Inhibition of platelet uptake of serotonin in plasma from patients treated with clomipramine and amitriptyline.

The inhibition of serotonin uptake by platelets has been measured in blood from 20 patients on amitriptyline (50--225 mg daily), 14 patients on clomipramine (25--200 mg daily), and in an untreated group of 21 depressed patients. A complete kinetic analysis was carried out in each patient. Using the increase in the kinetic parameter Km as a measure of uptake inhibition, there was high correlation between the daily dose and inhibition within each drug group, clomipramine being about 10 times more potent than amitriptyline. The inhibition did not vary with age, sex, duration of treatment (up to 3 years), or concomitant use of moderate doses of benzodiazepines, neuroleptics or lithium. In the amitriptyline group the inhibition was significantly smaller in smokers than in non-smokers. The kinetic parameter Vmax was essentially unchanged in the amitriptyline group, and was markedly reduced in the clomipramine group, but without any correlation with dose. The mixed competitive-noncompetitive effect of clomipramine confirms previous in vitro findings.