ENDOCRINE AND ULTRASOUND EVALUATION OF THE RESPONSE TO PGF 2-ALPHA AND GNRH GIVEN AT DIFFERENT STAGES OF THE LUTEAL-PHASE IN CYCLIC EWES

To investigate the effects of prostaglandin (PGF 2 alpha) plus GnRH at different stages of the luteal phase 13 ewes received PGF 2 alpha on Day 9 of the synchronized cycle, followed 36 h later by GnRH. This con trol regimen resulted in ovulation and normal corpus luteum (CL) funct ion. In the next cycle, the ewes were treated simultaneously with PGF 2 alpha and GnRH either on Day 4 (early, n=7) or Day 9 (late, n=6). Ov arian activity was monitored daily by ultrasonography, and blood sampl es were obtained to monitor hormonal patterns. Size of the largest fol licle present when GnRH was administered was similar in all groups, bu t the preceding growth rate was greatest for the early group. In the 3 6 h after injection of PGF 2 alpha, serum progesterone (P4) had declin ed to basal levels in the control cycles when GnRH was administered, b ut P4 concentrations were higher in the early group and were highest i n the late group when the GnRH was administered with PGI; 2 alpha. The LH surges induced by GnRH were highest in the control cycles, and wer e lower in the 2 treated groups. In the early group, 6 of 7 ewes demon strated ovulation within 48 h of GnRH, resulting in the formation of n ormal CL. In the late group, ovulation was delayed for about 5 d in 4 of 6 ewes, and subsequent luteal function was normal; no ovulation was detected in the other 2 ewes of this group, but the follicles became luteinized, resulting in a normal P4 profile in one and subnormal in t he other. These results suggest that follicles present during the earl y luteal phase are capable of ovulating and forming fully functional C L in response to exogenous GnRH. In contrast, follicles present during the late luteal phase fail to ovulate in response to GnRH while P4 le vels are high, even though the LH stimulus is adequate; however, these follicles persist and subsequently ovulate after P4 levels have decre ased. Therefore, the endocrine milieu to which a follicle was exposed may be more important than its size in determining its ability to unde rgo ovulation and development into a normal CL. (C) 1997 by Elsevier S cience Inc.