Serum cortisol levels affect the metabolic clearance rate of progesterone in female baboons.

Since interactions between progesterone (P₄), Cortisol (F), cortisone (E) and corticosteroid binding globulin (CBG) may influence the metabolic clearance rates (MCR) of these steroids, the effect of altering circulating F concentrations on clearance of the steroids was determined. MCR of P₄, F and E were determined by the iv constant infusion method in 6 pregnant and 6 nonpregnant baboons (Papio papio). Serum F concentrations were altered by iv infusion of 5 mg F/90 min or im injection of betamethasone (3 mg bi-daily for 2 days). Mean MCR-P₄ (1/d/kg ± SE) was greater (P < 0.01) in pregnant (92.8 ± 8.5) than in nonpregnant (53.9 ± 4.4) animals while mean MCR-F was similar in both groups ($\text{10.8 ± 1.2}\text{vs}\text{13.0 ± 1.5}$, respectively). Mean MCR-E was also similar in pregnant (30.8 ± 4.9) and nonpregnant (34.1 ± 4.5) baboons. Mean serum F concentrations (/gmg/100 ml ± SE) in 4 nonpregnant (42.0 ± 8.6) and 4 pregnant (52.2 ± 10.0) baboons were increased (P < 0.05) 60% by F administration but MCR-P₄, -F and -E were unaltered. Betamethasone treatment reduced (P < 0.05) serum F 75% in both groups. In nonpregnant baboons, betamethasone treatment reduced (P < 0.01) MCR-P₄ (37.3 ± 3.9), MCR-F (7.4 ± 1.6) and MCR-E (18.5 ± 3.7). Betamethasone treatment of pregnant animals reduced (P < 0.01) MCR-P₄ (56.5 ± 7.4), MCR-F (6.3 ± 0.8) and MCR-E (14.6 ± 2.6). Infusion of F into betamethasone-treated animals increased serum F levels and increased MCR-P4, -F and -E. It is concluded that variations in serum F levels affect the clearance of F, E and P₄ presumably because of the mutual interactions of these steroids with CBG.