Effects of the Prostaglandin F_2α Analogues Cloprostenol and Luprostiol in Combination With hCG on Synchronization of Estrus and Ovulation in Mares

Highlights

• We evaluated differences of two PGF_2α analogues in combination with human chorionic gonadotropin (hCG) in cyclic mares.

• Time to estrus and ovulation was equal for luprostiol and d-cloprostenol.

• Induction of ovulation was more reliable in luprostiol-induced cycles.

• Fertility was not impaired by either PGF_2α analogue in combination with hCG.

Abstract

The aim of the study was to evaluate differences between two PGF_2α analogues—luprostiol and d-cloprostenol—in combination with human chorionic gonadotropin (hCG) in their ability to induce estrus and ovulation and their impact on fertility. Breeding records of 155 mares (in 274 estrous cycles) were analyzed. Luteolysis was either induced with luprostiol (group LUP, 3.75 mg i.m.) or d-cloprostenol (group CLO, 30 μg i.m.) or mares came into estrus spontaneously (group CON). Ovulation was induced in all cycles by injection of hCG (1,500 IU i.v.). There was no significant difference between LUP and CLO in the interval from induction of luteolysis to injection of hCG (LUP: 4.4 ± 0.3 days, CLO: 5.1 ± 0.3 days) and from induction of luteolysis to ovulation (LUP: 6.5 ± 0.4 days, CLO: 7.2 ± 0.4 days). The time from injection of hCG until detection of ovulation was shorter in LUP than in CLO and CON (LUP: 2.2 ± 0.2 days, CLO: 2.8 ± 0.2 days, CON: 2.5 ± 0.1 days, CON and CLO vs. LUP P < .01). The percentage of cycles in which ovulation occurred within 48 hours after induction of ovulation was higher in LUP than in CLO and CON cycles. Pregnancy rate was not affected by either treatment. In summary, induction of estrus with luprostiol leads to a better synchronization of ovulation after treatment with hCG than cloprostenol without impairing fertility.

Introduction

In the mare, synchronization of estrus and ovulation is a challenge due to a highly variable response to pharmacologic induction of luteolysis and ovulation (reviewed by Squires [1]). The increasing application of reproductive biotechnologies in this species would benefit from more reliable schemes for the timing of ovulation. The interval from prostaglandin injection to the onset of estrus and to ovulation depends on various factors including the dose of PGF_2α analogue applied and follicle size of the mare at the time of induction of luteolysis [2]. It has been suggested that the dose-dependent effect of cloprostenol on the interval from treatment to ovulation might be the result of a pituitary response to the PGF analogue [2]. A stimulatory effect of the PGF analogue luprostiol on the hypothalamus and pituitary had already been reported in mares during the transition from winter anoestrus to the ovulatory season. In these mares, luprostiol resulted in release of GnRH as well as luteinizing hormone and follicle-stimulating hormone [3]. Similarly, injection of PGF_2α analogues induced cortisol secretion most probably via pituitary adrenocorticotropic hormone release [4], [5]. Again, differences in the response among PGF_2α analogues were suggested because in estrous mares, cortisol release in response to luprostiol was more pronounced than to cloprostenol [5]. However, a single injection of fenprostalene [6], but not of luprostiol [7], was effective to hasten ovulation in estrous mares.

In summary, the question arises if careful selection of the most appropriate prostaglandin analogue may help optimizing protocols for induction of estrus and ovulation in mares. In a retrospective analysis of breeding records, we therefore evaluated differences between two PGF_2α analogues—luprostiol and d-cloprostenol—in combination with human chorionic gonadotropin (hCG) in their ability to induce estrus and ovulation as well as their impact on fertility in client mares presented for breeding at an artificial insemination center. We hypothesized that luprostiol leads to a narrower and therefore more predictable time window of ovulation without negative effects on fertility in mares.