SEROTONIN-EVOKED CALCIUM TRANSIENTS IN AIRWAY SMOOTH-MUSCLE CELLS

Serotonin (5-HT) is an important mediator of allergic airway narrowing in several animal species. The present study was designed to characterize the receptor subtypes that mediate 5-HT-induced airway responses in the rat. To do this, we measured Ca2+ transients and adenosine 3',5'-cyclic monophosphate (cAMP) production evoked by 5-HT in cultured rat tracheal smooth muscle cells as well as 5-HT-induced contractions of isolated tracheal rings. 5-HT (10(-6) to 10(-4) M) triggered a rapid increase in intracellular Ca2+ concentration ([Ca2+](i)) followed by a second phase of sustained, elevated, and sometimes oscillating levels of [Ca2+](i). Sustained but not peak [Ca2+](i) levels were dependent on Ca2+ influx but were not attenuated by nifedipine (10(-5) M). Oscillations were observed in cells in Ca2+-free medium, suggesting Ca2+-induced Ca2+ release independent of Ca2+-influx. The effects of 5-HT were inhibited by thapsigargin (10(-6) M) and ketanserin (10(-7) M). In cells incubated with LY-278,584 (5-HT3 antagonist) and (-)pindolol (5-HT1 antagonist), 5-HT-evoked responses were not significantly different from the control values. 5-methyltryptamine (5-MT), a ligand with higher affinity for 5-HT2c receptors than ''classical'' 5-HT2 receptors, elicited higher responses than dipropyl-5-carboxamidotryptamine (DP-5-CT), which possesses lower affinity for 5-HT2c receptors than 5-MT, butan affinity for the classical 5-HT2 receptor similar to 5-MT. 5-HT (10(-7) to 10(-4) M) had no effect on basal levels of cAMP, and 10(-6) M 5-HT did not change forskolin-induced elevations of cAMP in the presence or absence of (-)pindolol. 5-HT increased isometric force of tracheal rings in a concentration-dependent manner. Ketanserin (10 (-7) M) caused 100% relaxation and (-)pindolol caused 40% relaxation of rings precontracted with 5 x 10(-6) M 5-HT. Our data suggest that in rat airway 5-HT acts via 5-HT2c-like receptors expressed on smooth muscle cells, causing Ca2+ release from intracellular stores, followed by Ca2+ influx, possibly through Ca2+ release-activated channels.