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EFFECTS OF MEDROXYPROGESTERONE ACETATE (MAP) ON OVARIAN ANTRAL FOLLICLE DEVELOPMENT, GONADOTROPIN SECRETION AND RESPONSE TO GnRH IN SEASONALLY ANOVULAR EWES.
Bartlewski, Pawel1, Aravindakshan, Jayaprakash2, Beard, Andrew3, Nelson, Michelle4, Batista-Arteaga, Miguel5, Cook, Susan4, Rawlings, Norman4, 1 2 3 4 5
ABSTRACT- When ovulation is induced with GnRH in anestrous ewes, a proportion of animals develop short-lived corpora lutea (CL). Progesterone treatment before GnRH prevents luteal inadequacy. Whether a similar effect can be achieved with medroxyprogesterone acetate (MAP) and if it is mediated by influences on growing ovarian follicles is not known. Two experiments were performed, on 13 and 11 anestrous Western white-faced ewes, respectively. Seven and 6 ewes (TRT), respectively, received MAP-releasing intravaginal sponges (Veramix®, Upjohn; 60 mg) for 14 d; the remaining ewes served as controls (CON). GnRH (250 ng every 2 h for 24 h followed by a bolus injection of 125 g of GnRH i.v.) was given either immediately (Exp. 1) or 24 h after sponge removal (Exp. 2). Follicular dynamics (follicles ≥3 mm in size) and development of luteal structures were monitored by transrectal ultrasonography for 41 d, starting 7 d before sponging. Blood samples were taken each day and also every 15 min for 6 h, 1 d before sponging and 1 d before sponge withdrawal, and every 15 min for 8 h after GnRH. None of the characteristics of follicle growth differed between TRT and CON, and there was no correlation between the number and age of follicles ≥5 mm at GnRH injections and ovulation rate. There were no differences between TRT and CON in mean daily FSH concentrations, and no differences in LH/FSH secretion based on the 2 intensive bleeds (P>0.05). In Exp. 1, the mean ovulation rate and proportion of ovulating ewes did not vary (P>0.05) between TRT and CON. In Exp. 2, the mean ovulation rate (2.3±0.2 and 1.2±0.6, respectively) and percentage of ewes that ovulated (100% and 60%, respectively) were higher (P⩽0.08) for TRT than CON. Normal (full-lifespan) CL were detected in 29% TRT and 76% CON (Exp. 1), and in 100% TRT and 60% CON (Exp. 2). In Exp. 1, the mean peak concentration of GnRH-induced LH surge was lower (P<0.05) in TRT compared to CON. In conclusion, treatment of anestrous ewes with MAP had no effect on the tonic secretion of LH/FSH, but the GnRH-induced LH surge was attenuated at 24 h after sponge removal. Ovulatory response and CL formation were greater when GnRH was given from 24 h after treatment with MAP. No differences in follicular gross morphology could be found to explain differences in ovulation rate and luteal outcome.
KEY WORDS: anestrous ewe, medroxyprogesterone acetate, follicular dynamics, gonadotropins
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