The dopamine D_2 receptor is expressed and sensitizes adenylyl cyclase activity in airway smooth muscle

摘要

Dopamine receptors are G protein-coupled receptors that are divided into two subgroups, "D_1-like" receptors (D_1 and D_5) that couple to the G_s protein and "D_2-like" receptors (D_2, D_3, and D_4) that couple to G_i. Although inhaled dopamine has been reported to induce bronchodilation in patients with asthma, functional expression of dopamine receptor subtypes has never been described on airway smooth muscle (ASM) cells. Acute activation of Gj-coupled receptors inhibits adenylyl cyclase activity and cAMP synthesis, which classically impairs ASM relaxation. In contrast, chronic activation of Gi-coupled receptors produces a paradoxical enhancement of adenylyl cyclase activity referred to as het-erologous sensitization. We questioned whether the dopamine D_2-like receptor is expressed on ASM, whether it exhibits classical Gi-coupling, and whether it modulates ASM function. We detected the mRNA encoding the dopamine D_2 receptor in total RNA isolated from native human ASM and from cultured human airway smooth muscle (HASM) cells. Immunoblots identified the dopamine D2 receptor protein in both native human and guinea pig ASM and cultured HASM cells. The dopamine D_2 receptor protein was immu-nohistochemically localized to both human and guinea pig ASM. Acute activation of the dopamine D_2 receptor by quinpirole inhibited forskolin-stimulated adenylyl cyclase activity in HASM cells, which was blocked by the dopamine D_2 receptor antagonist L-741626. In contrast, the chronic pretreatment (1 h) with quinpirole potentiated forskolin-stimulated adenylyl cyclase activity, which was inhibited by L-741626, the phospholipase C inhibitor U73122, or the protein kinase C inhibitor GF109203X. Quinpirole also stimulated inositol phosphate synthesis, which was inhibited by L-741626 or U73122. Chronic pretreatment (1 h) of the guinea pig tracheal rings with quinpirole significantly potentiated forskolin-induced airway relaxation, which was inhibited by L-741626. These results demonstrate that functional dopamine D_2 receptors are expressed on ASM and could be a novel therapeutic target for the relaxation of ASM.