A bioassay for
progesterone activity in dogs was established based on uterine weight (mg/kg
body weight) in immature beagles administered
progesterone for 10 days starting 9 days after priming with
estradiol cypionate (50 micrograms/kg, im).
Progesterone doses of 0, 0.17, 0.5, 1.5, 13.5 and 40.5 mg/kg per day, im, produced dose-dependent increases in the weights of uterine horns obtained after 5 or 10 days of treatment. The total uterine responses (horn removed at 5 days plus horn and fundus removed
at 10 days) to those were (mean +/- SEM) 374 +/- 33, 465 +/- 97, 684 +/- 68, 795 +/- 96, 1005 +/- 38, 1232 +/- 15 mg/kg, respectively. Responses to the 13.5 mg/kg per day dose of
progesterone in dogs given the
steroid antagonist
RU486 at daily oral doses of 5, 20 and 50 mg/kg were reduced to values of 634 +/- 24, 464 +/- 74 and 468 +/- 18 mg/kg, respectively, vs 1005 +/- 38 mg/kg in controls. Mean
progesterone levels were 27 +/- 1 micrograms/l. The
RU486 did not produce any consistent alterations in serum
cortisol levels. The results suggest that, in immature bitches, uterine weight changes can be used to bioassay
progestin activity following
estrogen priming,
RU486 is more potent as an antiprogestin than as an antiglucocorticoid, and
RU486 at oral doses of 5 and 20 mg/kg exerts submaximal and maximal antiprogestin activity, respectively, in the presence of physiological levels of
progesterone.