Circadian variation in platelet imipramine binding during the day in healthy subjects

Platelet tritiated imipramine binding values in healthy controls vary considerably from study to study. A possible contributor to such variation might be a circadian rhythm affecting binding, although previous studies of this have been contradictory. Platelet imipramine binding was examined in 12 healthy, medication-free subjects studied at 8 a. m., 11 a. m., 4 p. m., and 10 p. m. during one day. Imipramine binding was determined on platelet membranes, using 0.8-8 nM 3H-imipramine, and nonspecific binding was defined by 50 microM desipramine. All samples from a given individual were assayed simultaneously. The intra-assay coefficient of variation was 6.3 percent. There was no evidence of significant differences in binding capacity or affinity (Bmax or Kd) at different times of day. Circadian variation was explored using COSINOR analysis (DeMet et al., 1989). There was no evidence of circadian variation in binding using this model, even when only the variable portion of binding was considered for each individual. Intraindividual variation in binding was substantial, with a mean coefficient of variation of 29 percent for Bmax and 38 percent for Kd. The possible basis of this variation is unclear, but may reflect the presence of "occult" binding sites in the membrane, or the effect of endogenous modulators of binding. The interrelationship of Bmax and Kd may also be a factor. It was considered that low-affinity binding did not account for a significant part of the variation in Kd in this assay. The utility of imipramine binding as a biological marker of depression may be limited by such levels of intraindividual variation in binding parameters.