The dopamine D2receptor is expressed and sensitizes adenylyl cyclase activity in airway smooth muscle

Abstract

Dopamine receptors are G protein-coupled receptors that are divided into two subgroups, “D1-like” receptors (D1and D5) that couple to the Gsprotein and “D2-like” receptors (D2, D3, and D4) that couple to Gi. 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 Gi-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 heterologous sensitization. We questioned whether the dopamine D2-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 D2receptor in total RNA isolated from native human ASM and from cultured human airway smooth muscle (HASM) cells. Immunoblots identified the dopamine D2receptor protein in both native human and guinea pig ASM and cultured HASM cells. The dopamine D2receptor protein was immunohistochemically localized to both human and guinea pig ASM. Acute activation of the dopamine D2receptor by quinpirole inhibited forskolin-stimulated adenylyl cyclase activity in HASM cells, which was blocked by the dopamine D2receptor 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 D2receptors are expressed on ASM and could be a novel therapeutic target for the relaxation of ASM.