[3H]imipramine binding of protein nature in human platelets: inhibition by 5-hydroxytryptamine and 5-hydroxytryptamine uptake inhibitors.

The nature of [3H]imipramine binding to human platelets was investigated. Desipramine and 5-hydroxytryptamine (5-HT) displaced the same amount of binding and the binding was sensitive to protease treatment. The nature of pharmacological inhibition of [3H]imipramine binding was investigated in saturation experiments. Increases in KD without changes in Bmax were noted with the addition of 5-HT, desipramine, norzimeldine, or 5-methoxytryptoline. Reductions in Bmax without alterations in KD were obtained when citalopram or clomipramine was added. It is concluded that the [3H]imipramine binding site in human platelets is of protein nature and that this binding site contains the substrate recognition site for 5-HT uptake. In addition, [3H]imipramine and other 5-HT uptake inhibitors have bonds to other parts of the 5-HT uptake carrier or to the surrounding lipid membrane. This additional binding outside the substrate recognition site is not one single site but most likely represents sites that are specific for the chemical structure of each uptake inhibitor, respectively.