Some Circulatory Effects of 5-Hydroxytryptamine in Man

Continuous infusions of small quantities of 5-hydroxytryptamine (5-HT) were made into the pulmonary artery in 7 human subjects. In only 1 instance was there an unequivocal rise in pressure in the pulmonary artery and this was accompanied by a rise in brachial arterial pressure. The most consistent effect of the drug was to raise the heart rate. There was no effect on the cardiac output. Single rapid injections of 5-HT were made into the right atrium in 3 other subjects. In 1 of these subjects the injection of 4 µg. per Kg. caused transient ventricular arrest and systemic hypotension followed by a sustained rise in pressure in the pulmonary and brachial arteries. The results of this study were thought to indicate the presence of a Bezold-like reflex. The rapid injection of 5-HT was therefore thought to be dangerous and the investigation was discontinued.