microM) did not alter vanadate (200 microM)-induced contraction. Ouabain (10 microM) abolished the K+-induced relaxation of human bronchus bathed in K+-free PSS. This action was not shared by vanadate (200 microM). The tissue content of Na+ was increased and the tissue content of K+ was decreased by ouabain (10 microM). In contrast, vanadate (200 microM) did not alter the tissue content of these ions. Tissue incubation in a Na+-deficient (25 mM) PSS or in PSS containing amiloride (0.1 mM) markedly inhibited the spasmogenic effect of vanadate (200 microM). 5. Vanadate (200 microM)-induced contractions were markedly reduced by tissue treatment with each of the protein kinase C (PKC) inhibitors H-7 (10 microM), staurosporine (1 microM) and calphostin C (1 microM). Genistein (100 microM), an inhibitor of protein tyrosine kinase, also reduced the response to vanadate. 6 Vanadate (0.1-3 mM) and ACh (1 microM- 3 mM) each increased inositol phosphate accumulation in bronchus. Such responses were unaffected by a Ca2+-free medium either alone or in combination with ryanodine (10 microM). 7. In human cultured tracheal smooth muscle cells, histamine (100 microM) and vanadate (200 microM) each produced a transient increase in intracellular Ca2+ concentration ([Ca2+]i). 8. Intracellular microelectrode recording showed that the contractile effect of vanadate (200 microM) in human bronchus was associated with cellular depolarization. 9. It is concluded that vanadate acts directly on human bronchial smooth muscle, promoting the release of Ca2+ from an intracellular store. The Ca2+ release mechanism involves both the production of inositol phosphate second messengers and inhibition of Ca-ATPase. The activation of PKC plays an important role in mediating vanadate-induced contraction at values of [Ca2+]i that are close to basal.

H-7, a protein kinase C inhibitor, inhibits spontaneous tone and spasmogenic responses in normal and sensitized guinea pig trachea

1. H-7, a protein kinase C inhibitor, fully inhibited the spontaneous and stimulated (KCl 20 mM or histamine 0.5 mM) tone of trachea from normal and sensitized guinea pig. 2. H-7 depressed the concentration-contraction curves to KCl, histamine or 5-hydroxytryptamine in epithelium-denuded, indomethacin-treated, trachea from normal and sensitized guinea pigs while responses to CaCl2 (in Ca2+ -free, K+ -depolarized tissues) and acetylcholine were not affected. 3. H-7 (100 microM did not depress Ca2+ (20 microM-induced contraction of Triton X-100 skinned trachea. 4. These results suggest the involvement of PKC in the maintenance of spontaneous tone and spasmogenic responses of guinea pig trachea.

Effects of phorbol 12,13-diacetate and its influence on spasmogenic responses in normal and sensitized guinea-pig trachea

We have studied the effects of phorbol 12,13-diacetate (PDA) and its influence on a variety of spasmogenic responses in trachea isolated from normal and sensitized guinea-pigs. Tracheal preparations were denuded of epithelium, treated with indomethacin (2.8 microM), and cooled to 20 degrees C. In these experimental conditions, tracheal strips contracted to PDA (0.1 nM-1 microM). Contractions to PDA (1 microM) were greater in sensitized tissues. In normal trachea, contractions to PDA (0.1 microM) were depressed by H-7, 1-(5-isoquinolinyl-sulphonyl)-2-methylpiperazine, (50 microM), amiloride (10 microM), verapamil (10 microM) and Ca(2+)-free exposure. Similar effects were obtained in sensitized trachea except that PDA-induced contraction was resistant to verapamil and Ca(2+)-free exposure. Cooling (20 degrees C) of normal trachea substantially depressed the response to CaCl2 (in K(+)-depolarized tissues), KCl, histamine and 5-hydroxytryptamine without affecting the spasm induced by acetylcholine. This inhibitory effect of cooling was not observed in sensitized trachea. PDA (0.1 microM) did not affect spasmogenic responses at 37 degrees C but counteracted the inhibitory effect of cooling in normal trachea. PDA had no effect on sensitized tissues. PDA (0.1-1 microM) did not alter Ca(2+)-induced contraction of skinned normal and sensitized trachea. These results support the hypothesis that intracellularly stored Ca2+ plays an important role in the activation of sensitized tracheal muscle.

Epithelium modulates the kinetics of the response to substance P and its intrinsic activity in the guinea-pig trachea

The contractile response of guinea-pig tracheal preparations with or without epithelium to substance P has been studied in the presence or absence of thiorphan, an endopeptidase 24.11 inhibitor, paying special attention to the kinetics of the response. Without thiorphan, the response to substance P was greater in tracheal preparations without epithelium than in tracheal preparations with epithelium. The concentration-response curve was shifted to the left in the absence of the epithelium. In the presence of 10 microM thiorphan, the maximal contractile response induced by single doses of substance P (0.1 to 10 microM) was lower in tracheal preparations without epithelium. The maximal responses required 10 min in tracheal preparations with epithelium and 2 min in tracheal preparations without epithelium. These epithelium-dependent differences of reactivity remained in the presence of lipoxygenase or cyclooxygenase inhibitors and of selective antagonists of muscarinic, serotoninergic and histaminergic receptors, after the pre-treatment of tissues with capsaicin or compound 48/80 and in the presence of tetrodotoxin. The profile of the cumulative concentration-response curves for substance P was largely dependent on the time between two successive doses. When this time was short (2-4 min), curves established with or without the epithelium were parallel and both reached similar maximal values (2696 +/- 214 mg and 2780 +/- 62 mg, respectively). The curve in tracheal preparations without epithelium was slightly shifted to the left (EC50s: 24 +/- 10 nM and 78 +/- 19 nM). When this time was longer (10 min, ie corresponding to the time required for a full response to a single dose in intact trachea) the potency of substance P was not modified (EC50s: 13 +/- 3 nM and 52 +/- 11 nM), but a lower maximal response was observed with tracheal preparations without epithelium (1440 +/- 182 mg and 2832 +/- 209 mg). Similar results were observed with neurokinin A and neurokinin B. Thus, the removal of the epithelium led to a more rapid contraction and to a decrease of the maximal response to neurokinins, ie a decreased intrinsic activity, a property known to be drug- and tissue-dependent. These data suggest that the intrinsic activity of drugs depends on the cellular environment of the target cells in a tissue and is partly related to the diffusion and metabolism of drugs and to drug-induced hyporeactivity of the target cells.

Effects of sensitization on vasoactive intestinal polypeptide-induced relaxation and its concentration and binding in guinea-pig airways

We investigated the relaxant effect of vasoactive intestinal polypeptide (VIP) in trachea and lung parenchyma from normal and sensitized guinea-pigs. A technique by which drug access was restricted to either the mucosal or the adventitial surface of tracheal rings was used. In intact trachea, concentration-response curves for VIP entering from the mucosal surface (pD2 = 6.61 +/- 0.06) were displaced to the right compared with those for adventitial entry (pD2 = 6.78 +/- 0.04). Epithelium removal produced a leftward shift (approximately 2.8-fold) in the mucosal VIP concentration-response curve. Sensitization did not alter the responsiveness (maximal effect) or sensitivity (pD2 values) of tracheal rings to VIP irrespective of the surface of drug entry and of the absence or presence of epithelium. VIP-induced relaxation of normal and sensitized lung strips was also similar. Sensitization resulted in a significant decrease in tracheal VIP content (from 2.16 +/- 0.07 in normal to 0.60 +/- 0.08 nmol/mg protein in sensitized trachea; P < 0.05; n = 7) whereas the affinity of both high- and low-affinity binding sites for VIP increased as compared to that of normal trachea. Differences were not found in the binding capacities of normal and sensitized trachea. VIP content and binding did not differ in normal and sensitized lung. In conclusion, immunological sensitization produced changes in VIP tracheal content and binding but neither VIP-induced relaxation of isolated airways nor the influence of epithelium in this response was altered.

Epithelial factors: modulation of the airway smooth muscle tone

The airway epithelium is composed of a heterogeneous population of cells. This epithelial layer is not only a physical barrier but also a target responding to a variety of inflammatory mediators. These cells can respond by releasing contracting and relaxing factors to modulate airway responsiveness. They can also metabolize some of the inflammatory mediators. Epithelial damage is a consistent feature of some respiratory conditions, but whether or not such damage contributes to airway disease is for the moment unknown. This review summarizes the literature on the known and proposed roles of the epithelium in the modulation of the airway smooth muscle tone.

Relaxation by calcium antagonists of potassium-contracted trachea from normal and sensitized guinea-pigs: influence of epithelium and the surface of drug entry

A technique by which drug access was restricted to either the mucosal or the adventitial surface of tracheal rings, isolated from normal (unsensitized) or sensitized guinea-pigs, was used to study the role of the epithelium in the relaxation produced by calcium antagonists (verapamil, nifedipine, cinnarizine and flunarizine) of K(+)-induced contraction. In trachea from normal guinea-pigs, the relaxation to verapamil for unrestricted or mucosal drug entry was reduced in the absence of epithelium, whereas the relaxation produced by nifedipine, cinnarizine or flunarizine was unchanged. In sensitized trachea, the relaxation elicited by the calcium antagonists tested was similar in intact and epithelium-denuded tracheal rings irrespective of the surface of drug entry. These results confirm that the epithelium influences the relaxation to verapamil. This modulatory effect is absent in sensitized trachea and is not shared by other calcium antagonists.

Effect of molsidomine and linsidomine on the human isolated bronchus and the guinea-pig isolated trachea

The effects of molsidomine and its metabolite linsidomine were studied on the guinea-pig isolated trachea and on the human isolated bronchus. These effects were compared with those of nitrate derivatives (sodium nitroprusside, isosorbide dinitrate), theophylline, zardaverine and isoprenaline. Linsidomine exerted a relaxant effect similar to that of sodium nitroprusside on the two types of preparations precontracted with acetylcholine, histamine or potassium chloride. Molsidomine was about one-hundredth as potent as linsidomine, and less efficacious. The effects of the two substances were not modified by removal of the human bronchial epithelium. The concentration-response curves of linsidomine and sodium nitroprusside were significantly shifted to the right by methylene blue (3 x 10(-5) M) but the effects of isoprenaline were unmodified. The effects of linsidomine and sodium nitroprusside were potentiated specifically by zaprinast (10(-6)-10(-5) M), an inhibitor of type Ia or V phosphodiesterases, whereas the effects of isoprenaline were potentiated by zardaverine (10(-9)-10(-8) M), an inhibitor of class III and IV phosphodiesterases. The effects of all three substances (linsidomine, isoprenaline and sodium nitroprusside) were potentiated equally by theophylline (10(-5)-10(-4) M), a nonspecific inhibitor of phosphodiesterases. It is concluded that linsidomine is a potent relaxant of the smooth muscle of the guinea-pig isolated trachea and human isolated bronchus. In terms of potency and efficacy, its effect is much superior to that of the parent compound molsidomine. It is suggested that linsidomine acts, like nitrate derivatives, through the guanylate cyclase-cGMP system.

Thrombin stimulates release of endothelin and vasopressin, but not substance P, from isolated rabbit tracheal epithelial cells

Release of endothelin, vasopressin and substance P from isolated rabbit tracheal epithelial cells was monitored after incubation for 2 h with thrombin (10 U/ml) in the presence and absence of cycloheximide (100 micrograms/ml) and 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC, 0.6 mM). Thrombin stimulated the release of endothelin and vasopressin. Inhibition of the thrombin-stimulated release of endothelin and vasopressin by cycloheximide (44 and 56%, respectively) and NCDC (59 and 88%, respectively) indicates that the release of these peptides is at least partially dependent on the ability of thrombin to stimulate protein synthesis and activate the phospholipase C pathway. Substance P, which is also present in tracheal epithelial cells, was not released by thrombin.

The effects of intraluminal and extraluminal drug application on secretion and smooth muscle tone in the ferret liquid-filled trachea in vitro

With the ferret liquid-filled trachea in vitro, intraluminal methacholine (MCh), phenylephrine (PE) and histamine (Hist) increased smooth muscle tone and salbutamol (Salb) decreased tone. Lysozyme output was increased by intraluminal MCh and PE. Albumin transport into the lumen was not altered by intraluminal Hist, Salb or PE. The concentration-response curves for smooth muscle contraction and for lysozyme output to extraluminal MCh lay to the left of those for intraluminal MCh. Indomethacin shifted the smooth-muscle response curves to MCh significantly to the left but did not significantly alter lysozyme output. Extraluminal MCh produced a concentration-dependent increase in albumin output whilst intraluminal MCh did so in one of three studies. Albumin output in response to MCh was not significantly altered by indomethacin. Thus, MCh has a less potent effect on smooth muscle and lysozyme secretion and, to a lesser extent, on epithelial albumin transport when given intraluminally. This may be because the epithelium restricts diffusion of the drug or due to the production of a non-prostanoid factor which inhibits smooth muscle responsiveness. Smooth muscle responsiveness is enhanced by blocking cyclooxygenase activity, suggesting MCh-induced release of a prostanoid with relaxant activity.

Role of acetylcholinesterase in airway epithelium-mediated inhibition of acetylcholine-induced contraction of guinea-pig isolated trachea

To seek evidence for the involvement of acetylcholinesterase activity in the modulatory influence of the airway epithelium, we examined responses to acetylcholine (ACh), bethanechol, histamine or KCl in isolated epithelium-intact and epithelium-denuded guinea-pig trachealis preparations. The concentration-response curves to ACh were shifted 26-fold to the left by epithelial denudation but the contractile response to KCl was not altered. The response to histamine in epithelium-denuded preparations increased 4-fold with no attenuation in the presence of physostigmine (30 nM). Physostigmine (30 nM) potentiated the response to ACh in epithelium-intact tissues more (about 26-fold) than in epithelium-denuded tissues (about 3.5-fold). Thus, in the presence of physostigmine removing the epithelium had only a slight effect (not statistically significant) on the potency of ACh to contract the trachea. Removing the epithelium had no effect on the potency of bethanechol, a muscarinic receptor agonist that is not a substrate for cholinesterases. Physostigmine itself contracted the trachealis muscle but the pD2 values and maximum responses in epithelium-intact and denuded preparations were not significantly different. The frequency-response curves to electrical field-stimulated cholinergic contractions were unaffected by removing the epithelium. In conclusion, the principal mechanism by which the epithelium inhibits contraction of guinea-pig trachea to exogenously applied ACh is via epithelium-derived acetylcholinesterase activity.

Contractile activity of big endothelin-1 on the human isolated bronchus

1. We have studied the contractile activity of the 39 amino acid precursor of endothelin-1 (ET-1), big endothelin-1 (big ET-1), on human isolated bronchi. The contribution of the metalloproteases, neutral endopeptidase (NEP) and angiotensin converting enzyme (ACE), in the presence or absence of the epithelium lining, by use of specific inhibitors, was also evaluated on the effects of big ET-1. 2. Big ET-1 elicited a potent contraction of human isolated bronchus. The -log EC50 value for big ET-1 was 7.53 +/- 0.08 (n = 11) and Emax 78.5 +/- 3.8% (% of ACh 3mM). 3. Incubation of human isolated bronchi with the NEP inhibitor phosphoramidon (10(-5) M) induced a rightward shift of the concentration-response curve induced by big ET-1 (10(-9) M to 3 x 10(-7) M). Similar results were observed when human bronchi were incubated with thiorphan (10(-5) M), but the shift to the right was significantly less (P less than 0.01) than that observed in the case of phosphoramidon (-0.35 +/- 0.05 vs -0.67 +/- 0.07 log unit). 4. The two inhibitors of angiotensin I converting enzyme (ACE), captopril or enalapril diacid, did not affect the concentration-response curve for contraction induced by big ET-1. 5. When the epithelium was removed, a leftward shift of the concentration-response curve of big ET-1 (10(-9) M to 3 x 10(-7) M) was observed. Incubation of human isolated bronchi with phosphoramidon or thiorphan (10-5M) or with enalapril diacid or captopril did not modify the leftward shift of the concentration-response curve for big ET-1 after epithelium removal.6. These results suggest that big ET-1 elicits potent contractile activity in the human isolated bronchus and that its effect is the consequence of the conversion to ET-1 by a phosphoramidon-sensitive metalloprotease which, although different from NEP and ACE, appears to be similar to the endothelinconverting enzyme (ECE) described in other studies in animals.

Influence of benzodiazepines on the response of the guinea-pig isolated trachea to the contractile action of adenosine

The effects of diazepam and other agonists of central or peripheral benzodiazepine receptors were studied on the contractile action of adenosine, 2-chloroadenosine and R-PIA (N6-(L-2-phenylisopropyl)-adenosine) on the guinea-pig isolated trachea. These effects were compared to those of dipyridamole. Diazepam 10(-7) to 10(-5) M potentiated the efficacy of adenosine; the maximal contractile effect of adenosine (% vs. acetylcholine 10(-3) M) was 20.4 +/- 4.2 (n = 21) in control conditions and 45.5 +/- 3.7 (n = 6; P less than 0.001) in the presence of diazepam 10(-5) M. Ro5-4864 (10(-7) to 10(-5) M) or alpidem (10(-7) to 10(-5) M), both agonists of peripheral benzodiazepine receptors, potentiated the contractile effects of adenosine to the same extent as diazepam. Clonazepam and zopiclone, both agonists of central benzodiazepine receptors, did not modify these effects. Antagonists of central (flumazenil) or peripheral (RP 52028) benzodiazepine receptors had no influence on the interaction between diazepam or Ro5-4864 and adenosine. Conversely, dipyridamole significantly reduced (10(-7) M) or suppressed (10(-6) M) the contractile effects of adenosine. The contractile effects of 2-chloroadenosine and R-PIA were weakly affected in presence of high concentrations of diazepam and dipyridamole. Epithelium removal potentiated the contractile effect of adenosine on the guinea-pig isolated trachea and increased the potentiating effect of diazepam. It is concluded that benzodiazepines and related compounds can potentiate the contractile effect of adenosine on the guinea-pig isolated trachea through the activation of a peripheral receptor for the benzodiazepines and the resulting inhibition of adenosine uptake.

Epithelium modulates the reactivity of sensitized guinea-pig trachea: influence of the surface of drug entry

A technique by which drug access was restricted to either the mucosal or the adventitial surface of tracheal rings isolated from sensitized guinea-pigs was applied to study the role of the epithelium in modulating responses to KCl, acetylcholine, histamine and antigen (bovine serum albumin, BSA). Epithelium removal did not alter the responsiveness or sensitivity of tracheal rings to KCl. In contrast, a leftward shift occurred for concentration-response curves to acetylcholine (concentration ratio (CR) = 4.1), histamine (CR = 2.9) and BSA (CR = 33.9) entering from the mucosal surface of de-epithelialized trachea. This shift was not associated with changes in the maximal effect of the spasmogens. Response to the adventitial entry of these spasmogens was not altered by epithelial stripping. These results confirm that the epithelium modulates tracheal responses to certain spasmogens including antigen challenge. This role was exclusively exerted for mucosal drug entry. The mechanism underlying this protective effect of epithelium remains to be determined.

Modulation by the epithelium of the extent of bronchial narrowing produced by substances perfused through the lumen

1 Airway narrowing was determined in vitro as a measure of bronchial reactivity. A bronchial segment from pig lung was perfused with a Krebs solution and the change in flow rate to drugs and small ions perfused intraluminally was compared with that obtained by application to the serosal surface. 2 The sensitivity (EC50) to acetylchloline was 30 times greater on the serosal surface than on the luminal surface. Concentrations of histamine and carbachol which had threshold responses on flow rate when perfused intraluminally virtually stopped flow on the serosal surface. Potassium depolarizing solutions (containing either KCl or K2SO4) and vanadate (VO3-) had little or no effect intraluminally but completely stopped flow through the bronchial segment when applied to the serosal surface, i.e. they closed off the airway. 3 After removal of the epithelium the sensitivity to drugs and K+ perfused intraluminally was increased to equal that on the serosal surface. 4 No evidence for suppression of smooth muscle contraction by a putative epithelium-derived inhibitory factor (EpDIF) could be obtained: no inhibition of smooth muscle contractility was seen when the agents listed above were perfused intraluminally and their perfusion continued while they were applied to outside. 5 It was concluded that the epithelium plays a crucial role as an impermeant barrier in modulating the responsiveness of the airways smooth muscle.