The uptake of [14Cityramine by rat platelets was less efficient than the uptake of [14C]-5-HT. [14C]-Tyramine was taken up in a two phase curve which could be resolved into a rapid saturable phase and a slower non-saturable phase that was linear with concentration. The value of the non-saturable component was ∼ 12 pmoles/103 platelets/15 sec at 10μM tyramine concentration. The km of active transport was ∼ 3 μM and the Vmax was ∼ 25 pmoles/108 platelets/15 sec.[14C]Tyramine active uptake was inhibited by both serotonin (ki ∼ 0.8μM) and fennuramine (k1 ∼ 2.0μM). These results strongly suggest that the active uptake of tyramine by rat platelets is accomplished by means of the 5-HT transport mechanism. The affinity of tyramine for the 5-HT receptor (∼ 3 μM) is substantially higher than the affinity reported for dopamine (∼ 70 μM). This indicates that the 3-hydroxyl group of dopamine diminishes its affinity for the 5-HT receptor of rat platelets.
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