Redistribution by 5‐hydroxytryptamine of carotid arterial blood at the expense of arteriovenous anastomotic blood flow

Abstract

SUMMARY1. The effects of 5-hydroxytryptamine by intravenous (1, 5 and 10 ,ug kg-' min' in cats) and intracarotid (0 5 and 2 jug kg-' min' in pigs) routes were studied on the complete distribution ofcommon carotid artery blood flow, measured with radioactive microspheres (15 ,um). In addition, the amine was also infused (0-75-3 ,ug kg-' min-') 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-') 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.

Nimodipine‐induced changes in the distribution of carotid blood flow and cardiac output in pentobarbitone‐anaesthetized pigs

“…Local infusions of nimodipine, but not of its solvent, redistributed carotid arterial blood flow in favour of the nutrient (tissue, arteriolar) component at the expense of the non-nutrient (AVA) component. Unlike 5-HT where the increased nutrient flow is mainly distributed to the skin and ears (Saxena and Verdouw, 1982;, nimodipine enhanced primarily that to the skeletal muscles and tongue.…”

“…administration of nimodipine on the distribution of cardiac output, a mid-sternal thoracotomy was performed. A cannula was inserted into the left atrial appendage for injection of the microspheres and a catheter in the femoral artery was employed to withdraw a reference blood sample during microsphere injection (Saxena et al, 1980 Saxena & Verdouw, 1982. Prior to use, the spheres were deaggregated by mechanical agitation.…”

“…The values determined for lungs, when microspheres were injected into the carotid artery, represent the AVA part of the carotid circulation (see Johnston & Saxena, 1978;Saxena & Verdouw, 1982). In the case of left atrial injection, the lungs receive microspheres via both peripheral AVAs and bronchial arteries; however, the contribution via the latter route appears to be only about 1% (Baile et al, 1982).…”

“…in a particular tissue and that detected in all tissues (i.e. tissues of the head, including the complete brain, and the neck and lungs) collectively, and Qca, is carotid blood flow (ml min ') (see Saxena & Verdouw, 1982). The amount of cardiac output distributed to the various tissues .Qtis) was calculated as: Qti, (ml min l) = (Itis/ lart) X Qart and Qts (%) = (Qi,/CO) x 100, where Itis and Iart are, respectively, the radioactivity (c.p.m.)…”

Investigations with GMC2021 in experimental models predictive of antimigraine activity and coronary side-effect potential

“…Since little or no radioactivity was detected in the heart and kidneys, all microspheres trapped in lungs reached this tissue from the venous side after escaping via carotid arteriovenous anastomoses. Therefore, the amount of radioactivity in the lungs was used as an index of the arteriovenous anastomotic fraction of carotid blood flow (Johnston and Saxena, 1978;Saxena and Verdouw, 1982).…”