Radioenzymatic assay of platelet serotonin, dopamine and norepinephrine in subjects with normal and increased serotonin production.

Platelet serotonin, dopamine and norepinephrine concentrations were determined with sensitive and specific radioenzymatic methods in normal volunteers and in sick subjects with both normal and increased serotonin production. In the assays, a 3H-labeled methyl group is transferred from [3H]S-adenosyl methionine to the monoamine under investigation by a semi-purified enzyme. The platelet serotonin concentration (expressed as pmol serotonin/mg platelet protein) was significantly higher in the patients with increased serotonin production from carcinoid tumors (5400 +/-0 500) than in the normal volunteers (900 +/- 100) or the sick subjects with normal serotonin production (900 +/- 100). Although eight of the subjects with increased serotonin production had symptoms of the carcinoid syndrome such as flushing and diarrhea, two of the subjects with increased serotonin production did not have these symptoms. The radioenzymatic method of platelet serotonin measurement was as effective as a conventional fluorometric technique for serum serotonin in detecting patients with serotonin overproduction from carcinoid tumors, and had the advantages of being more sensitive and specific. The great sensitivity and specificity of the radioenzymatic techniques suggest that it might be useful in evaluating disease states in which platelet serotonin is decreased. Using these techniques we found that dopamine and norepinephrine were also present in platelets, although in much lower amounts than serotonin. Despite the markedly increased serotonin concentration in the platelets of subjects with serotonin overproduction from carcinoid tumors, there was no alteration in the dopamine or norepinephrine concentration of their platelets.