β2-adrenoceptor agonist-induced upregulation of tachykinin NK2 receptor expression and function in airway smooth muscle

Abstract

Neurokinin A (NKA) induces bronchoconstriction mediated by tachykinin NK2 receptors in animals and humans, and may be increased in asthma. Because β2-adrenoceptor agonists are the most widely used bronchodilators in asthma, we investigated the effects of the β2-adrenoceptor agonist fenoterol on NK2 receptor messenger RNA (mRNA) and receptor density as well as the functional responses of bovine tracheal smooth muscle to the NK2 receptor agonist [β-Ala8]-NKA(4-10) in vitro, using Northern blot analysis, receptor binding, and organ bath studies. Incubation with fenoterol induced a time-and concentration-dependent upregulation of NK2 receptor mRNA (71% increase after 12 h at 10 7 M fenoterol), which was abolished by propranolol (a nonselective β-adrenoceptor agonist) and IC11 18551 (a selective β2-adrenoceptor antagonist), but not by CGP20712A (a selective β1-adrenoceptor antagonist), indicating that fenoterol acts via β2-adrenoceptors. These effects were mimicked by forskolin and prostaglandin E2 (PGE2), both agents that increase cyclic adenosine monophosphate (cAMP), and by the cAMP analogue 8-bromo-cAMP. The upregulation was blocked by cycloheximide, indicating that it requires new protein synthesis, and was accompanied by an increase in both the stability of NK2 receptor mRNA and the rate of NK2 receptor gene transcription. Radioligand binding assay using the selective NK2 receptor antagonist [3H]SR48968 showed a significant increase in the number of receptor binding sites after 12 h and 18 h, which was accompanied by an increased contractile responsiveness to the NK2 receptor agonist [β-Ala8]-NKA(4-10). Dexamethasone completely prevented the fenoterol-induced increase in NK2 receptor mRNA and in the contractile response. We conclude that β2-adrenoceptor agonists induce upregulation of functional NK2 receptors in airway smooth muscle by increasing cAMP, and that this can be prevented by a corticosteroid. The increased responsiveness could be relevant to asthma control and mortality.