Can the epithelium affect allergen-evoked histamine release from the perfused guinea pig trachea?

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.

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.

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

The aim of the study was to investigate whether or not the epithelium plays an active role in histamine metabolism, via the histaminase and/or methyltransferase pathways. Isolated tracheas from guinea pigs sensitized to egg albumin (EA) were used. The epithelium was either left intact or removed from the preparations. The tracheal tubes were mounted in a chamber, allowing estimation of smooth muscle tension, and perfused with buffer. In some experiments the perfusate was collected for determination of histamine and methylhistamine. Mepyramine was used to evaluate the contribution of histamine to the EA-induced contraction. Mepyramine reduced the contraction by 90% when the epithelium was removed; with intact epithelium the reduction was 47%. Aminoguanidine, a histaminase inhibitor, significantly potentiated the response to EA when the epithelium was left intact. Traces of methylhistamine were detected in tissue extracts and perfusates. We conclude that histaminase is present in the preparation and that it can contribute to the inhibitory effect of the epithelium by means of histamine degradation.