Evidence for two distinct serotonin transport systems in platelets

Two transport sytems for serotonin were measured independently in osmotic lysates of porcine platelets. After pre-equilibration with potassium phosphate and dilution into NaCl medium, only the plasma membrane transport system was observed. Under these conditions, serotonin accumulation is inhibited by imipramine, but not by reserpine or norepinephrine, and requires high (100 meq/liter) concentrations of Na⁺. In the presence of ATP at low (10 meq/liter) concentrations of Na⁺, only the transport system of the intracellular serotonin storage organelles was observed. Under these conditions, transport is inhibited by reserpine, norepinephrine, and epinephrine, but not by imipramine, and requires either ATP or an artificially imposed pH gradient (interior acidic) across the organelle membrane. These and other results suggest that accumulation of serotonin in platelet storage organelles is coupled predominantly to the pH gradient across the membrane.