The effects of N, N’-bis-(O-methoxybenzylaminohexyl)-cystamine tetrahydrochloride (BHC), 2-brom-d-lysergic acid diethylamide (BOL) and prazosin on 5-hydroxytryptamine (5-HT) dose-response curves in rabbit ear artery and thoracic aorta were determined. BHC, an irreversible alpha-adrenoceptor antagonist, and prazosin had no substantial effect while BOL shifted the 5-HT dose-response curve to the right in aorta. BHC shifted the curve to the right and reduced maximal response to 5-HT in ear artery. BOL shifted the curve to the right only above 10–6M 5-HT in control, but at all concentrations studied in BHC-pretreated ear arteries. In vitro denervation of the ear artery with 6-hydroxydopamine did not significantly alter the 5-HT dose-response curve, nor the effect of BHC on that curve. On the other hand, desipramine decreased the contractile response to high concentrations of 5-HT in non-denervated ear arteries. Prazosin increased the 5-HT threshold and slope of the curve in the ED50 region, but had no inhibitory effect on contractile responses above 3 × 10–7M 5-HT or on maximal response. It is concluded that 5-HT acts exclusively on 5-HT receptors in aorta, but on both alpha-adrenergic and 5-HT receptors in ear artery. 5-HT also possesses a small indirect sympathomimetic action at high concentrations in the ear artery. Prazosin has no effect on 5-HT receptors in either vessel and blocks the alpha-adrenoceptor stimulation by 5-HT in ear
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