[Membrane mechanisms of the excitatory action of serotonin on the smooth muscles of the rabbit pulmonary artery].

Serotonin induced dose-dependent tonic contractions of the rabbit pulmonary artery smooth muscles with KED50, of 2.7 X 10(-7) mol/l. More than 80% of these contractions were found to be dependent on extracellular calcium. Hyperpolarization of cell membrane by inwardly applied electrical current caused nearly 50% reduction in serotonin-induced contractions. The same portion of contractions was inhibited by verapamil and Ca2+. Serotonin-, but not potassium-induced contractions were completely inhibited by sodium nitroprusside which is thought to be selective inhibitor of receptor-operated calcium channels. These findings could indicate that Ca2+ ions, responsible for serotonin-induced contractions enter the cell from the outer surface of the cellular membrane via receptor-operated calcium channels. Nearly half of serotonin-operated Ca2+ channels appear to be also potential-operated.