Reversal of serotonin vasodilatation in the dog external carotid bed by sympathetic denervation.

In view of the conflicting reports of both constrictor and dilator effects of serotonin on the external carotid vascular bed of dogs, the influence of intraarterial infusions of the amine on blood flow through this territory was assessed by electromagnetic flowmeter techniques. In anesthetized intact dogs, serotonin produced dose-related increases in flow. These vasodilator responses were markedly diminished after ipsilateral vagotomy and were followed by delayed vasoconstriction, which then became the predominant response. Pretreatment with atropine did not modify dilator responses, while resection of the ipsilateral stellate ganglion reversed them to pure constriction. In the internal carotid, serotonin elicited constriction, and this effect was unaffected by vagotomy. These results were interpreted in terms of the hypothesis relating serotonin reactivity to vascular tone. The amine would elicit dilatation through an effect on tonically constricted small vessels. Removal of this tone by section of the vagosympathetic trunk or stellectomy would unmask the constrictor effect of serotonin on large vessels. Such reversal of responses would not occur in the internal carotid, a territory normally devoid of important sympathetic tone.