Uptake of 5-hydroxytryptamine in blood platelets and its inhibition by drugs: role of plasma membrane and granular storage.

The initial uptake of 3H-5-hydroxytryptamine (3H-5-HT) showed linearity for short time intervals in normal and reserpinized blood platelets of guinea-pigs, but was lower in reserpinized platelets. Teh Km values for the 3H-5-HT uptake were virtually identical in normal and reserpinized platelets, whereas Vmax was lower in the latter. Imipramine and chlorpromazine caused the same percentage inhibition of 3H-5-HT uptake in normal and reserpinized platelets; the reserpine-like compound Ro 4-1284 inhibited the uptake of 3H-5-HT in the normal, but not markedly in the reserpinized platelets. Haloperidol, prenylamine and Ro 4-9040 were more potent inhibitors in normal than in reserpinized platelets. It is concluded that (a) the Km of the initial uptake of 5-HT by platelets is probably determined by the mechanism at the plasma membrane, whereas Vmax may be codetermined by the intracellular storage capacity, (b) platelets are models for differentiating the site of action (plasma membrane or storage organelles) of drugs interfering with 5-HT uptake, and (c) neuroleptics- and reserpine-like compounds may either act selectively on the plasma membrane or on the intracellular storage organelles, or affect both of these subcellular sites.