Uptake and liberation of mepacrine in blood platelets.

1. Isolated platelets of guinea pigs incubated in Tyrode showed a rapid accumulation of 14C-mepacrine which was less dependent on temperature than that of 14C-5-hydroxytryptamine (5HT). 2. The uptake of mepacrine was not inhibited by imipramine, cocaine, ouabain, KCN, NaF, and reserpine. 3. Various 5HT-liberating drugs, e.g. Ro 4-1284 (benzoquinolizine derivative with reserpine-like action), amphetamine, tyramine and imipramine did not markedly affect the 14C-mepacrine content of platelets previously loaded with this compound. 4. Thrombin and chlorpromazine caused a liberation of 14C-mepacrine from the platelts which was however, less pronounced than that of 14C-5HT. 5. From these and previous findings it is concluded that 14C-mepacrine accumulates in the 5HT storage organelles by a reserpine-insensitive mechanism not dependent on an active transport at the cytoplasmatic membrane level.