Antidepressant binding to the porcine and human platelet serotonin transporters

The ability of four antidepressant drugs, imipramine, alaproclate, norzimelidine, and fluvoxamine, to inhibit serotonin transport into platelet plasma membrane vesicles was tested over a range of external Na+ concentrations. Imipramine affinity, as we previously reported [J. Biol. Chem. 258:6115-6119 (1983)] increases sigmoidally with Na+. When measured by inhibition of serotonin transport, the affinity for alaproclate and norzimelidine is much less sensitive to Na+ and fluvoxamine actually inhibits more avidly at lower Na+. All of the drugs competitively inhibit serotonin transport. Moreover, alaproclate, norzimelidine, and fluvoxamine all competitively displace [3H]imipramine from platelet plasma membranes. The Ki for fluvoxamine inhibition of transport is 16-fold higher than its Ki for inhibition of imipramine binding. In contrast, alaproclate inhibits transport at concentrations lower than those required to block imipramine binding. In the case of fluvoxamine, and possibly also alaproclate, these differences are not due to separate sites mediating substrate and imipramine binding but rather to differences in the nature of binding and transport measurements. The results suggest that these antidepressant drugs and serotonin all bind to the same site, or to overlapping sites on the serotonin transporter, or to sites on the transporter whose occupation is mutually exclusive with substrate site occupation. The observation that binding of each ligand reacts differently to changes in Na+ suggests that distinct subsites are involved in each case. As reported previously by Wennogle and Myerson [Eur. J. Pharmacol. 86:303-307 (1983)] serotonin decreases the rate of imipramine dissociation from human platelet membranes. This effect is not observed in porcine platelets, is not Na+ dependent, and requires serotonin concentrations over 100 times the Km for transport. It is likely, therefore, to result from serotonin binding to a site distinct from the transport active site.