The influence of methysergide on 5-hydroxytryptamine-induced changes in regional distribution of blood flow.

Systemic and regional haemodynamic variables were measured at the baseline and after saline or 5-HT infusions (5 microgram kg-1 min-1, i.v.) or methysergide injections (0.5 mg kg-1, i.v.). Cardiac output and its complete distribution were measured by the radioactive microsphere (15 micrometer diam) technique. Although 5-HT did not change the systemic variables, methysergide caused a moderate increase in systolic and mean blood pressure and heart rate. 5-HT caused a substantial increase in gastric and a moderate increase in cerebral and myocardial blood flow at the expense of that to the lungs (arteriovenous shunt + bronchial flows), kidneys and skin. While methysergide was able to reduce the vascular responses to 5-HT in stomach, skin, kidneys, heart, lungs and brain, the drug itself, like 5-HT, decreased the number of microspheres reaching the lungs. Since a large number of 15 micron microspheres can escape through the arteriovenous anastomoses to lodge in the lungs it seems likely that both 5-HT and methysergide can reduce the 'non-nutrient' flow through these anastomoses.