High plasma concentrations of metoclopramide are not detected by radioreceptor assay

Plasma samples from individuals treated with equivalent doses of metoclopramide or thioridazine were tested for their potency in the radioreceptor assay. Samples of plasma from patients treated with thioridazine actively displaced 3H-spiroperidol from striatal membranes, while plasma from patients treated with metoclopramide were virtually inactive. The lack of activity in the radioreceptor assay is of particular interest as the concentration of metoclopramide determined by gas chromatography, in the same plasma samples, indicates a mean value of 1,102 ng/ml.