Human platelets possess multiple [3H]imipramine binding sites

Scatchard analysis of [³H]imipramine binding to human platelets over the concentration range of 0.1-250 nM revealed a biphasic concave upward curve. Computer-assisted analysis indicated the best fit of the data by a two-site model with apparent K_d values of 0.68 and 293 nM and apparent B_max values of 802 fmol/mg protein and 12.72 pmol/mg protein for the high- and low-affinity components, respectively. These results demonstrate the complex manner in which [³H]imipramine binding to human platelets, suggesting a reason for inconsistencies in B_max values reported in the literature. Further studies of these low- as well as high-affinity sites may help elucidate the functional role of [³H]imipramine binding sites in affective disorders.