Validation of a method to measure the uptake of [3H]dopamine by human platelets

We have assessed the validity of using platelet-rich plasma (PRP) to measure dopamine uptake by platelets. In addition, we report on a pilot study comparing [3H]dopamine uptake by PRP from psychotic patients to that by PRP from healthy volunteers. Uptake of radiolabelled dopamine by PRP was related to the concentration of [3H]dopamine added and correlated with platelet number. [3H]dopamine uptake by PRP was not altered by varying pH (6.8 to 8.0) or by the time PRP was incubated prior to the addition of the radiolabelled dopamine. 11.7 +/- 0.34% (mean +/- SEM) of [3H]dopamine added to plasma was precipitated by polyethylene glycol (PEG) and appeared to be associated with a large molecular mass component of plasma. The amount of PEG precipitable [3H]dopamine did not correlate with dopamine uptake (r = 0.02) or differ between patients and controls. Uptake of [3H]dopamine by PRP from 52 volunteers (26 M, 26 F; age, 18-75 yr), expressed as the area under the dopamine uptake curve up to 60 min in arbitrary units, ranged from 72-285 for males and 59-455 for females. Comparing [3H]dopamine uptake by PRP from 15 psychotic patients to these sex-specific reference ranges 9 of 13 PRP from schizophrenic patients had [3H]dopamine uptake outside the normative values whereas the two non-schizophrenic patients did not differ from normal. Dopamine uptake by PRP may be useful in the study of diseases with altered dopaminergic activity in the CNS.