The dopamine D(1) receptor is expressed and facilitates relaxation in airway smooth muscle

BACKGROUND: Dopamine signaling is mediated by G(s) protein-coupled “D(1)-like” receptors (D(1) and D(5)) and G(i)-coupled “D(2)-like” receptors (D(2-4)). In asthmatic patients, inhaled dopamine induces bronchodilation. Although the G(i)-coupled dopamine D(2) receptor is expressed and sensitizes adenylyl cyclase activity in airway smooth muscle (ASM) cells, the G(s)-coupled dopamine D(1)-like receptor subtypes have never been identified on these cells. Activation of G(s)-coupled receptors stimulates cyclic AMP (cAMP) production through the stimulation of adenylyl cyclase, which promotes ASM relaxation. We questioned whether the dopamine D(1)-like receptor is expressed on ASM, and modulates its function through G(s)-coupling. METHODS: The mRNA and protein expression of dopamine D(1)-like receptor subtypes in both native human and guinea pig ASM tissue and cultured human ASM (HASM) cells was measured. To characterize the stimulation of cAMP through the dopamine D(1) receptor, HASM cells were treated with dopamine or the dopamine D(1)-like receptor agonists (A68930 or SKF38393) before cAMP measurements. To evaluate whether the activation of dopamine D(1) receptor induces ASM relaxation, guinea pig tracheal rings suspended under isometric tension in organ baths were treated with cumulatively increasing concentrations of dopamine or A68930, following an acetylcholine-induced contraction with or without the cAMP-dependent protein kinase (PKA) inhibitor Rp-cAMPS, the large-conductance calcium-activated potassium (BK(Ca)) channel blocker iberiotoxin, or the exchange proteins directly activated by cAMP (Epac) antagonist NSC45576. RESULTS: Messenger RNA encoding the dopamine D(1) and D(5) receptors were detected in native human ASM tissue and cultured HASM cells. Immunoblots confirmed the protein expression of the dopamine D(1) receptor in both native human and guinea pig ASM tissue and cultured HASM cells. The dopamine D(1) receptor was also immunohistochemically localized to both human and guinea pig ASM. The dopamine D(1)-like receptor agonists stimulated cAMP production in HASM cells, which was reversed by the selective dopamine D(1)-like receptor antagonists SCH23390 or SCH39166. A68930 relaxed acetylcholine-contracted guinea pig tracheal rings, which was attenuated by Rp-cAMPS but not by iberiotoxin or NSC45576. CONCLUSIONS: These results demonstrate that the dopamine D(1) receptors are expressed on ASM and regulate smooth muscle force via cAMP activation of PKA, and offer a novel target for therapeutic relaxation of ASM.