Characterization of the GnRH-induced corpus luteum in the cycling heifer

Although recent studies in other laboratories have greatly increased our understanding of luteal function in cattle, we are still unable to accurately synchronize luteolysis, preovulatory follicular growth and ovulation with commercially available pharmacological preparations. The aim of these studies was to investigate the factor(s) which controls the formation, function and regression of the GnRH-induced corpus luteum (CL).

Initially, we sought to develop an experimental model to examine the characteristics of the CL induced in intact heifers with normal oestrous cyclicity, by GnRH injection in the early luteal phase. Administration of GnRH on Day 6 after the synchronized oestrus resulted in ovulation and formation of an additional CL in >70% of animals. Following prostaglandin F₂α (PGF₂α) in the mid-luteal phase, the spontaneously-formed CL underwent luteolysis while the induced CL did not, leading to a delay in return to oestrus associated with a persistence of luteal function.

Having demonstrated the ovulatory competence of the dominant follicle of the first follicular wave, and the formation of a functional CL, it was of interest to examine the reason for the premature demise of the induced CL in the GnRH responders when compared to the spontaneouslyformed CL. Administration of steroid-stripped bovine follicular fluid (bFF), which suppressed the growth of the dominant follicle, and reduced serum oestradiol concentrations, resulted in a further delay in the return to oestrus and a significant extension in the lifespan of the induced CL, when compared to the untreated responders, and provided circumstantial evidence that oestradiol was the endogenous agent responsible for regression of the induced CL.

Although luteal lifespan was extended by the use of bFF, the function of the induced CL was still limited compared to the spontaneously-formed CL. A comparison of the structure and composition of the induced CL with the spontaneously-formed CL of the same age, revealed no differences in concentrations of progesterone and oxytocin 7 days after formation, but a significant increase in LH receptor concentration in the induced CL. This suggests that the limitation in lifespan of the induced CL was not due to a lack of luteotrophic support. Furthermore, a subsequent study examining the interaction between the induced CL and the uterine endometrium, demonstrated that the GnRH responders with an induced CL were competent to respond to an oxytocin challenge with a 13,14-dihydro-15-ketoprostaglandin F₂α (prostaglandin F metabolite) response of similar magnitude to animals with a spontaneous CL. The similarity in response suggested that the premature demise of the induced CL was not the result of advanced release of PGF₂α, or of oxytocin-induced reduction in luteotrophic support

To investigate the possibility of an additional direct effect of bFF on the induced CL, the development of a simple procedure for the dispersion and culture of bovine luteal cells in our laboratory, enabled a study of the effect of bFF on luteal cell function in vitro. The study demonstrated that bFF contained a factor(s) that inhibited progesterone production by mixed populations of cultured luteal cells in vitro. The inhibitory effect was seen in both steroid-stripped, inhibin-enriched and inhibin-depleted fractions of bFF. Thus, bFF appears to inhibit luteal function in vitro by a direct effect on the CL, but extend luteal funcion in vivo indirectly by suppression of follicular growth and a reduction in peripheral oestradiol concentrations.

Finally, we compared the steroidogenic capacity in vitro, of dispersed luteal cells from induced and spontaneous CL 7 days after ovulation, in response to treatment with LH (the primary luteotrophin in the bovine), and dibutyryl cyclic adenosine 3',5'-monophosphate (dbcAMP), which is independent of receptor integrity. Both spontaneous and induced dispersed cells were non-responsive to LH, but spontaneous luteal cells dramatically increased progesterone production in the presence of dbcAMP, suggesting that dispersion had damaged the LH receptor. Progesterone production in response to dbcAMP by luteal tissue from induced CL was markedly reduced, suggesting a failure of the steroidogenic response at an as yet unidentified point in the second messenger pathway

Collectively, we have demonstrated that administration of GnRH in the early luteal phase results in formation of an additional CL of limited functional capacity. The premature demise of the induced CL appears to be a consequence of an endogenous signal, which is probably oestradiol, from the dominant follicle. This functional association between luteal and follicular elements in the ovary is a target for future research attempting to improve oestrous synchronization techniques in cattle.