1. The involvement of plasma 5-hydroxytryptamine in normal subjects during sympathetic stimulation and in patients with Raynaud's phenomenon was studied. 2. Arterial and venous plasma levels of 5-hydroxytryptamine were measured in normal subjects in a warm room, during reflex sympathetic stimulation by body cooling and during intra-arterial infusions of tyramine. Normal subjects ( n = 19) had significantly higher levels of 5-hydroxytryptamine in venous plasma [mean 1.42 (SEM 0.23) ng/ml] than in arterial plasma [0.67 (0.12) ng/ml; P < 0.01]. Body cooling ( n = 10) or tyramine infusion ( n = 8) did not increase venous levels of 5-hydroxytryptamine despite significant decreases in blood flow and increases in vascular resistance. 3. Venous plasma levels of 5-hydroxytryptamine were also determined in patients with primary Raynaud's phenomenon ( n = 12) or secondary Raynaud's phenomenon due to scleroderma ( n = 11). Patients with primary or secondary Raynaud's phenomenon did not have significantly higher venous plasma levels of 5-hydroxytryptamine than normal subjects, even during vasospastic attacks ( n = 3). 4. It is concluded that either 5-hydroxytryptamine is not involved in sympathetic nerve vasoconstriction or in Raynaud's phenomenon, or 5-hydroxytryptamine released in the microcirculation is largely taken up or metabolized by endothelial cells or platelets.
1. The effect of intra-arterial histamine on fingertip blood flow (FBF) and vascular resistance (FVR) was studied in normal subjects during reflex sympathetic vascoconstriction induced by body cooling and vasoconstriction caused by intra-arterial noradrenaline. 2. In a room at 20°C, FBF increased from 15.3 ± 35.5 (sd) to 28.3 ± 55.9 ml min −1 100 ml −1 of tissue and FVR decreased from 23.7 ± 17.7 to 11.9 ± 9.9 mmHg·min −1 100 ml −1 ( P < 0.01) during infusions of histamine (0.5–4 μg/min) in 14 subjects. In nine of these subjects, the disappearance half times of local injections of Na 131 I were measured and decreased from 19.8 ± 10.9 to 12.9 ± 7.3 min during histamine infusions, indicating an increase in nutritional flow. Arteriovenous shunt flow was also probably affected, for increases in FBF were sometimes large and FBF increased without a change in the radioisotope half time in two subjects. 3. Neither cimetidine nor pyrilamine (mepyramine) consistently prevented the FBF responses to histamine. Administration of both antihistamines together attenuated the response. 4. During noradrenaline infusions in four subjects, a large increase in FBF (8.9 ± 10.9 to 39.0 ± 8.2 ml, P <0.005) occurred at the smallest dose (0.5 μg/min) of histamine. 5. It is concluded that histamine can vasodilate fingertips and increase nutritional blood flow during reflex sympathetic vasoconstriction. This vasodilatation may be mediated via both histamine H 1 and H 2 receptors.