A bolus injection of 5-HT (32μg/kg) into the right atrium immediately induced Bezold-Jarisch reflex (hypotension, bradycardia and apnea) and delayed bronchoconstriction (assumed by decreased air flow and increased intrapleural pressure) in anesthetized, spontaneously breathing rats. This bronchoconstrictor response was mimicked by α-methyl-5-HT (5-HT2-receptor agonist : 32μg/kg), and 5-methoxytryptamine (5-HT2- and 5-HT4-receptor agonist : 32μg/kg), but not by 1-phenylbiguanide (5-HT3-receptor agonist : 16μg/kg) or cisapride (5-HT4-receptor agonist : 32μg/kg). To study the effects of 5-HT and α-methyl-5-HT on the airway dynamics, lung conductance (GL) and lung compliance (CL) were calculated from flow rate, tidal volume and intrapleural pressure curves. Dose response curve (DRC) of 5-HT on GL or CL was not affected by granisetron (5-HT3-receptor antagonist : 0.05 mg/kg), but DRC of α-methyl-5-HT was significantly shifted to the right. Subsequently, both the DRCs of 5-HT and α-methyl-5-HT were shifted to the right by sarpogrelate (5-HT2-receptor antagonist : 0.3, 1.0 mg/kg) in a dose-dependent manner. Moreover, the bronchoconstriction of 5-HT and α-methyl-5-HT (respectively 32μg/kg) were either abolished or potently reduced by bilateral, cervical vagotomy, but the bronchoconstriction response to 5-HT was partly recovered at 128μg/kg. Peak response latency of GL were 9.14±0.39 sec in 5-HT (32μg/kg), suggesting that the response might be originated at the bronchial vascular bed. These results suggest that, at least in the rat, 1) 5-HT-induced bronchoconstriction is mediated by 5-HT2-receptor, but not by 5-HT3- or 5-HT4-receptor, 2) sarpogrelate antagonizes 5-HT-induced bronchoconstriction in a dose-related manner, 3) 5-HT-induced bronchoconstriction is potently augmented by vagal reflex, but is primarily mediated by a direct action in the airway, 4) the possible site of action of intravenous 5-HT is the bronchial, not the pulmonary vascular bed
