AN ASSAY TO MEASURE THE SIMULTANEOUS UPTAKE OF [H-3] DOPAMINE AND [C-14] SEROTONIN BY THE HUMAN PLATELET REVEALS AN IMIPRAMINE-INSENSITIVE [H-3] DOPAMINE UPTAKE MECHANISM

To determine whether a specific dopamine uptake mechanism is present on the human platelet the simultaneous uptake of [H-3]dopamine and [C-14]serotonin by platelets was measured. Utilising a dual radiolabel uptake technique, platelets have been shown to take up serotonin more rapidly and to a greater extent than they take up dopamine. Furthermore, at high concentrations serotonin was able to reduce dopamine uptake by platelets by 60% whereas dopamine had no effect on serotonin uptake. Similarly, imipramine and reserpine reduced (97% and 74% respectively) serotonin uptake by platelets in a dose-dependent manner, but did not affect the uptake of dopamine. our data show that platelets take up dopamine by a mechanism independent of the imipramine-sensitive serotonin uptake mechanism. Furthermore, the increased capacity of platelets to store serotonin is because serotonin, unlike dopamine, is transported into the dense granules of the platelet.