15-ketodihydro-PGF(2 alpha), progesterone and cortisol profiles in heifersafter induction of parturition by injection of dexamethasone

In order to study rapid changes in 15-ketodihydro-PGF(2 alpha), cortisol an d progesterone in the period preceding parturition in cattle, pre-term part urition was induced in 4 late pregnant heifers. Parturitions were induced b y 2 intramuscular injections of 20 mg dexamethasone with a 24-h interval. T he first injection was made on days 254, 258, 264 and 265 in gestation, res pectively. Twenty-four h before the first injection an intravenous polyuret hane cannula was inserted. Blood samples were collected at least every hour until 12 h after parturition and during the second stage of labour at leas t 6 times per hour. Plasma was analysed for 15-ketodihydro-PGF(2 alpha) and progesterone by radioimmunoassays, and for cortisol by an ELISA. The avera ge 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 t he experiment the levels of PGF(2 alpha) metabolite were low (< 300 pmol/L) and increased slowly to levels between 1000 and 2000 pmol/k at lone day be fore parturition. During the last day, however, the levels increased rapidl y and the highest levels (> 10000 pmol/L) were reached at the time of deliv ery. No pulsatile release was seen. Immediately after foetal expulsion the PG-metabolite levels decreased rapidly in all animals. In the 2 animals wit h RFM, however, this decline ceased within a few h. The PC-metabolite level s in these animals then started to increase and reached levels as high as d uring parturition. Luteolysis occurred between 1.6 and 0.4 days before part urition 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 PGF(2 alpha) release at prepartal luteo lysis and parturition is not pulsatile in cattle and that cortisol profiles in heifers with retained foetal membranes might differ from the profiles i n non-RFM heifers at the time of parturition.