Coupling of airway smooth muscle bitter taste receptors to intracellular signaling and relaxation is via G_αi1,2,3

Bitter taste receptors (TAS2Rs) are expressed on human airway smooth muscle (HASM) and evoke marked relaxation. Agonist interaction with TAS2Rs activates phospholipase C and increases compartmentalized intracellular Ca²⁺([Ca²⁺]i) via inositol 1,4,5 triphosphate. In taste cells, theGprotein gustducin couples TAS2R to phospholipase C; however, we find very low levels ofGagustmRNAor protein in HASM.Wehypothesized that anotherGprotein inHASM transmits TAS2R function. TAS2R signaling to [Ca²⁺]i, extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the Gi family. a subunit expression in HASM was G_αi2.G_αi1=G_αi3. G_αtrans1≈G_atrans2, with Gagust and Gao at the limits of detection (.100-fold lower than Gai2). Small interfering RNA knockdowns in HASM showed losses of [Ca21]i and ERK1/2 signaling when G_ai3, G_ai2, orG_ai3 were reduced. G_αtrans1 and G_atrans2 knockdowns had no effect on [Ca21]i and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G_αgust and G_αoknockdowns did not affect any TAS2R signaling. In overexpression experiments in human embryonic kidney-293T cells, we confirmed an agonistdependent physical interaction between TAS2R14 and Gai2. ASM cells from transgenic mice expressing a peptide inhibitor ofG_αi2 had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G_αi1, G_αi2, and G_ai3, with no evidence for functional coupling to G_αgust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of Gagust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.