Binding of [3H]Imipramine to Human Platelet Membranes with Compensation for Saturable Binding to Filters and Its Implication for Binding Studies with Brain Membranes

Abstract

Apparent specific binding of [³H]imipramine to human platelet membranes at high concentrations of imipramine showed deviation from that expected of a single binding site, a result consistent with a low-affinity binding site. The deviation was due to displaceable, saturable binding to the glass fibre filters used in the assays. Imipramine, chloripramine, desipramine, and fluoxetine inhibited binding to filters whereas 5-hydroxytryptamine and ethanol were ineffective. Experimental conditions were developed that eliminated filter binding, allowing assay of high and low-affinity binding to membranes. Failure to correct for filter binding may lead to overestimation of binding parameters, B_max and K_D for high-affinity binding to membranes, and may also be misinterpreted as indicating a low-affinity binding component in both platelet and brain membranes. Low-affinity binding (K_D < 2 μM) of imipramine to human platelet membranes was demonstrated and its significance discussed.