Aglepristone (RU534) administration to non-pregnant bitches in the mid-luteal phase induces early luteal regression

The effect of the antiprogestagen aglepristone (10 mg/kg bw), administered at days 29 and 30 following the estimated day of LH surge (day 0), on corpora lutea (CL) function was examined during the diestrus phase of non-pregnant bitches. Aglepristone shortened (P 0.01) the luteal phase and complete luteolysis (progesterone 2 ng/mL) was observed at days 40.8 3.5 and 71.5 4.6 (means SD; n 9/group) in treated and control bitches, respectively. Peripheral estradiol-17 concentrations declined from 91.5 14.3 pg/mL at day 9 to 50 pg/mL at day 18, remaining at approximately the same levels thereafter in both treated and control bitches. Intraluteal in vitro synthesis of progesterone and estradiol-17 released by CL explanted at day 38 from control bitches (511.9 285.6 and 40.7 17.2 pg/mg protein, respectively) did not differ from that of treated. From day 38, intraovarian hemodynamic variables (arterial blood flow, systolic peak, and end-diastolic velocities), monitored by color-coded and pulsed Doppler, decreased more steeply (P 0.01) in aglepristone-treated (n 4) than in control (n 4) bitches, whereas the resistance index increased (P 0.01) in treated animals. All the blood flow parameters were undetectable at 60 3.6 and 68 2.0 days (medians SD) after LH peak in treated and control bitches, respectively. In conclusion, aglepristone administration to dogs during the mid-luteal phase markedly accelerates the luteolytic process which is accompanied by a parallel decline in ovarian blood flow supply with a shift from approximately 8 to 10 days.