title: The cellular and molecular basis of bitter tastant-induced bronchodilation.
abstract: Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca(2+) signals, as revealed in cultured ASM cells, to activate large-conductance Ca(2+)-activated K(+) channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca(2+) events nor alter spontaneous local Ca(2+) transients. Interestingly, they increase global intracellular [Ca(2+)]i, although to a much lower level than bronchoconstrictors. We show that these Ca(2+) changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca(2+)]i induced by bronchoconstrictors, and this lowering of the [Ca(2+)]i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca(2+) channels (VDCCs), resulting in reversal in [Ca(2+)]i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca(2+) signaling pathways via Gβγ to increase [Ca(2+)]i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca(2+)]i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.
authors.affiliation: Department of Microbiology and Physiological Systems, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America.
authors.forename: Cheng-Hai
authors.initials: CH
authors.lastname: Zhang
authors.forename: Lawrence M
authors.initials: LM
authors.lastname: Lifshitz
authors.forename: Karl F
authors.initials: KF
authors.lastname: Uy
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Ikebe
authors.forename: Kevin E
authors.initials: KE
authors.lastname: Fogarty
authors.forename: Ronghua
authors.initials: R
authors.lastname: ZhuGe
mesh_terms: D000818:Animals; D001980:Bronchi / Q000187:drug effects* / Q000378:metabolism*; D001993:Bronchodilator Agents / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D002738:Chloroquine / Q000494:pharmacology; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D051379:Mice; D008810:Mice, Inbred C57BL; D009130:Muscle, Smooth / Q000187:drug effects* / Q000378:metabolism*; D013649:Taste
chemical_list: D001993:Bronchodilator Agents; D002738:Chloroquine; D002118:Calcium