Premature prostaglandin F2α secretion causes luteal regression in GnRH-induced short estrous cycles in cyclic dairy heifers
Introduction
Premature luteolysis shortening the normal length of the bovine estrous cycle is a common phenomenon following first ovulation in puberty or immediately after calving [1], [2], [3], [4]. While mechanisms behind these short estrous cycles are not yet fully understood, recent findings indicate that early demise of the corpus luteum (CL) is the result of premature release of prostaglandin F2α (PGF2α) from the uterus [3], [4], [5]. The reason for this premature release remains to be elucidated. Oxytocin and progesterone (P4) receptors in the endometrium and uterine PGF2α response to oxytocin obviously play important roles [6], [7]. Exogenous administration of progestagens prior to the first ovulation prevents premature luteolysis and the short estrous cycles that follow [4], [8], [9].
We have previously found short estrous cycles in normal-cycling dairy cows and heifers after cloprostenol (Clp) and gonadotropin-releasing hormone (GnRH) treatment [10], [11]. Estruses were induced with Clp during early diestrus (7–10 days after estrus), followed by 0.1 mg of gonadorelin 24 h later. This regimen caused short estrous cycles in about 30% of the animals. These cycles resemble in duration, timing of luteolysis, and P4 secretion those occurring after the first postpartum ovulation. However, these short cycles occurred in normally cycling animals and were preceded by normal luteal phases, which—by definition—should have prevented their occurrence.
The aim of this study was to confirm that CL regression in normally cycling dairy heifers seen during short estrous cycles induced with Clp and GnRH administered 24 h apart is caused by a premature release of PGF2α (monitored by its main metabolite, 15-ketodihydro-PGF2α). A further aim was to study the PGF2α release pattern more closely to determine whether it resembles the spontaneous release occurring during normal luteolysis or whether PGF2α is continuously secreted after the induced ovulations, leading to short estrous cycles. In the case of continuous secretion, luteolysis is expected to occur at about the same time as during these short estrous cycles, when CL has just reached responsiveness to PGF2α.
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Animals
Twenty-four Ayrshire heifers were randomly allotted to four groups of equal size. The heifers were between 11 and 20 months of age at the beginning of the experiment, with group averages of 13.5, 13.7, 14.0, and 14.2 months. All animals had normal estrous cycles and were clinically healthy. They were loose housed until the start of the experiment and then stanchioned. They were fed grass silage, dry hay, and concentrates according to Finnish standards. The experiment ran between September 1999
Results
Three heifers in group C-d6 had an incomplete luteolysis after Clp treatment. In addition, another heifer in the same group formed a cystic follicle. The entire data for these four heifers were rejected from the analyses. Thus, only two heifers were left in group C-d6. The control groups (C-d6 and C-d7) were combined (group C) since no obvious differences were found after spontaneous ovulations for Clp treatments given either on Day 6 or on Day 7. One heifer (no. 228) from group T-d6 was
Short estrous cycles
Of 12 heifers, 7 showed a short estrous cycle after induced ovulation with GnRH 24 h after Clp administration, while no short cycles were detected in control groups after spontaneous ovulations in Clp-induced estruses. This significant difference confirms our earlier findings that GnRH administered 24 h after PG treatment given 7–9 days after estrus causes short estrous cycles in some cyclic dairy cows and heifers ([10], [11]; Taponen, unpublished data). The 58% incidence of short cycles in the
Conclusions
We have demonstrated that the luteal regression seen in short estrous cycles induced with cloprostenol and GnRH administered 24 h apart in normally cycling dairy heifers is related to a premature release of PGF2α. This release closely resembles the release of PGF2α during normal spontaneous luteolysis. In heifers with a short estrous cycle as compared with those with an estrous cycle of normal length, no disturbances or changes in ovulatory follicles and ovulation preceding premature luteolysis,
Acknowledgements
The authors wish to thank Viikki Research Farm and especially herd manager Juha Suomi for helping in arranging the experiment. This study was supported by the Research Foundation of Veterinary Medicine, Helsinki, Finland.
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