Endocrine control of parturition

Abstract

A sizable problem in our understanding of the control of parturition has been a desire by different investigators to explain the whole mechanism in terms of their current theory. We have been offered explanations in terms of factors such as withdrawal of the systemic progesterone block, the local progesterone block, the volume:progesterone ratio, the progesterone:estrogen ratio, the oxytocin effect, the progesterone:oxytocin effect, the fetal role, and the prostaglandin role. With the coming of age of endocrinology and the ability to reduce hormone assays to a routine laboratory service, we have at last begun to understand that many, if not all, rather than anyone of these individual explanations may be implicated in the control of parturition. We now recognize that in late pregnancy a train of events is initiated that ultimately results in the delivery of the fetus. However, we still do not know exactly how and where that train starts, or exactly how it exerts its ultimate action on the myometrial cell. It is a major objective of this chapter to try to bring together some of the abovementioned theories, and particularly to show that, for example, the role of the fetus in the initiation of parturition and the concept of progesterone withdrawal may be part of the same mechanism. It would be folly to say that at the present time either or both of these concepts explain parturition in all species, but by examining a number of different species we hope to illustrate as many of the discrepancies as similarities.

Effects of periconceptional undernutrition on the initiation of parturition in sheep

“…Estrogen in turn upregulates maternal endometrial PGHS-2 expression and PGF 2␣ output at the onset of parturition, suggesting that the rise in placental PGE 2 synthesis should precede the rise in maternal PGFM and 17␤-estradiol concentrations (31). In sheep, fetal plasma PGE 2 concentrations rise progressively over the last 15-20 days of pregnancy, whereas maternal plasma PGF 2␣ concentrations rise sharply only 12-24 h before delivery (6,30). However, the observation from the present study could be explained by the following mechanism.…”

Investigations into the mechanisms controlling parturition in cattle

“…For many mammalian species, the signal cascade leading to the initiation of parturition is still poorly understood with the information available pointing toward distinct species-specific differences (Thorburn & Challis 1979, Ló pez Bernal 2001, Zakar & Hertelendy 2007, Mitchell & Taggart 2009). So far the sheep has served as an important animal model and detailed information on the regulatory mechanisms leading to birth is available for this species (for review see Whittle et al (2001)); in the ovine fetus at the end of gestation, maturation of the hypothalamo-pituitary-adrenal (HPA) axis leads to a substantial increase in fetal cortisol, triggering in uninucleated trophoblast cells (UTC) an upregulation of cyclooxygenase 2 (COX2) and of the steroidogenic key enzyme 17a-hydroxlyase-C17, 20-lyase (CYP17).…”

Influence of meclofenamic acid on the initiation of parturition and lactation in the sow

“…In the sow, as in other corpus luteum-dependent species, the onset of parturition (Thorburn & Challis, 1979) and lactation (Kuhn, 1969) are controlled by the factors which cause corpus luteum regression. The administration of prostaglandin (PG) F-2rx or its analogues causes a decline in plasma progesterone and induces premature parturition in sows in late pregnancy (Gustafsson, Einarsson, Larsson & Edqvist, 1976;Wetteman, Hallford, Kreider & Turman, 1977) while the inhibition of PG synthesis by indomethacin in late pregnancy delays the onset of parturition (Sherwood, Nara, Crnekovic & First, 1979).…”