Changes in Luteal Life Span in Large DomesticAnimaIs Occur Because of Changes in Prostaglandin F2α Synthesis by the Uterus
Significant changes in the length of the life span of the CL in nonpregnant large domestic species occur only because of changes within the uterus. As discussed in Chapter 38, the presence of an embryo results in the blockage of PGF2α synthesis and a continuance in luteal activity.
Prolonged luteal phases also typically occur in mares in the absence of uterine infection. This deficit in mares appears to be a genetic propensity toward inadequate synthesis and release of PGF2rt. The absence of a uterine horn can also result in a lengthened luteal phase in animals in which the ipsilateral horn controls the CL (local control). In this situation (e.g., in the cow), if ovulation occurs in the ovary ipsilateral to the missing horn, the luteal phase is prolonged because of the need for the ipsilateral uterine horn to control the life span of the CL.In nonpregnant large domestic animals, inflammatory responses of the endometrium caused by bacterial contamination can result in significant synthesis and release of PGF2α, leading to premature Iuteolysis and a shortening of the estrous cycle. It should be emphasized that luteal activity is almost always normal in the absence of uterine abnormality in large domestic species. Thus, short estrous cycles in large domestic animals are pathognomonic for uterine infection.
must be pulsatile, with pulses occurring at about 6-hour intervals, in order for Iuteolysis to be affected (Figure 36-5). The concept has developed that a minimum of four to five pulses within 24 hours is required to cause complete Iuteolysis. If pulse intervals increase significantly before complete Iuteolysis (e.g., to 12 hours), the CL can recover and continue to function, even if at a lower level of steroid synthetic activity. The uterus must be exposed to estrogen and progesterone to synthesize and release PGF2rt. Although the initiation of PGF2rt synthesis that leads to Iuteolysis is not completely understood, one possible explanation is that estrogen (from an antral follicle) causes the initial synthesis and release of PGF2rt. In sheep it is thought that an interplay occurs between the uterus and ovary after the initial PGF2rt pulse. PGF2rt affects the CL to cause both a reduction in progesterone production and the release of luteal oxytocin. Oxytocin then interacts with receptors within