1. The systemic circulation at rest and during exercise was studied in ten normal male volunteers, after placebo on one occasion and after acute intravenous administration of the serotonergic antagonist ketanserin on another occasion. The effects of ketanserin on the components of the renin—angiotensin—aldosterone system, on plasma catecholamines and on exercise capacity for graded uninterrupted exercise were also investigated.

2. At rest in recumbency rapid intravenous injection of 10 mg of ketanserin, followed by a continuous infusion of 2 mg/h, produced an acute but transient fall in mean intra-arterial pressure of 6 mmHg compared with placebo. After 15 min the mean arterial pressure with ketanserin was within 2 mmHg of the mean pressure with placebo. In the sitting position both at rest and up to 30% of maximal work rate, the mean arterial pressure during ketanserin did not differ from the pressure on placebo. However, at higher levels of physical activity the rise in mean arterial pressure was lower with ketanserin; the pressure achieved with placebo was 7.5 mmHg higher at maximal work rate. Heart rate and cardiac output were significantly higher during ketanserin.

3. When the subjects were lying down and resting, plasma noradrenaline and adrenaline levels, plasma renin activity and angiotensin II concentration were not affected by ketanserin; however, these values were higher in the sitting position both at rest and during exercise. Plasma aldosterone was reduced by ketanserin during exercise and also when the subject was resting in the recumbent position.

4. Exercise capacity as measured by peak oxygen uptake was similar during ketanserin (3.09 ± se 0.12 litres/min) and during placebo (3.11 ± 0.13).

5. The data suggest that 5-hydroxytryptamine can have only a small role, if any, in pressure homoeostasis in sodium replete man at rest in recumbency. At moderate and heavy levels of exercise, the results are compatible with a role for 5-hydroxytryptamine in pressure regulation. Activation of the sympathetic nervous system by ketanserin is suggested by increases of plasma catecholamines, heart rate, cardiac output and plasma renin. The suppression of plasma aldosterone suggests that 5-hydroxytryptamine may have a role in the regulation of aldosterone secretion which is independent of angiotensin II.

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