Pleiotropic Effects of Bitter Taste Receptors on Ca2+i Mobilization, Hyperpolarization, and Relaxation of Human Airway Smooth Muscle Cells

Asthma is characterized by airway inflammation and airflow obstruction from human airway smooth muscle (HASM) constriction due to increased local bronchoconstrictive substances. We have recently found bitter taste receptors (TAS2Rs) on HASM, which increase Ca2+i and relax the muscle. We report here that some, but not all, TAS2R agonists decrease Ca2+i and relax HASM contracted by G-protein coupled receptors (GPCRs) that stimulate Ca2+i. This suggests both a second pathway by which TAS2Rs relax, and, a heterogeneity of the response phenotype. We utilized eight TAS2R agonists and five procontractile GPCR agonists in cultured HASM cells. We find that heterogeneity in the inhibitory response hinges on which procontractile GPCR is activated. For example, chloroquine inhibits Ca2+i increases from histamine, but failed to inhibit Ca2+i increases from endothelin-1. Conversely, aristolochic acid inhibited Ca2+i increases from endothelin-1 but not histamine. Other dichotomous responses were found when Ca2+i was stimulated by bradykinin, angiotensin, and acetylcholine. There was no association between Ca2+i inhibition and TAS2R subtype, nor whether Ca2+i was increased by Gq- or Gi-coupled GPCRs. Selected studies revealed a correlation between Ca2+i inhibition and HASM cell-membrane hyperpolarization. To demonstrate physiologic correlates, ferromagnetic beads were attached to HASM cells and cell stiffness measured by magnetic twisting cytometry. Consistent with the Ca2+i inhibition results, chloroquine abolished the cell stiffening response (contraction) evoked by histamine but not by endothelin-1, while aristolochic acid inhibited cell stiffening from endothelin-1, but not from histamine. In studies using intact human bronchi, these same differential responses were found. Those TAS2R agonists that decreased Ca2+i, promoted hyperpolarization, and decreased HASM stiffness, caused relaxation of human airways. Thus TAS2Rs relax HASM in two ways: a low-efficiency de novo Ca2+i stimulation, and, a high-efficiency inhibition of GPCR-stimulated Ca2+i. Furthermore, there is an interaction between TAS2Rs and some GPCRs that facilitates this Ca2+i inhibition limb.