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

In vitro characterisation of the duration of action of the histamine-1 receptor antagonist azelastine

Azelastine is a selective antagonist at the human histamine-1 receptor and is used clinically in the treatment of allergic rhinitis. In this study we have investigated its duration of action in vitro in an effort to characterise the receptor and tissue components involved. Chinese hamster ovary cell membrane fragments were used to determine the kinetics of azelastine at the H₁ receptor in a radioligand binding assay. Further duration of action studies were completed in tissue preparations using guinea-pig trachea and human bronchus. In radioligand binding studies, azelastine reached steady state at the H₁ receptor after approximately 41 min and exhibited a significantly slower dissociation rate constant from the receptor than the first generation antihistamine, diphenhydramine. In washout studies completed in guinea-pig and human airway in vitro tissue preparations, azelastine continued to antagonise the effects of histamine at the H₁ receptor for at least 18 h post-washout of the antagonist. This outcome was reversed following removal of the epithelium from guinea-pig isolated tracheal strips. These studies indicate there is a tissue component contributing to azelastine's duration of action, in addition to its direct H₁ receptor binding, with evidence suggesting a role for the epithelial layer.

Effect of histamine on lung contractile elements in growing cattle

OBJECTIVE

To determine the effect of histamine on the contractile elements of the respiratory tract in neonatal calves and young adult cattle.

SAMPLE POPULATION

Samples of trachealis muscle, bronchi, and intrapulmonary arteries and veins dissected from the respiratory tracts of healthy bovids (2 to 8 days and 16 to 20 months old).

PROCEDURE

Histamine cumulative concentration-effect curves (10(-6) to 10(-3) M) were constructed in duplicate smooth muscle samples mounted in organ baths. Contractile responses to histamine were compared with reference contractions elicited by methacholine (10(-5) M) for airways or KCl (127 mM) for vessels.

RESULTS

In young adult cattle, trachealis muscle had a substantial contractile response to histamine (84% of methacholine-induced contraction), whereas bronchi reacted slightly (15 and 20% for large and small bronchi, respectively). Although contractile responses to KCl were comparable in arteries and veins, histamine-induced contractions were greater for intrapulmonary veins than for arteries (202 vs 48% of KCl-induced contraction). In neonatal calves, histamine-induced contraction of veins also exceeded that of arteries (230 vs 54% of KCl-induced contraction); however, unlike in young adult cattle, histamine produced notable contraction of large and small bronchi (48 and 60% of methacholine-induced contraction, respectively).

CONCLUSIONS AND CLINICAL RELEVANCE

Compared with intrapulmonary arteries, intrapulmonary veins have greater contractile responses to histamine in neonatal and young adult cattle. Data suggest loss of histamine responsiveness in bronchial smooth muscle as neonatal calves grow to young adults. Venodilation may be useful in treatment of lung edema in cattle.

Functional identification of epithelial and smooth muscle histamine-dependent relaxing mechanisms in the bovine trachea, but not in bronchi

Theoretically, the overall effect of histamine on respiratory smooth muscle is the result of a subtle balance of contraction and relaxation. The aim of the study was to identify histamine type 2 (H2) and 3 (H3) receptor-dependent relaxing mechanisms in the contractile elements of the bovine tracheobronchial tree. In bronchial preparations, histamine induced very weak contractions, which were not exacerbated with the H2-antagonist cimetidine. Moreover, precontracted bronchial rings never relaxed in response to cumulative doses of histamine or amthamine (H2-agonist). In intact tracheal preparations, histamine induced strong contractions that were exacerbated by cimetidine (E(max): +17.2+/-6.6%) but not by thioperamide (H3-antagonist). Precontracted tracheal bundles did not relax in response to cumulative doses of the H3-agonist R-alpha-methylhistamine. The tracheal contractile response was higher in denuded compared to intact preparations (11.0+/-1.2 vs. 6.0+/-1.7 g). Cimetidine effect was dramatically potentiated in denuded tracheal strips (+40.0+/-11.7%). It is concluded that the weak response of bovine bronchi to histamine is due to a relative scarcity of H1 receptors on bronchial smooth muscle rather than to H2- or H3-dependent relaxation. In the bovine trachea, the smooth muscle possesses relaxing H2 but no H3 receptors. The epithelium exercises a relaxation, which is independent from H2 and H3 receptors.