Abstract
In order to study rapid changes in 15-ketodihydro-PGF2α, cortisol and progesterone in the period preceding parturition in cattle, pre-term parturition was induced in 4 late pregnant heifers. Parturitions were induced by 2 intramuscular injections of 20 mg dexamethasone with a 24-h interval. The first injection was made on days 254, 258, 264 and 265 in gestation, respectively. Twenty-four h before the first injection an intravenous polyurethane cannula was inserted. Blood samples were collected at least every hour until 12 h after parturition and during the second stage of labour at least 6 times per hour. Plasma was analysed for 15-ketodihydro-PGF2α and progesterone by radioimmunoassays, and for cortisol by an ELISA. The average time from injection to parturition was 7.7 (6.6–8.9) days (mean (range)). Two of the heifers had retained foetal membranes (RFM). At the start of the experiment the levels of PGF2α metabolite were low (< 300 pmol/L) and increased slowly to levels between 1000 and 2000 pmol/L at one day before parturition. During the last day, however, the levels increased rapidly and the highest levels (>10000 pmol/L) were reached at the time of delivery. No pulsatile release was seen. Immediately after foetal expulsion the PG-metabolite levels decreased rapidly in all animals. In the 2 animals with RFM, however, this decline ceased within a few h. The PG-metabolite levels in these animals then started to increase and reached levels as high as during parturition. Luteolysis occurred between 1.6 and 0.4 days before parturition in all animals. The cortisol profile showed a distinct peak at the time of parturition in the RFM heifers. This peak was absent in the non-RFM heifers. This study shows that the PGF2α release at prepartal luteolysis and parturition is not pulsatile in cattle and that cortisol profiles in heifers with retained foetal membranes might differ from the profiles in non-RFM heifers at the time of parturition.
Highlights
The foetal pituitary-adrenal axis is the route by which parturition is initiated in cattle (Flint et al 1979)
In contrast to the pulsatile pattern observed in the oestrous cycle, the PGF2α metabolite levels increased in a continuous way, showing a completely different profile
The absolute levels of PGF2α metabolite at the time of prepartal luteolysis (1.6-0.4 days antepartum) are comparable to those observed during the luteolytic pulses in the oestrous cycle (Basu & Kindahl 1987), but the levels observed after progesterone decline differ between prepartal and preovulatory luteolysis
Summary
The foetal pituitary-adrenal axis is the route by which parturition is initiated in cattle (Flint et al 1979). ACTH from the foetal pituitary stimulates the foetal adrenals to produce increased amounts of cortisol This increase induces synthesis of placental 17α-hydroxylase and aromatase and increases production of oestrogen at the expense of progesterone (Anderson et al 1975). In cattle reaching the end of pregnancy progesterone is produced mainly by the corpus luteum and parturition does not occur until this progesterone production has ceased (Lindell et al 1981, Janszen et al 1990) The aim of this experiment was to study the profile of the PGF2α metabolite, 15-keto-13,14dihydro-PGF2α, and to relate this to cortisol and progesterone levels as well as clinical findings during the period after induction of parturition by dexamethasone in cattle
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