Silastic implants of progesterone produce high circulating levels of both progesterone and 20...

Previous studies indicated that circulating 20 alpha-hydroxyprogesterone (20 alpha-OH-P) in cyclic female rats derived from enzymatic conversion of progesterone in corpus luteum tissue (Hashimoto and Wiest, 1969). A possibility of extraovarian conversion of progesterone to 20 alpha-OH-P also was revealed by our recent finding that placement of Silastic implants of progesterone produced not only high plasma progesterone but also 20 alpha-OH-P in ovariectomized (OVX), previously pseudo-pregnant, rats (Gilman et al., 1981). This study was performed to measure the amounts of serum 20 alpha-OH-P and progesterone in intact or adrenalectomized (ADX), cyclic female rats and in OVX, ADX or OVX-ADX animals with or without progesterone implants. Twenty-four-hour patterns of serum progestins revealed that OVX rats exhibited low progesterone secretion (3-6 ng/ml) and undetectable levels of 20 alpha-OH-P in the absence of ovaries. In intact, cyclic females, circulating concentrations of 20 alpha-OH-P were 4- to 6-fold greater than those or progesterone. ADX resulted in a partial reduction of progesterone but not 20 alpha-OH-P in the circulation. Placement of 4 cm Silastic implants of progesterone produced high serum concentrations of both progesterone (29 +/- 4 ng/ml, mean +/- SEM) and 20 alpha-OH-P (27 +/- 1 ng/ml) in OVX and OVX-ADX rats. These findings demonstrate that placement of progesterone implants into female rats produces high circulating concentrations of both progesterone and 20 alpha-OH-P, and indicate that an extraovarian and extra-adrenal system(s) for progestin conversion is functioning under persistently high circulating progesterone conditions. By this system, large amounts of circulating progesterone are transformed into a less potent progestational compound, 20 alpha-OH-P.