Responsiveness of isolated canine cerebral and peripheral arteries to ergotamine.

Changes in tension of spiral strips from dog saphenous, external carotid and basilar arteries were monitored isometrically. Serotonin (5-HT) contracted the 3 arterial preparations in about the same concentration range. Compared with noradrenaline (NA) (100%) the efficacy of 5-HT was about 60% on saphenous, 150% on external carotid and about 500% on basilar arteries. Ergotamine (E) stimulated the 3 vascular preparations in concentrations about 100 times lower than 5-HT. Compared with NA the efficacy of E was about 20% on saphenous, 50% on external carotid and 200% on basilar arteries. Compared with 5-HT (100%) however, E had similar constrictor activities in the 3 arteries. It seemed likely therefore that 5-HT receptors are involved in the vasoconstrictor activity of E. Evidence for this was obtained in experiments in which cyproheptadine (Cy), a 5-HT antagonist, was used. For antagonism of E concentrations of Cy about 6 times higher than those required to inhibit 5-HT were necessary, whereas for antagonism of NA about 500 times higher Cy concentrations were necessary. The results suggest that on canine arterial vascular smooth muscle the constrictor activity of E is mediated mainly through serotoninergic receptor sites.