Subclasses of platelet 3H-imipramine binding sites.

The possible presence of multiple high affinity 3H-imipramine (3H-IMI) binding sites on blood platelets was studied using trypsin digestion and cyanoimipramine (CNIMI), a pseudo-irreversible inhibitor of 3H-IMI binding and serotonin uptake. Increasing concentrations of CNIMI resulted in a discontinuous curve with a plateau at intermediate concentrations (0.05-0.35 nM). CNIMI sensitive (0.25 nM) sites accounted for approximately half of total high affinity 3H-IMI binding as defined by displacement with 100 microM desipramine. Similar results were obtained when platelet membranes were pretreated with trypsin (0.21-0.84 mg/ml), and no additional inhibition was evident with a combination of both treatments. The present results suggest that 3H-IMI may bind to two separate types of high affinity sites. One subclass is apparently proteinaceous and sensitive to low concentrations of CNIMI, whereas the other is apparently nonproteinaceous and is CNIMI resistant.