Dopamine uptake in platelets: Two different low-affinity, saturable mechanisms

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Abstract
Uptake of dopamine (DA) in human blood platelets was found to encompass two different saturable components, one chloride-dependent and one non-chloride-dependent. The chloride-dependent uptake had an apparentK m of about 4×10−5 M, was strongly inhibited by serotonin (5HT), and moderately inhibited by ouabain, PHMB and by substituting K+ for Na+ in the incubation medium. The antidepressants imipramine, clomipramine, desipramine and nomifensine showed approximately the same inhibitory potency against this uptake as against 5HT uptake in platelets. This chloride-dependent uptake mechanism is probably identical with the 5HT uptake mechanism in platelets. The non-chloride-dependent uptake had an apparentK m of about 1.4×10−4 M, and was not inhibited by metabolic inhibitors or antidepressants, and only moderately by 5HT. Its characteristics seem to be in accordance with facilitated diffusion. When platelets preloaded with DA were reincubated in fresh medium without chloride, the efflux curve indicated a distribution between one ‘superficial’ and one ‘deep’ compartment, containing 68% and 32% of total platelet DA, respectively. The deep compartment probably corresponds to the dense osmiophilic granules. The efflux kinetics are similar to those found for 5HT.