Ultrastructural Localization of 5-Hydroxy-tryptamine in Blood Platelets

THE blood platelets of various animals, including man, contain 5-hydroxytryptamine (5HT) and can take up this amine from plasma and other incubation media. The amine seems to be stored by a

mechanism which requires metabolic energy, because metabolic inhibitors (such as potassium cyanide and chloro-mercuribenzoate) or elimination of glucose from the incubation medium lead to a

rapid loss of platelet 5HT (refs. 1 and 2). On the basis of the results of differential centrifugation experiments, as well as of electron microscope investigations, it has been postulated

that platelet 5HT is localized in special sub-cellular organelles in a similar way to aromatic mono-amines in nervous tissue2–6. Some investigators think that the 5HT might be associated

with the α-granules or vacuoles of the platelets4,6. 5HT storing particles have not, however, been clearly demonstrated up to now at an ultrastructural level. The present communication shows

that platelet 5HT is at least partially localized in special submicroscopic organelles which are not identical with α-granules.

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