The effects of preovulatory administration of oral letrozole on ovulatory follicle and the subsequent corpus luteum in mares

Progesterone acts as an important factor in maintenance of pregnancy and development of equine embryo. Treatment with letrozole, a nonsteroidal inhibitor of aromatase enzyme regulating the synthesis of estrogens, during the preovulatory period has been reported to result in larger corpus lutea (CLs) with greater progesterone production in human and bovine. The present study was conducted to evaluate the effect of preovulatory administration of letrozole on the size of CL and progesterone concentration. In the present study, mares (n = 7) received PGF2α (250 μg) 14 days after the observed ovulation and were subjected to daily ultrasonography to evaluate the development of the largest follicle and the resultant CL for two consecutive estrous cycles. In addition, progesterone concentration was measured over the course of luteal phases. During the first cycles, as control cycles, mares received no treatment. During the second cycles, mares received letrozole (0.5 mg/kg) orally beginning from PGF2α administration for 3 days. Letrozole reduced the diameter of ovulatory follicle (P < 0.05) and advanced the time of ovulation (P < 0.05). There was also a tendency for formation of smaller CLs following letrozole administration (P = 0.065). Moreover, concentration of progesterone during midluteal phase was lower following letrozole treatment (P < 0.05). In conclusion, the present study showed that letrozole treatment during the preovulatory period could result in premature ovulation and impair the function of subsequent CL.