[³H] dopamine uptake by platelet storage granules in Parkinson's disease

[³H] Dopamine (DA) uptake by platelet storage granules was determined in 10 never-treated patients with Parkinson's disease (PD) (in 6 of them, also after 6 months of levodopa treatment), in 18 long-term levodopa-treated patients and in 15 age-matched normal controls. Maximum velocity (Vmax) of DA uptake was significantly lower in the group of never-treated PD subjects compared to healthy controls (P<0.001) and also lower compared to the PD long-term treated patients (P<0.025). The Vmax of PD never-treated changed after submitting the patients to 6 months treatment (P<0.025) and became similar to the Vmax in the treated group. The apparent Michaelis constant (Km) of DA uptake in PD never-treated was also significantly different from controls (P<0.005) and also different (but not significantly) from PD "long-term" treated patients. Decreased DA uptake by platelet storage granules in PD may reflect a generalized defect of dopaminergic cells with an impairment in the vesicularization of DA which may contribute to the symptomatology of this motor disease.