Plasma concentrations of prolactin, progesterone, relaxin and oestradiol-17 beta in sows treated with progesterone, bromocriptine or indomethacin during late pregnancy.

Pregnant gilts (3/group) were given no treatment, 10 mg bromocriptine twice daily by mouth, from Day 111 of pregnancy to 1 day post partum, 25 mg progesterone s.c. at 6-h intervals from Days 111 to 116 inclusive or 400 mg indomethacin by mouth at 6-h intervals from Day 111 to 116 inclusive. Before spontaneous delivery maternal plasma prolactin and relaxin concentrations started to rise almost simultaneously between 58 and 47 h before the first piglet and both hormones reached peak values when the plasma progesterone concentration had started to decline rapidly (approximately 21-23 h). Suppression of prolactin levels by bromocriptine prevented the onset of lactation completely but had no obvious influence on changes of the other hormone concentrations and the course of parturition. Progesterone treatment delayed the onset of expulsion of the piglets but did not delay the simultaneous increase in prolactin and relaxin concentrations. These changes in hormone levels were prevented by indomethacin treatment but occurred essentially unchanged when the treatment was ended. The results support the concept that parturition in the pig is preceded by a biphasic increase of plasma prostaglandin levels.