Characterization of platelet alpha 2 adrenoceptors and measurement in control and depressed subjects.

The alpha-adrenoceptor on platelets has been characterized using 3H-yohimbine, 3H-dihydroergocriptine, and 3H-clonidine. The receptor, which exhibits the characteristics of an alpha 2-type, has a Bmax for dihydroergocriptine of 330 fmoles/mg protein, for yohimbine of 178 fmoles/mg protein, and for clonidine of 38 fmoles/mg protein. Clonidine, but not yohimbine binding, is decreased by the presence of K+, Na+, or Li+. Adenosine triphosphate (ATP) and the guanosine triphosphate (GTP) analogue, Gpp(NH)p, reduce the affinity of clonidine for its binding site. Acute exercise, such as playing squash, does not apparently alter the Bmax or Kd of 3H-yohimbine binding to platelet membranes, and in vitro studies, with intact platelets or platelet membranes, show that incubation with adrenalin (10 microM) does not induce alterations in Bmax or Kd. In the present study, no correlation was found between age and alpha 2-adrenoceptor numbers. There was no significant difference in the Bmax for 3H-yohimbine binding to platelet membranes from control and depressed subjects, although the mean value for the depressed group was some 10% lower than that for the corresponding control group. There were no overall significant and consistent effects of desipramine (DMI) treatment. After 2-3 days of treatment, the Bmax was reduced by 20%, after 7 days by 14%, and after 21 days it was 8% above the control value. When an additional group of patients on DMI (7 days of treatment) was analyzed using one supramaximal concentration of 3H-yohimbine, there was a significant decrease (25%) in binding.