Abstract
1. The effects of 5-hydroxytryptamine by intravenous (1, 5 and 10 mug kg(-1) min(-1) in cats) and intracarotid (0.5 and 2 mug kg(-1) min(-1) in pigs) routes were studied on the complete distribution of common carotid artery blood flow, measured with radioactive microspheres (15 mum). In addition, the amine was also infused (0.75-3 mug kg(-1) min(-1)) into the carotid artery of cats to observe its influence on the shunting of microspheres in the jugular venous blood.2. The basal total common carotid blood flow was distributed ipsilaterally mainly to extracerebral tissues and only little blood entered the brain. As shown by the presence of microspheres in the lungs after injection into the carotid artery (52% in cats; 82% in pigs), a major fraction of the carotid blood by-passed the capillary bed through arteriovenous anastomoses in the head (non-nutrient fraction).3. 5-Hydroxytryptamine redistributed the blood in favour of the nutrient compartment at the expense of arteriovenous anastomotic fraction. In cats, tissue blood flow did not significantly change but, in the pig, blood flow to all tissues, particularly to skin and ears, was substantially increased despite a reduction in total carotid blood flow. This reduction was entirely due to a change in the non-nutrient fraction.4. Intracarotid infusion of 5-hydroxytryptamine in vagosympathectomized intact or spinal cats decreased the number of microspheres appearing in the jugular venous blood, again indicating a reduction in arteriovenous anastomotic flow due to a constriction of these non-nutrient vessels.5. Cyproheptadine (1 mg kg(-1)) completely reversed the effect of 5-hydroxytryptamine on the total carotid blood flow. However, the vasoconstriction of arteriovenous anastomoses was only partially attenuated and the vasodilatatory response was either unchanged (muscle) or even enhanced (skin, ear and bones).6. It is suggested that 5-hydroxytryptamine causes vasoconstriction of the large arteries via D-receptors which are only partly involved in the constriction of arteriovenous anastomoses. A part of the vasoconstriction in these non-nutrient vessels and the vasodilatation in extracerebral tissues are mediated by atypical 5-hydroxytryptamine receptors that are yet to be fully characterized.