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Serotonin uptake in human platelets was inhibited by cytochrome P450 inhibitors such as miconazole and econazole but not clotrimazole. There was a correlation between inhibition of serotonin uptake and inhibition of imipramine binding, suggesting that these P450 inhibitors may inhibit serotonin uptake via direct binding to the transporter. P450 inhibitor effects on serotonin uptake did not seem to be related to the effects of these compounds on intracellular calcium mobilization. Additionally, nitric oxide pathway stimulation does not appear to be involved.

Uptake of tritiated serotonin into human platelets was found to be rapidly inhibited by the tyrosine kinase inhibitors, genistein and methyl 2,5-dihydroxycinnamate. Binding studies indicated that uptake inhibition did not correlate with direct binding of these inhibitors to the transporter. Chelation of mobilizable intracellular Ca2+ did not inhibit the effects of genistein on uptake. These results suggest a more direct, non-Ca2+ mediated effect of tyrosine kinase inhibitors on uptake.

The serotonin uptake inhibitors sertraline, paroxetine and fluoxetine were compared with imipramine and the calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7) and calmidazolium, for their effects on intracellular Ca2+ mobilization in human platelets. All serotonin uptake inhibitors and calmodulin antagonists augmented thrombin-mediated increases in intracellular Ca2+. Sertraline, calmidazolium and W-7 also caused large dose-dependent increases in baseline levels of intracellular Ca2+. There was a rough correlation between the ability to elevate intracellular Ca2+ and potencies for inhibition of calmodulin. Neomycin, an inhibitor of inositol trisphosphate (IP3) generation, significantly inhibited the effects of sertaline. This is consistent with a role of IP3 and calmodulin in the effects of these drugs.

Lectin affinity chromatography was used to demonstrate the glycoprotein nature of the serotonin (5-HT) transporter. The human platelet transporter protein was solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), labelled with [3H]cyanoimipramine and chromatographed on lectin columns. Wheat germ, gorse seed and lentil lectin-agarose columns specifically retained 5-HT transporter. Peanut and horse gram lectins were ineffective. Concanavalin A showed high non-specific adsorption. Binding of [3H]imipramine to platelet membranes or solubilized 5-HT transporter was not affected by lectins. These data suggest that the lectin interaction with 5-HT transporter is extrinsic to the antidepressant binding site.