Human platelet imipramine recognition sites: biochemical and pharmacological characterization

The influence of the membrane environment on the integrity of the human platelet [3H]-imipramine recognition site was examined. When platelet membranes were isolated in a buffer containing enzyme inhibitors (EDTA, EGTA and antiprotease) a significantly greater number of high affinity [3H]-imipramine binding sites was observed. A calcium-stimulated degradation of imipramine sites was also demonstrated. This degradation occurred in vitro over physiologically relevant time periods. Furthermore, inactivation of imipramine binding was achieved by very low concentrations (IC50 = 5 microgram/ml) of phospholipase A2. Specific serotonin reuptake inhibitors were potent displacers of [3H]-imipramine binding; histamine (H1), alpha-adrenergic (alpha 1), and muscarinic agents were much less active. The receptor was shown to be proteinaceous in nature due to its sensitivity to protease, heat denaturation and chemical modification with N-ethylmaleimide. From these results it is proposed that membrane lipid perturbations, catalyzed by calcium, may control expression of platelet [3H]-imipramine sites. The relation of this recognition site to aminergic systems and the possible relevancy to the action of antidepressants are addressed.