The uptake of dopamine by human platelets has been shown to be temperature- and energy-dependent and not to occur as a result of dopamine binding to and then being internalised with the dopamine D-1 or D-2 receptor. However, occupancy of these receptors could affect dopamine uptake by platelets through their second messenger systems. We have therefore studied the effect of dopamine receptor agonists, which stimulate receptor second messenger systems, on dopamine uptake by platelets. Uptake of [3H]dopamine by human platelets was not affected by the dopamine D-1 receptor agonist SKF 38393 or the dopamine D-2 receptor agonists, quinpirole and bromocriptine. In contrast, the uptake of [3H]dopamine was decreased by the mixed dopamine receptor agonists dopamine and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN). Furthermore [3H]ADTN, like [3H]dopamine, was taken up by platelets. In conclusion ADTN, a compound structurally similar to dopamine appears to compete for the dopamine uptake system on the human platelet. Thus, our data further support to the hypothesis that a selective dopamine uptake system is present on the human platelet and that this system is not influenced by the dopamine D-1 or D-2 receptor.
- Dopamine receptor agonists