Is the airway epithelium responsible for histamine metabolism in the trachea of guinea pigs?

Histamine N-methyltransferase Modulates Human Bronchial Smooth Muscle Contraction

To elucidate the modulatory role of histamine-degrading enzymes in airway constrictor responses, human bronchial strips were studied under isometric conditions in vitro. Pretreatment of tissues with the histamine N-methyltransferase (HMT) inhibitor SKF 91488 specifically potentiated the contractile responses to histamine, causing a leftward displacement of the concentration response curves, whereas the diamine oxidase inhibitor aminoguanidine had no effect. This potentiation was attenuated by mechanical removal of the epithelium. The HMT activity was detected in the human bronchi, which was less in the epithelium-denuded tissues than the epithelium-intact tissues. These results suggest that HMT localized to the airway epithelium may play a protective role against histamine-mediated bronchoconstriction in humans.

Increase in inflammatory mediator concentrations in exhaled breath condensate after allergen inhalation

BACKGROUND

Although a number of studies have been carried out to examine the baseline concentrations of inflammatory mediators in asthmatic patients, the clinical utility of exhaled breath condensate (EBC) in allergen-induced bronchoconstriction has not yet been clarified.

OBJECTIVE

We examined whether the release of inflammatory mediators can be detected in EBC after allergen-induced bronchoconstriction in asthmatic patients.

METHODS

We quantified mast cell-associated mediators in EBC and their corresponding urinary metabolites before and after allergen inhalation.

RESULTS

Early asthmatic responses (EARs) caused significant increases in the concentrations of cysteinyl leukotrienes (CysLTs; median, 10.4 vs 99.0 pg/mL; P < .0001) and prostaglandin D(2) (PGD(2); median, 2.26 vs 8.72 pg/mL; P = .0077), but not that of histamine, from baseline concentrations. Significant increases in the concentrations of urinary leukotriene E(4) and 9alpha, 11beta-prostaglandin F(2) were detected in patients with EARs. However, the percentage increases in the concentrations of CysLTs and PGD(2) in EBC did not correlate with those of their corresponding urinary metabolites. The increases in concentrations of CysLTs and PGD(2) in EBC in patients with EARs correlated with each other and correlated with the extent of decrease in FEV(1). An insignificant difference in tyrosine concentration before and after the inhalation test demonstrated that errors caused by dilution of inflammatory mediators are negligibly small in EBC collected over a short period.

CONCLUSION

In patients with allergen-induced EARs, pulmonary generation of mast cell-associated mediators can be evaluated by quantifying CysLTs and PGD(2) in EBC, suggesting that the quantification of EBC mediators might be useful in monitoring acute asthmatic airway inflammation.

Epithelial modulation of cholinergic responses in rabbit trachea is partly due to neutral endopeptidase activity

By the simultaneous measurement of acetylcholine release and smooth muscle contraction in rabbit tracheal segments with and without epithelium, pre- as well as postsynaptic effects of this cell layer were studied on cholinergic neurotransmission. The epithelial cell layer exerted a presynaptic inhibitory influence on acetylcholine release, induced by KCl and electrical stimulation, with a concomitant decrease in the smooth muscle contractions. The responses elicited by exogenous acetylcholine, acting postsynaptically, were also inhibited in the presence of the epithelium. The epithelial effect was not accounted for by the production of inhibitory prostaglandins or a nitric oxide-synthase product. Furthermore, the epithelium did not function as a metabolic site for the degradation of acetylcholine. Phosphoramidon, an inhibitor of neutral endopeptidase, mimicked the effects of epithelium removal on the cholinergic responses to high frequency stimulation and on the acetylcholine-induced effects. Neutral endopeptidase inhibition did not further enhance the responses in epithelium-denuded segments. We therefore suggest that the inhibitory function of the epithelium can be partly explained by the activity of neutral endopeptidase, limiting the excitatory effects of tachykinins on cholinergic responses. An alteration in the neutral endopeptidase activity as a result of inflammatory responses and epithelial damage can contribute to the mechanism of airway hyperreactivity in asthma.

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.

Influence of epithelial removal on the antihistaminic activity of azelastine in the guinea-pig airway smooth muscles

The influence of epithelial removal on the antihistaminic activity of azelastine in the guinea-pig isolated cervical trachea and main bronchus was examined. In both preparations with an intact epithelium, histamine produced a concentration-dependent contraction, but its potency (pD2) and contractility (Emax) were significantly higher in the main bronchus than in the cervical trachea. Epithelial removal from these preparations significantly increased the pD2 values in both tracheal and bronchial preparation, while the Emax value was significantly increased only in the cervical trachea. When these preparations were pretreated with azelastine (20 or 200 nM), concentration-response curves for histamine shifted downward, but were not influenced by the presence of an intact epithelium. Neither concentration of azelastine significantly modified the contractile responsiveness of tracheal and bronchial preparations to carbachol or neurokinin A. These results indicate that azelastine selectively antagonizes the histamine-induced contraction of the guinea-pig airway smooth muscles with or without an intact epithelium.

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.

Inhaled A23187 produces a preferential sensitization to substance P

We examined the effect of A23187-induced lung injury on airway responses to a variety of bronchoconstrictive aerosols in conscious guinea pigs. Guinea pigs were exposed to aerosolized A23187 or vehicle for 12 min or until labored breathing began. Animals were allowed to recover for 24 h, and then they were challenged with inhaled histamine, leukotriene D4 (LTD4), platelet-activating factor (PAF), or substance P. Eight minutes after start of the bronchoprovocative aerosol, the guinea pigs were killed and excised lung gas volume (ELGV) measurements were used as an index of in vivo airway obstruction. No differences in ELGV dose-response curves to LTD4 were seen in A23187- and vehicle-exposed animals. A23187 exposure produced small increases in both histamine and PAF sensitivity. However, A23187 caused a much more pronounced leftward shift in the dose-response to substance P. Coadministration of the neutral endopeptidase inhibitor, thiorphan, did not reduce the A23187-related airway responses to substance P. Histologic evaluation of A23187-treated lungs revealed peribronchiolar inflammation, bronchiolar epithelial injury, and mild alveolitis. We conclude that A23187 treatment produces differential airway responses to bronchoactive agents, with a preferential sensitization to substance P.

Relationship between histamine, lipoxygenase and cyclooxygenase products in antigen-induced contraction in guinea-pig tracheal tube preparations

We have used a tracheal tube preparation to study antigen-induced contraction in sensitized guinea pig airways. Treatment with both the cyclooxygenase inhibitor indomethacin and the lipoxygenase inhibitor MK-886 (L-663,536) affected this contraction in preparations with intact epithelium. Indomethacin potentiated and MK-886 inhibited part of the contraction. Leukotriene release from tracheal tubes was measured after antigen challenge, and was found to be significant in preparations with an intact epithelium. When the epithelium was removed, the histamine receptor antagonist mepyramine reduced antigen-induced contraction by 90%. Our results show that when the epithelium is absent, histamine is the most important mediator in the contraction. With the epithelium left intact, the contraction is more complex: both the cyclooxygenase and lipoxygenase pathways are involved, and our findings indicate that eicosanoid production is associated with the airway epithelium.