Platelets and biogenic amines. 2. Indications for a discrete low affinity uptake mechanism shared by norepinephrine and 5-hydroxytryptamine in human platelets.

The uptake of norepinephrine (NE) by human platelets at 10(-9)-5 X 10(-4) M of labelled amine concentration was investigated. At physiological concentrations of NE the uptake was unsaturable and could not be inhibited by imipramine or ouabain. At NE concentrations between 25 and 485 microM the uptake also comprised a saturable component that could be completely blocked by imipramine and partly by ouabain. The saturable uptake of NE had an apparent Km of 273 +/- 50 microM and a Vmax of 0.19 +/- 0.05 pmole/10(6) platelets/min. The affinity of NE (IC50) for the 5-HT transporting carrier was 2.3 mM, 8.4 times higher than the apparent Km for saturable NE uptake. The affinity of 5-HT (IC50) for the NE-transporting carrier was 5.8 microM, 5.8 times higher than the apparent Km for saturable 5-HT transport. Imipramine and norzimeldin were equipotent inhibitors of saturable NE uptake, the potency being of the same degree as that for saturable 5-HT uptake. The tertiary amine amitriptyline was 6 times more effective in inhibiting saturable NE uptake than its demethylated product nortriptyline. Nortriptyline and its hydroxylated E- and Z-isomers had a stronger inhibitory effect on saturable NE uptake than on uptake of 5-HT. The results suggest that human platelets possess two separate amine-transporting carriers, both having their highest affinity for 5-HT. The one with the lowest affinity for 5-HT can also accept NE as a substrate. The human platelet does not possess a high-affinity uptake system for NE comparable to that in adrenergic tissue.