Regional and systemic haemodynamic changes evoked by 5-hydroxytryptamine in awake and anaesthetized rabbits

The radioactive microsphere method has been used to study the effects of the infusions of 5-hydroxytryptamine (5-HT) on cardiac output and its distribution in both pentobarbital-anaesthetized and conscious rabbits. Doses of 5 and 10 microgram kg-1 min-1 caused progressive decreases in diastolic pressure and total peripheral resistance in the anaesthetized rabbits but not in conscious ones. There was no significant effect on systolic pressure, cardiac output or heart rate. However, in spite of minimal systemic changes, in both groups 5-HT evoked increases in blood flow to the brain, heart and stomach at the expense of the kidneys, skin, liver (hepatic artery) and lungs (bronchial artery plus arteriovenous anastomotic flow). The 10 microgram kg-1 min-1 dose at 5-HT also increased blood flow and decreased resistance in the extracerebral tissues of the head. The significance of these findings in relation to the possible role of 5-HT in the migraine syndrome has been discussed.