Epithelium-derived Relaxing Factor Research Articles

Airway epithelium: more than just a barrier!

Airway hyper-responsiveness and epithelial cell damage are associated commonly with asthma. The airway epithelium is a physical barrier that protects sensory nerves and smooth muscle from stimulation by inhaled irritants. In addition, epithelial cells release mediators that can inhibit bronchoconstriction by relaxing the underlying smooth muscle: so-called 'epithelium-derived relaxing factors' (EpiDRFs). Clear functional evidence for EpiDRFs is provided by experiments where different endogenous mediators induce the relaxation of tracheas containing epithelium, but cause a contraction in preparations lacking this layer. Here, Gert Folkerts and Frans Nijkamp describe the pharmacological relevance of the putative EpiDRFs, prostaglandin E2 and NO, in the modulation of airway tone under basal conditions in vitro and in vivo. Special attention is paid to the role of both EpiDRFs in the development of airway hyper-responsiveness in animal models and in patients with asthma.

気道の防御機構 気道上皮由来平滑筋弛緩因子による気道収縮の防御 Ｋ ｃｈａｎｎｅｌの役割

気道の防御機構 気道上皮由来平滑筋弛緩因子による気道収縮の防御 Ｋ ｃｈａｎｎｅｌの役割

Adenosine receptor-mediated relaxation of rabbit airway smooth muscle: a role for nitric oxide.

In this study, we investigated the relaxant effect of adenosine receptor agonists on KCl-precontracted airway smooth muscle from rabbits and characterized the type of receptor involved in bronchorelaxation in the presence and absence of epithelium. We further defined the role of epithelium-derived relaxing factor, i.e., nitric oxide (NO), on these responses. In both epithelium-intact and -denuded tertiary airway rings from rabbits, the adenosine receptor agonists 2-[p-(2-carboxyethyl)]phenylethylamino-5-N'-ethylcarboxamidoadenos ine (CGS-21680), 5'-(N-ethyl-carboxamido)adenosine (NECA), 2-chloroadenosine (CAD), and (-)-N6-(2-phenylisopropyl)adenosine (R-PIA) relaxed airway smooth muscle with a potency order of CGS-21680 > NECA > CAD > R-PIA. A 98.5, 89.7, 73.2, and 64.7% relaxation was observed at 10(-5) M by CGS-21680, NECA, CAD, and R-PIA in the epithelium-intact bronchial rings, respectively. The 50% maximum effective concentration (EC50; x 10(-7) M) values for CGS-21680, NECA, CAD, and R-PIA were 2, 4, 9, and 80, respectively. Denuded rings, however, showed much less relaxant responses to various adenosine agonists compared with epithelium-intact rings. The adenosine receptor antagonist 8-(sulfophenyl)theophylline significantly attenuated the relaxant responses to all the agonists in the epithelium-intact and -denuded rings. The epithelium-dependent relaxant effect of the agonists in airway rings was inhibited by NG-monomethyl-L-arginine (L-NMMA; 30 microM). The EC50 (x 10(-6) M) values for CGS-21680, NECA, CAD, and R-PIA in the presence of inhibitor were 5.5, 8, 30, and 200, respectively. The L-NMMA produced an insignificant inhibitory effect in the epithelium-denuded rings. L-Arginine but not D-arginine (100 microM) reversed the inhibitory effect of L-NMMA on adenosine agonist-induced relaxation. In primary epithelial cells in culture, CGS-21680 (10(-5) M) induced a fourfold increase in NO production over the control. The CGS-21680-induced NO production in epithelial cells was significantly inhibited by NG-nitro-L-arginine methyl ester (L-NAME). Moreover, L-arginine reversed the inhibitory effect of L-NAME in the epithelial cells. The data suggest that adenosine relaxes rabbit airway smooth muscle through an A2 adenosine receptor and the epithelium serves as a source of NO.

The relaxant effect of ketamine on guinea pig airway smooth muscle is epithelium-independent.

Airway epithelial cells and vascular endothelial cells modulate the tone of the underlying smooth muscle by releasing relaxing factors such as prostanoids and nitric oxide (NO). In the present study, we investigated whether the relaxant effect of ketamine depends on any of the epithelium-derived relaxing factors. Tracheae of female guinea pigs were cut spirally into strips (15 x 3 mm) and mounted in water-jacketed organ baths filled with Krebs-bicarbonate buffer aerated with a mixture of 95% O2 and 5% CO2 at 37 degrees C. Changes in the tension of the strips were measured isometrically with a force displacement transducer and recorded with a polygraph. In the first set of experiments, we examined the effect of ketamine on the concentration-response curves for histamine and carbachol in strips in which the epithelium was kept intact and in strips with denuded epithelium. In the second and third set of experiments, we studied the effect of indomethacin, a cyclooxygenase inhibitor, and N-omega-nitro-L-arginine methylester(L-NAME), a NO synthase inhibitor, on the relaxant activity of ketamine on tracheal strips contracted by histamine or carbachol. The following results were obtained: 1. Mechanical denudation of the tracheal epithelium shifted the concentration-response curve for histamine to the left (the 50% effective concentration [EC50] value of histamine decreased from 3.5 +/- 0.02 x 10(-6) M in the intact strips to 0.98 +/- 0.01 x 10(-6) M in denuded strips, P < 0.001). However, removal of the tracheal epithelium did not change the response to carbachol (the EC50 for carbachol was 1.1 +/- 0.02 x 10(-7) M in intact strips versus 0.88 +/- 0.01 x 10(-7) M after epithelial removal, P > 0.05). 2. Ketamine shifted to the right the concentration-response curves for histamine and carbachol in both intact and denuded tracheae. 3. Indomethacin did not alter the relaxant effect of ketamine on the tracheae contracted by either histamine (the concentration that inhibits 50% [IC50] of ketamine = 1.5 +/- 0.01 x 10(-3) M in control strips and 1.3 +/- 0.04 x 10(-3) M in strips pretreated with indomethacin, P > 0.05) or carbachol (the IC50 of ketamine was 2.5 +/- 0.02 x 10(-4) M in control strips and 2.4 +/- 0.01 x 10(-4) M in strips pretreated with indomethacin, P > 0.05). 4. L-NAME did not influence the relaxant effect of ketamine on tracheae contracted by either histamine (the IC50 of ketamine = 1.6 +/- 0.05 x 10(-3) M in control strips and 1.6 +/- 0.05 x 10(-3) M in strips pretreated with L-NAME, P > 0.05) or carbachol (the IC50 of ketamine = 2.6 +/- 0.04 x 10(-4) M in control strips and 2.3 +/- 0.01 x 10(-4) M in trips pretreated with L-NAME, P > 0.05). These results indicate that neither the mechanical removal of the tracheal epithelium nor the blockade of the release of potent mediators from tracheal epithelial cells influence the relaxant effect of ketamine on guinea pig tracheal strips contracted by histamine or carbachol. We conclude that ketamine relaxes the airway smooth muscle by an epithelium-independent mechanism.

Release and actions of inhibitory prostaglandins from canine tracheal epithelium

This study evaluated the sources and actions of inhibitory prostanoids on canine tracheal smooth muscle to determine if these accounted for epithelium derived relaxing factor activity. Concentration-response curves of canine tracheal smooth muscle were generated to carbachol in the presence and absence of the epithelium and (or) indomethacin, a cyclooxygenase inhibitor. Removal of the epithelium or addition of indomethacin (10(-5) M) in the presence of the epithelium shifted the curve significantly leftward compared with epithelial-intact tissue. Furthermore, addition of indomethacin to epithelial-denuded tissue produced the greatest shift in the curve. Removal of the epithelium increased contractility in response to electrical-field stimulation at intermediate frequencies compared with epithelium-intact tissues. The addition of indomethacin to epithelium-intact tissue also increased the contractile responses. Removal of the epithelium in the presence of indomethacin further increased responses. Radioimmunoassay of muscle bath fluid indicated that the inhibitory prostanoids prostaglandin I2 (PGI2) and prostaglandin E2 (PGE2) were released. PGI2 was released from the epithelium as well as from a nonepithelial source. PGE2 was released from the epithelium in response to electrical-field stimulation. The release of PGE2 was greatly reduced by the addition of indomethacin (10(-5) M), but not by the addition of omega-conotoxin (GVIA), an N-type Ca2+ channel antagonist, nor by the addition of tetrodotoxin, a Na+ channel blocker. Both toxins abolished contractions to electrical-field stimulation. We conclude that the inhibitory prostanoids PGI2 and PGE2 are released, along with a nonprostanoid factor from epithelium, and modulate airway smooth muscle contractility to stimulation of cholinergic nerves and muscarinic agonists. PGE2 is released from epithelium by electrical-field stimulation independent of nerve function.

Evidence that epithelium-derived relaxing factor released by bradykinin in the guinea pig trachea is nitric oxide.

Bradykinin, applied locally to the airways, is a weak bronchoconstrictor agent in guinea pigs in vivo and it may cause constriction or dilatation of guinea pig airways smooth muscle in vitro. We examined the motor effect of bradykinin perfused through the lumen of isolated guinea pig tracheal tubes with or without nitric oxide (NO) synthase inhibitors. In the presence of NG-nitro-D-arginine methyl ester (D-NAME) or NG-monomethyl-D-arginine (D-NMMA) intraluminal bradykinin caused a moderate concentration-dependent relaxation. In contrast, in the presence of NG-nitro-L-arginine methyl ester (L-NAME) or NG-monomethyl-L-arginine (L-NMMA) tracheas developed a sustained increase in tone, and bradykinin caused a marked, concentration-dependent contraction, both effects being reversible by pretreatment with L-arginine, but not with D-arginine. The ability of bradykinin to relax (in the presence of D-NAME) or contract (in the presence of L-NAME) guinea pig tracheal tubes was not affected by indomethacin. Bradykinin contracted epithelium-denuded tracheas in the presence of either L-NAME or D-NAME. Both contraction and relaxation by bradykinin were blocked by the kinin B2 receptor antagonist, HOE 140. Baseline production of guanosine 3',5'-cyclic monophosphate (cyclic GMP) in strips of guinea pig trachealis in vitro was markedly reduced by L-NAME, but not by D-NAME. Bradykinin increased baseline cyclic GMP concentration. These results indicate that bradykinin releases NO or a NO-related molecule, which, possibly by increasing cyclic GMP concentrations, mediates relaxation and opposes contraction induced by bradykinin itself, and further, that bradykinin releases NO from the tracheal epithelium.

The epithelial barrier and airway responsiveness.

Epithelial injury and bronchial hyperresponsiveness are commonly associated with airway disease, and are widely considered to occur as the result of inflammatory changes in the airway wall. Mechanistically, the airway epithelium may influence the sensitivity of the airways to provocative stimuli through its primary function as a cellular barrier between the air and the interstitium, or by liberating a variety of bronchoactive mediators, e.g., lipoxygenase and cyclooxygenase products, nitric oxide, and an epithelium-derived relaxing factor (EpDIF). Much attention has focused on the latter function of the epithelium, particularly the putative EpDIF, which has an action considered to be analogous to that of endothelium-derived relaxing factor in blood vessels. The modulation of airway calibre by the epithelium has recently been investigated in vitro using tubular preparations of bronchi, where removal of, or damage to, the epithelium increases the sensitivity to agonists by several orders of magnitude. This contrasts with the effect of removing the epithelium on strips or rings of airway wall, where the increase in sensitivity is small and rather variable, but this has been the primary observation for proposing a putative EpDIF. This review evaluates the barrier or protective function of the airway epithelium and the major role it plays in the modulation of airway responsiveness. A role of a putative EpDIF seems, at best, to be of minor functional significance.

Epithelial modulation of cholinergic responses in human tracheal smooth muscle.

The aim of this study was to test the hypothesis that the increase in maximal responses to histamine, acetylcholine, and cholinergic electrical field stimulation and decreased relaxant responses to isoprenaline reported in asthmatic tracheal smooth muscle result from the epithelial damage observed in asthma. The effect of mechanical removal of the epithelium on contractile and relaxant responses was examined in normal human postmortem tracheal smooth muscle strips. The epithelium was removed from alternate tracheal strips obtained from 25 subjects within 14 h of sudden death from nonrespiratory causes. In paired samples, contractile cholinergic and inhibitory nonadrenergic, noncholinergic (i-NANC) neural responses to electrical field stimulation and responses to exogenous histamine, potassium chloride, theophylline, and isoprenaline were unaffected by removal of the epithelium. However, the maximal isometric tension (Tmax) induced by methacholine increased by 70.1 +/- 19.8% (mean +/- SE, p < 0.005, n = 9), without alteration in EC50. These data suggest that disruption of the epithelium is unlikely to be the explanation of the abnormalities observed in trachea in fatal asthma. Explanations of the increase in maximal response to methacholine following removal of the epithelium include loss of an epithelium-derived relaxant factor released via an epithelial muscarinic receptor or loss of a specific permeability or metabolic barrier imposed by the epithelium for methacholine.

Interleukin-1 beta inhibits airway smooth muscle contraction via epithelium-dependent mechanism.

To determine whether the cytokine interleukin (IL)-1 beta directly affects airway smooth muscle functions and, if so, what the mechanism of action is, we studied canine isolated bronchial segments under isometric conditions in vitro. Incubation of tissues with human recombinant IL-1 beta (10 ng/ml) for 150 min decreased the contractile responses to acetylcholine, histamine, and KCl. The inhibitory effect of IL-1 beta on the acetylcholine (10(-3) M)-induced contraction was concentration-dependent, the maximal decrease from the baseline contraction being 52 +/- 8% (mean +/- SD, p < 0.001) observed with 10 ng/ml IL-1 beta. Intracellular levels of cyclic AMP and cyclic GMP were not significantly altered by IL-1 beta. The IL-1 beta-induced inhibition of the contractile responses was not affected by pretreatment of tissues with indomethacin or propranolol, but it was greatly attenuated by mechanical removal of epithelium. These results suggest that IL-1 beta may play a protective role against bronchoconstrictor responses via epithelium-dependent mechanism such as the release of epithelium-derived relaxing factor.

Do Perfused Small Caliber Airways of Guinea-pig Release an Epithelium-dependent Relaxing Factor?

Summary: The influence of epithelium removal on the effects of contractile substances on airway responsiveness was investigated on the guinea-pig perfused bronchioles. A gentle rubbing of the luminal surface with a pipe cleaner significantly shifted to the left the concentration-response curves evoked by histamine (3 × 10 -12-10 -4 M) and acetylcholine (10 -9-10 -3 M) and decreased the relaxation response to fenoterol (10 -12-2 × 10 -5 M). In contrast, removal of epithelium did not alter the responses to K + (4.7 × 10 -3-1.2 × 10 -1 M), theophylline (10 -8-10 -2 M), sodium nitroprusside (4 x 10 -10-4 × 10 -5 M) or papaverine (10 -4 M). In intact preparations treated with indomethacin (10 -5 M), histamine and acetylcholine induced contractions similar to that produced by rubbed tissues whereas relaxation induced by fenoterol was not modified. 10 -5 M tranylcypromine (inhibitor of prostacyclin synthesis) or 10 -6 M L-NAME (N G Nitro-L-Arginine Methyl Ester, a nitric oxide synthesis inhibitor) did not alter any concentration-response curves. Whereas prostaglandin E 2 had no effect, prostaglandin E 1 (10 -12-10 -5 M) induced concentration-dependent relaxation, indicating that this prostanoid could be an epithelium-derived relaxing factor. These results suggest that epithelium of small caliber airways could release a cyclooxygenase product, namely a prostanoid, involved in the epithelium-dependent modulation in response to contractile drugs.

The regulatory effect of substances released from porcine bronchial epithelial cells on hypoxic pulmonary vasoconstriction.

In this study, porcine pulmonary artery segments (intact or denuded) and bronchial segments (with or without epithelium) were tested in organ bath. Hypoxia caused a bronchial epithelium dependent relaxation of intact or denuded pulmonary artery preconstricted with phenylephrine. Constriction in intact pulmonary artery coated with epithelium denuded bronchus was observed during hypoxia. The bronchial epithelium-dependent relaxation could be inhibited by indomethacin but not be blocked by the following agents: atropine, propranolol, gossypol, methylene blue and chlorpheniramine. The results suggested that HPV was endothelium-dependent. Hypoxia could cause the production of an epithelium-derived relaxing factor (EpDRF), which acted directly on smooth muscle. The effect of EpDRF was not mediated by pulmonary endothelium, but might be related to arachidonic acid cyclooxygenase pathway.

L-arginine-dependent nitric oxide synthase: a new metabolic pathway in the lung and airways

An L-arginine-dependent pathway, metabolising L-arginine to citrulline and nitrogen oxides, has been described in many cell types in different species, including man. Two subtypes of this nitric oxide synthase have been reported: a constitutive enzyme type, releasing nitric oxide after stimulation, is typically found in endothelial and neural cells; another subtype can be induced in macrophages after cytokine treatment. This review summarizes the literature on the known and proposed roles of this L-arginine-dependent nitric oxide production in different pulmonary processes. Nitric oxide has been reported to act as a neurotransmitter in the inhibitory nonadrenergic, noncholinergic nerves in the airways of guinea-pig and man. It is released in cytostatic processes by immune-stimulated alveolar macrophages. Recent data on the role of L-arginine-dependent processes in immune-complex-mediated lung injury, histamine-induced activation of guanylate cyclase or cytokine networks in the lung are also discussed. Finally, similarities and differences between tracheal epithelium-derived relaxing factor and nitric oxide are analysed. The details of the role and distribution of nitric oxide synthase in the (human) lung and airways are not yet completely understood. Nitric oxide is believed to play a role in various pulmonary physiological processes, such as bronchodilation and the cytotoxic action of certain cells. The modulation of nitric oxide release will therefore, most probably lead to application of novel therapies in diseases such as asthma and inflammatory pulmonary diseases.

RELAXING ACTION OF VASOACTIVE INTESTINAL PEPTIDE (VIP) ON SMOOTH MUSCLE OF ISOLATED GUINEAPIG TRACHEA; ROLES OF EPITHELIAL CELLS

Relaxing action of VIP on acethylcholine (Ach)-induced pre-contraction of smooth muscle of isolated guineapig trachea was examined in a condition with or without epithelium. The action was compared with those of isoproterenol and theophyline. Dose-response curve produced by Ach was also made after VIP (10-8M) was added into a organ bath before applying Ach.(1) VIP relaxed Ach-induced pre-contraction of guinea-pig trachea in a dose dependent manner. The action was more potent than those of theophylline and isoproterenol in molar basis.(2) Relaxing action of VIP, theophylline and isoproterenol on Ach-induced pre-contraction of the tracheal strip was attenuated by removing epithelium, indicating spontaneous synthesis of epithelium derived relaxing factor in the epithelial cells.(3) Inhibitory action of VIP against Ach-induced contraction of the guineapig trachea was not apparent, when VIP was added in the organ bath prior to Ach. This result indicated that VIP might be inactivated by neutral endopeptidase which was produced in epithelial cells after a certain period of time.

Mechanism of H2O2-induced modulation of airway smooth muscle.

We investigated the effects of H2O2 generated by glucose (G) and glucose oxidase (GO) on the isolated rabbit tracheal smooth muscle suspended in Krebs-Ringer solution. H2O2 generated by G+GO was measured with luminol-dependent chemiluminescence. G+GO in the concentrations of 1x (1.80 microM G, 0.075 U/ml GO) and 2, 4, and 8x generated 1.35, 3.2, 6.10, and 6.00 microM of H2O2, respectively. H2O2 produced relaxation of rabbit tracheal smooth muscle, relaxed acetylcholine (ACh)-precontracted muscle, and reduced muscle responsiveness to ACh. These effects were concentration dependent. H2O2, however, produced contraction of guinea pig tracheal smooth muscle. Catalase completely inhibited the H2O2-induced relaxation of ACh-precontracted tracheal smooth muscle. H2O2-induced relaxation was greater in preparations with intact epithelium (65%) than in those denuded of epithelium (40%). The relaxant effects of H2O2 in the presence of an inhibitor of nitric oxide synthesis (NG-monomethyl-L-arginine), an inhibitor of guanylate cyclase (methylene blue), an inhibitor of cyclooxygenase (indomethacin), and an ATP-sensitive K+ channel blocker (glipizide) were 44, 44, 39, and 48%, respectively. H2O2-induced relaxation in the presence of indomethacin in preparations with denuded epithelium was 29%. These results suggest that H2O2-induced relaxation of tracheal smooth muscle is partly epithelium dependent and is mediated by inhibitory arachidonic acid metabolites, epithelium-derived relaxing factor (nitric oxide), ATP-sensitive K+ channels, and the synthesis and release of prostaglandins from epithelium and the underlying smooth muscle.

Potential sources of artefact in the co-axial bioassay

The apparent release of relaxant activity from airway epithelium (epithelium-derived relaxing factor, EpDRF) has been examined in a co-axial bioassay system. The endothelium-denuded rat aorta, placed inside either the epithelium-intact guinea-pig trachea or rabbit bronchus relaxed in response to acetylcholinc. In a modification of the standard preparation, the airway was slit longitudinally and immobilised inside a silicone rubber tube. Under these conditions, the acetylcholine-induced relaxation was abolished. Under the conditions of the co-axial bioassay, the oxygen tension in the lumen of either airway tube was lower than that of the bathing fluid. Upon addition of acetylcholine at concentrations which caused relaxation in the co-axial bioassay. the oxygen tension inside the epithelium-intact, but not the epithelium-denuded guinea-pig trachea was depressed to levels which would have affected the contractile reponse of a rat aorta. We suggest that the assay of relaxant activity from airways using co-axial preparations may be complicated by changes in volume and oxygen tension in the lumen of the donor airway and discuss how such problems might be avoided.

Effect of guinea pig tracheal epithelium on the contraction of rat vascular smooth muscle.

It has been well known that the integrity of airway epithelium is important in developing of bronchial hyperreactivity or bronchial asthma. But the mechanisms underlying this nonspecific airway hyperresponsiveness are not yet determined. To evaluate the ability of guinea pig trachea to release an epithelium derived relaxing factor (EpDRF) which relax rat vascular smooth muscle, we performed the coaxial bioassay using guinea pig trachea and rat aorta. And to evaluate the nature of EpDRF we investigate the influence of methylene blue and indomethacin on the coaxial bioassay. Results were as follows. 1) Vascular smooth muscle mounted into the epithelium intact trachea which was precontracted with phenylephrine was relaxed by addition of histamine or acetylcholine. But vascular smooth muscle mounted into epithelium denuded trachea failed to be relaxed. 2) Epithelium dependent relaxation of vascular smooth muscle was not affected by pretreatment of methylene blue or indomethacin. These results strongly suggests that guinea pig tracheal epithelium releases EpDRF which is able to relax rat vascular smooth muscle. And EpDRF released by airway epithelium is not related to endothelium derived relaxing factor (EDRF) or cyclooxygenase products.

Human Nasal and Bronchial Epithelial Cells in Culture: An Overview of Their Characteristics and Function

Our studies have demonstrated it is possible to culture both human nasal and bronchial epithelial cells to confluency under similar conditions in vitro. These cells are similar morphologically, histologically, and functionally and resemble the cells in vivo. Studies of ciliary activity demonstrate that these cells react toward various agonist and antagonist agents in a similar manner as the cells reported in vivo and consequently will prove to be a valuable model for the evaluation of various therapeutic agents used in the management of infective lung diseases. Similarly studies on the effect of histamine have demonstrated that histamine probably does lead to an increase in bronchial epithelial permeability in vivo and that this effect is likely to be mediated via stimulation of H-1 receptors. Again this could be of clinical relevance when formulating appropriate therapy. The finding that human tracheal and nasal epithelial cells in culture are capable of synthesizing PGE2 could have important clinical implications because this compound is thought to be a putative epithelium-derived relaxing factor. Although this finding awaits confirmation, similar studies with bronchial epithelial cells in vitro would provide a useful tool for study of the function of the bronchial epithelium and its role in the pathogenesis of diseases such as asthma and chronic bronchitis.

EFFECT OF HCL-TREATMENT ON THE CONTRACTILITY OF GUINEA-PIG TRACHEAL SMOOTH MUSCLE

It has been known that responsiveness of airway smooth muscle to various contracting substances is enhanced by destruction of epithelium. One of the proposed mechanism is impairment of production of epithelium derived relaxing factor (EpDRF) which is supposed to be produced in epithelial cells. Contraction of guinea-pig airway smooth muscle against acetylcholine (10-4M) was examined before and after the treatment of 1N HCl. One normal HCl (100μl, 300μl or 500μl) was added to 10ml organ bath for 5min in order to cause chemical damage to epithelial cells. No difference was demonstrated in contraction of airway smooth muscle against acetylcholine after the treatment of 100μl 1N HCl. However, contraction was increased from 7.1×10-2g to 8.×10-2g (N=9, p<0.05) after the treatment of 300μl 1N HCl and from 9.2×10-2g to 13.0×10-2g (N=6, p<0.05) after the treatment of 500μl HCl. Dose-response curve against acetylcholine (10-6-10-3M) and histamine (10-7-10-4M) shifted to the left and the maximal contraction increased after the treatment of 300μl 1N HCl. As arachidonic acid metabolite has been reported to be involved in this phenomena, 6keto-PGF1α, a stable metabolite of PGI2 and one of the relaxing prostaglandins, in the organ bath was measured by radioimmunoassay. No significant change in the concentration of 6keto-PGF1α was demonstrated after the treatment of 1N HCl, which indicates that PGI2 is not involved in this phenomenon as EpDRF.

気道上皮と気道過敏症

Bronchial asthma is characterized by eosinophilic inflammation, desquamation of airway epithelium and bronchial hyperresponsiveness. Eosinophil causes desquamations of epithelium and damages to epithelial cells, following the reduction of the mediators which relaxes bronchial muscle.Human epithelial cells can produce PGE2, 15-HETE, 8, 15-diHETE, 8, 15-LT. PGE2 causes the relaxation of airway smooth muscle. 15-HETE inhibits LT B4 release from rabbit neutrophil and LT C4 release from human eosinophil, respectively. Furthermore, epithelial cells may release epithelium-derived relaxing factor (s) (EpDRF) which cause airway muscle relaxation. Neutral endopeptidase (NEP, enkephalinase) in the epithelium destroys substance P and neurokinin A which induce bronchoconstriction.Desquamations and damages of airway epithelium reduce these relaxing factors and cause bronchial hyperresponsiveness to bronchoconstractive effects.

Heterogeneity in epithelium-dependent responses

The airway epithelium exerts a profound influence on the responsiveness of bronchial smooth muscle to both contracting and relaxing agents. This may be due to the release of an epithelium-derived factor or factors. There is a considerable heterogeneity in the effects of the epithelium between orders of bronchi, between species, and between pharmacologic agents. Such heterogeneity may reflect variations in the release and/or effect of the epithelium-derived relaxing factor(s). This report demonstrates that: (1) there is a basal and a stimulated release of the factor, (2) the prominence of different types of release varies between species, (3) the effect of the epithelium on relaxation of bronchial smooth muscle is greatest in the presence of high degrees of cholinergic tone, (4) the effects of the epithelium are not mediated via cyclic GMP, and (5) the epithelium-derived relaxing factor is not nitric oxide.