Mepacrine, a tool for investigating the 5-hydroxytryptamine organelles of blood platelets by fluorescence microscopy.

In the blood platelets of various species exposed to mepacrine, the average number of green-yellow fluorescent granules (probably identical with the 5-hydroxytryptamine [5-HT] storage organelles) corresponded to that of flashes emitted by the platelets on prolonged irradiation with violet-blue light. In platelets of fawn-hooded rats the number of granules did not markedly differ from that of normal rat platelets, but the fluorescence intensity and the uptake of mepacrine in vitro showed a marked decrease and the flashes were less numerous. The heavy population of human platelets exhibited considerably more granular structures than the light population. The data suggest that (1) in normal, mepacrine-loaded platelets one flash corresponds to one 5-HT organelle and (2) mepacrine is a useful tool for investigating the number and function of the 5-HT organelles in live platelets and possibly for studying platelet age.