Accumulation of dopamine by blood platelets from normal subjects and parkinsonian patients under treatment with L-DOPA.

1. Human blood platelets have been shown to take up dopamine by an energy-dependent, saturable process that is inhibited by 5-hydroxytryptamine (5-HT), desipramine and other drugs.2. Platelets from parkinsonian subjects receiving oral L-DOPA also took up dopamine.3. When the responses of normal and parkinsonian platelets were compared, the parkinsonian cells showed the following differences: increased affinity for the dopamine transport process; decreased equilibrium concentrations of dopamine after incubation for 90 min, and greater efflux of dopamine from loaded platelets during a 10 min incubation.4. There were no differences in the uptake of 5-HT by parkinsonian platelets, but endogenous 5-HT was significantly reduced; ATP was normal.5. In two out of three samples of platelets from parkinsonian subjects, traces of a dopamine-like substance were detected, but this finding requires confirmation.6. If the platelet is a valid model for dopaminergic brain neurones, then the results described would suggest that dopamine uptake and storage may be abnormal in brain neurones in Parkinson's disease.