American journal of physiology. Lung cellular and molecular physiology

Nocturnal asthma is characterized by heightened bronchial reactivity at night, and plasma melatonin concentrations are higher in patients with nocturnal asthma symptoms. Numerous physiological effects of melatonin are mediated via its specific G protein-coupled receptors (GPCRs) named the MT 1 receptor, which couples to both G q and G i proteins, and the MT 2 receptor, which couples to G i . We investigated whether melatonin receptors are expressed on airway smooth muscle; whether they regulate intracellular cyclic AMP (cAMP) and calcium concentrations ([Ca 2+ ] i ), which modulate airway smooth muscle tone; and whether they promote airway smooth muscle cell proliferation. We detected the mRNA and protein expression of the melatonin MT 2 but not the MT 1 receptor in native human and guinea pig airway smooth muscle and cultured human airway smooth muscle (HASM) cells by RT-PCR, immunoblotting, and immunohistochemistry. Activation of melatonin MT 2 receptors with either pharmacological concentrations of melatonin (10-100 µM) or the nonselective MT 1 /MT 2 agonist ramelteon (10 µM) significantly inhibited forskolin-stimulated cAMP accumulation in HASM cells, which was reversed by the Gα i protein inhibitor pertussis toxin or knockdown of the MT 2 receptor by its specific siRNA. Although melatonin by itself did not induce an initial [Ca 2+ ] i increase and airway contraction, melatonin significantly potentiated acetylcholine-stimulated [Ca 2+ ] i increases, stress fiber formation through the MT 2 receptor in HASM cells, and attenuated the relaxant effect of isoproterenol in guinea pig trachea. These findings suggest that the melatonin MT 2 receptor is expressed in ASM, and modulates airway smooth muscle tone via reduced cAMP production and increased [Ca 2+ ] i .