الفهرس 
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Abstract
Serotonin, (5-HT), a potent vasoco nstrictor, plays an important role m mediating pulmonary hypertension and probably pulmonary edema. The present study was under taken to investiga te the possible mechanism(s) of action, the role of arachidonic acid metabolites and calcium in mediating or modulating the prt;<ssor effect of seratonin in the pulmonary vasculature of the rabbit The main findings of this study can be summarized as follows:-* Serotonin produced a dose-dependent increase in the rabbit’s pulmonary arterial tension in in -vitro. Serotonin effect was similar in both endothelized and denuded rabbit’s pulmonary arterial rings which indicate that removal of endothelium did not significantly alter the pressor response of serotonin . On the other hand, the pressor . effect of serotonin was totaly abolished with ketanserin , a 5-HT2 antagonist, indicating that the pressor effect of serotonin was mediated through stimulation of serotonergic receptors mainly 5-HT2 receptors . In the in-situ study, serotonin produced increase in the pulmonary arterial tension and significant increase in calcium level which indicates that the action of 5-HT is mediated via calcium entry to the target cell.