Nitric oxide inhibits serotonin-induced calcium release in pulmonary artery smooth muscle cells

Nitric oxide (NO) is a potent endothelium-derived pulmonary vasodilator. Serotonin (5-HT; 10-50 μM) constricts pulmonary artery (PA) by releasing Ca²⁺ from intracellular stores and promoting Ca²⁺ influx through Ca²⁺ channels in PA smooth muscle cells (PASMC). The effect of NO on 5-HT-induced increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]) in rat PASMC was investigated to elucidate whether inhibition of agonist-mediated Ca²⁺ rise is involved in the NO-mediated pulmonary vasodilation. The 5-HT-induced increase in [Ca²⁺](i) was characterized by a transient (because of Ca²⁺ release from intracellular stores) followed by a plateau (because of Ca²⁺ influx). Removal of extracellular Ca²⁺ eliminated the 5-HT-induced [Ca²⁺](i) plateau, but insignificantly affected the [Ca²⁺](i) transient. In some of the PASMC bathed in the Ca²⁺-containing or Ca²⁺-free solution, 5-HT also induced Ca²⁺ oscillations. Pretreatment of the cells with 10 μM cyclopiazonic acid (CPA) abolished, whereas 10 mM caffeine negligibly affected, the 5-HT-induced [Ca²⁺](i) transients in the absence of external Ca²⁺. Authentic NO (~0.3 μM) reversibly diminished 5-HT-induced [Ca²⁺](i) transients but augmented CPA-induced Ca²⁺ release in the absence of extracellular Ca²⁺. NO also significantly inhibited 5-HT- induced [Ca²⁺](i) plateau in PASMC bathed in Ca²⁺-containing solution, suggesting that NO inhibits both agonist-induced Ca²⁺ release from the CPA- sensitive Ca²⁺ stores and Ca²⁺ influx from extracellular fluid. These data suggest that NO-induced inhibition of the evoked increases in [Ca²⁺](i) and augmentation of Ca²⁺ sequestration into intracellular stores in PASMC are involved in the mechanisms by which NO causes pulmonary vasodilation.