ANALYSIS OF THE CONTRACTILE EFFECT OF 5-HYDROXYTRYPTAMINE ON THE ISOLATED POSTERIOR COMMUNICATING ARTERY OF THE CAT

5‐Hydroxytryptamine (5‐HT) induced dose‐dependent increases in tension on the isolated posterior communicating artery (PCA) of the cat were significantly antagonized by lysergic acid diethylamide (LSD, 6 times 10−9 m). In the presence of phentolamine (10−6 m) the contraction induced by the two lowest doses of 5‐HT was significantly reduced. Pretreatment of the animals with reserpine (3 mg kg−1, i.p., total dose) did not modify the dose‐response curve to 5‐HT except for the lowest dose. Removal of both superior cervical sympathetic ganglia 15 days before the experiment brought about a significant increase in the vasoconstriction induced by 5‐HT at all the doses compared with the control. Cocaine (10−6m) induced a significant shift to the left of the dose‐response curve to 5‐HT but the maximum response was the same as in the control. The augmented response to 5‐HT after denervation was partially antagonized by LSD (6 times 10−9 m) but not by phentolamine (10−6 m). These results show that the vasoconstriction elicited by 5‐HT in the PCA of the cat is mainly due to direct stimulation of tryptaminergic receptors. The participation of an indirect adrenergic component in the contractile effects of 5‐HT seems to be negligible. 1979 Royal Pharmaceutical Society of Great Britain