Isolated 5-hydroxytryptamine organelles of rabbit blood platelets: physiological properties and drug-induced changes.

1. In isolated 5-hydroxytryptamine (5-HT) organelles of rabbit platelets, the concentrations of 5-HT, histamine and adenosine-triphosphate (ATP) respectively are about 200 times higher than in intact platelets. Organelles incubated in plasma at 37 degrees C gradually lose endogenous 5-HT, histamine and ATP and take up (14)C-5-HT against a considerable concentration gradient. Liberation and uptake of 5-HT markedly decrease with dimishing incubation temperature.2. Exposure to reserpine in vitro strongly counteracts the uptake of (14)C-5-HT by isolated organelles, whereas the (14)C-5-HT uptake of intact isolated platelets is less affected by the drug. 5-HT organelles of platelets from reserpinized rabbits also take up very little (14)C-5-HT.3. Imipramine inhibits the uptake of (14)C-5-HT in isolated organelles less markedly than in isolated platelets.4. It is concluded that in the organelles 5-HT and possibly histamine may be associated with ATP. Reserpine probably impairs the uptake of 5-HT at the level of the organelles (possibly by interfering with the association 5-HT/ATP), whereas imipramine seems to act preferentially on the cell membrane.