2 AFFINITY STATES FOR [H-3] IMIPRAMINE BINDING TO THE HUMAN PLATELET 5-HYDROXYTRYPTAMINE CARRIER - AN EXPLANATION FOR THE ALLOSTERIC INTERACTION BETWEEN 5-HYDROXYTRYPTAMINE AND IMIPRAMINE

5‐Hydroxytryptamine (5‐HT) showed a biphasic effect on the dissociation rate of [3H]imipramine from human platelet membranes: At low concentrations (EC50, ∼ 2.5 μM), 5‐HT stimulated the rate, as expected for mutually exclusive binding of 5‐HT and imipramine; at higher concentrations (EC50, ∼ 40 μM), 5‐HT reduced this stimulated rate, a result consistent with 5‐HT binding at a site that is physically distinct from both the imipramine binding site and the 5‐HT transport recognition site of the 5‐HT carrier. This modulatory effect could be mimicked by tryptamine, was saturable and independent of Na+ concentration, and could also be demonstrated for detergent‐solubilized carriers. Monophasic association kinetics for [3H]imipramine binding were found. Heat stability experiments showed biphasic thermal inactivation curves. These results are consistent with [3H]imipramine binding to two classes of binding sites at the 5‐HT carrier on human platelet membranes, with affinities three‐ to fivefold different. 5‐HT can convert the lower‐affinity state into the higher‐affinity state. Copyright © 1990, Wiley Blackwell. All rights reserved