Effect of dexamethasone treatment on sex steroid-binding protein, corticosteroid-binding globulin, and steroid hormones in cycling rhesus macaques.

We tested the hypothesis that dexamethasone lowers sex steroid-binding protein levels and observed the effect of dexamethasone on corticosteroid-binding globulin and specific steroid hormones in plasma. Four cycling rhesus macaques were studied during three consecutive menstrual cycles (first and third cycles served as controls). In the second cycle, each animal received 0.5 mg of dexamethasone intramuscularly at 8:00 AM and 8:00 PM daily for 21 consecutive days. Blood samples were taken at 1- to 3-day intervals during each cycle. Concentrations of sex steroid-binding protein, corticosteroid-binding globulin, cortisol, testosterone, progesterone, and estradiol were measured in plasma. The percentage and plasma concentration of free testosterone were also determined. Within 2 days of treatment, dexamethasone suppressed cortisol to 5% of baseline values, which returned in the third cycle. All cycles were ovulatory. Dexamethasone significantly lowered plasma levels of all the compounds except progesterone: sex steroid-binding protein, -30%; corticosteroid-binding globulin, -14%; testosterone, -36%; and estradiol, -45%. The percentage of free testosterone was significantly elevated, but free testosterone concentrations were unchanged. Although our data conclusively show that dexamethasone suppresses plasma sex steroid-binding protein levels in the rhesus macaque, it remains to be established whether this suppressive effect leads to an increase in the metabolic clearance rate of testosterone.