PLASMA-LEVELS OF 5-HYDROXYTRYPTAMINE DURING SYMPATHETIC-STIMULATION AND IN RAYNAUDS-PHENOMENON

1. The involvement of plasma 5-hydroxytryptamine in normal subjects du ring sympathetic stimulation and in patients with Raynaud's phenomenon was studied. 2. Arterial and venous plasma levels of 5-hydroxytryptam ine were measured in normal subjects in a warm room, during reflex sym pathetic stimulation by body cooling and during intra-arterial infusio ns of tyramine. Normal subjects (n = 19) had significantly higher leve ls 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 o f 5-hydroxytryptamine despite significant decreases in blood flow and increases in vascular resistance. 3. Venous plasma levels of 5-hydroxy tryptamine were also determined in patients with primary Raynaud's phe nomenon (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-hydroxytryptam ine than normal subjects, even during vasospastic attacks (n = 3). 4. It is concluded that either 5-hydroxytryptamine is not involved in sym pathetic nerve vasoconstriction or in Raynaud's phenomenon, or 5-hydro xytryptamine released in the microcirculation is largely taken up or m etabolized by endothelial cells or platelets.