Redistribution of carotid artery blood flow by 5-HT: effects of the 5-HT2 receptor antagonists ketanserin and Wal 1307.

The study concerned effects of ketanserin and a new 5-HT2 receptor antagonist, Wal 1307, on the responses to 5-hydroxytryptamine (5-HT) in the porcine common carotid vascular bed. More than 80% of the total carotid blood flow (208 +/- 18 ml; n = 12) bypassed the tissues via arteriovenous anastomoses. Intracarotid infusions of 5-HT (2 micrograms X kg-1 X min-1) reduced the total carotid blood flow by about 50% and arteriovenous anastomotic flow by 85% but extracerebral tissue (nutrient) blood flow more than tripled. The cerebral component did not change. Thus, 5-HT appears to constrict large conducting arteries and arteriovenous anastomoses but dilates arterioles. Ketanserin and Wal 1307 did not affect carotid blood flow distribution but completely blocked the amine-induced reduction of total carotid blood flow. The constriction of arteriovenous anastomoses was only slightly reduced but the 5-HT-induced arteriolar, vasodilation was enhanced. We conclude that vasoconstriction in the main trunk of the carotid artery and, to a smaller degree, in the arteriovenous anastomoses and arterioles, is mediated by mediated by 5-HT2 receptors. The major part of the constriction of arteriovenous anastomoses and arteriolar dilation elicited by 5-HT is, however, not mediated by 5-HT2 receptors. It is argued that these 'atypical' 5-HT receptors may be related to 5-HT1 binding sites.