Placental Steroid Metabolism in Late Pregnancy

Influence of the fetal pituitary-adrenal axis on fetal and maternal progesterone and unconjugated oestrogen concentrations in the pig

“…Activation of one of these enzymes, 17a-hydroxylase, is mediated by fetal cortisol in sheep and goats (Anderson, Flint & Turnbull, 1975;Flint, Kingston, Robinson & Thorburn, 1978) and forms part of the cascade by which parturition is induced by the fetal pituitary-adrenal axis. A similar mechanism has been suggested for the pig (Craig, 1982). Fetal decapitation (Stryker & Dziuk, 1975) or hypophysectomy (Bosc, du Mesnil du Buisson & Locatelli, 1974) are known to prolong gestation in the pig and during the development of these procedures we found that progesterone concentrations were higher in hypophysectomized than in intact fetuses.…”

“…The major sources of fetal progesterone are the placenta (Silver et al, 1979;Craig, 1982) or active transfer from the maternal circulation (Barnes et al, 1974), whereas little is produced by the fetal adrenal during late gestation (Macdonald, Colenbrander, Elsaesser & Heilhecker, 1980).…”

“…Maternal oestradiol-17ß concentrations have been shown to be lower in gilts carrying small numbers of hypophysectomized fetuses than in those carrying small numbers of normal fetuses (Kendall, Dziuk, Nelson & Thurmon, 1985). The placenta is the major source of oestrogens during gestation in the pig (Fèvre, Léglise & Rombauts, 1968;Robertson & King, 1974) and Craig (1982) showed that the placenta has limited enzymic ability to synthesize oestrogens from pregnenolone by the A4-pathway in vitro. Moreover, the activity of those enzymes essential for the metabolism of progesterone to oestrone increased at term.…”

Relaxin and progesterone are myometrial inhibitors in the ovariectomized non-pregnant mini-pig

“…Moreover, the effect is similar to that demonstrated in animals of other species (Porter, 1979) in that frequency of intrauterine pressure cycles is lowered while amplitude is not significantly reduced. A single dose of relaxin, which for a brief period pro¬ duced plasma concentrations comparable to those found normally pre partum (Sherwood, Chang, Bevier & Dziuk, 1975;Taverne et al, 1982) (Docke & Worch, 1963;Zerobin & Spörri, 1972;Bower, 1974), and suggests that the ability of progesterone, or synthetic progestagens (Craig, 1982), to maintain pregnancy in the sow after bilateral ovariectomy is, at least in part, due to its inhibition of myometrial contractions. As in the rabbit (Csapo, Takeda & Wood, 1963) and the ewe , the effect of progesterone was gradual in onset since the reduction in the frequency of pressure cycles occurred progressively over a period of 6-24 h. Progesterone also abolished the myometrial responsiveness to oxytocin.…”

INHIBITION OF 3β‐HYDROXYSTEROID DEHYDROGENASE IN THE SOW NEAR TERM: EFFECTS ON FETAL AND MATERNAL STEROIDS AND ON DELIVERY

“…In fact the levels of progesterone in fetal arterial plasma are twice those in the maternal circulation, while umbilical venous blood contains even greater concentrations (Barnes, Comline & Silver, 1974). Whether the high circulating progesterone in the fetal pig has any function is not known, although a reduction of maternal levels by ovariectomy in late gestation results in some apparent transfer from fetus to mother (Craig, 1982).…”