RELAXIN CONCENTRATIONS IN PIG PLASMA AFTER THE ADMINISTRATION OF INDOMETHACIN AND PROSTAGLANDIN-F-2-ALPHA DURING LATE PREGNANCY

The present study was conducted to determine the influence of prostaglandins (PGs) on relaxin levels during late pregnancy in the pig. Concentrations of relaxin in peripheral plasma were measured in pigs treated as follows: 1) controls, 2) 10 h infusion of PGF2a (0.5 mg/h) on day 110, 3) indomethacin (4 mg/kg) twice each day from days 109–116, 4) indomethacin plus PGF2o infusion (treatments 2 and 3), and 5) indomethacin (treatment 3) plus PGF2a(1 continuous infusion from day 110 until the end of parturition. Infusion began at 0.5 mg/h for 24 h, followed by 3 mg/h for 3 h, 6 mg/h for 3 h, and 9 mg/h until parturition. In the three treatment groups infused with PGFPGF2a, there was a sharp surge in relaxin levels during the infusion of PGF2(, (P < 0.005). In these three treatment groups, the mean relaxin concentrations obtained 1 h after the beginning of PGF2a infusion (>100 ng/ml) was greater (P < 0.01) than preinfusion levels (<15 ng/ml), and within 10 of the beginning of PGF2a infusion, the relaxin levels had begun to decline. Sows injected with indomethacin only (treatment 3) did not experience elevated levels of relaxin until 2–4 days after the termination of the administration of indomethacin. Maximal relaxin levels in the indomethacin-treated sows were obtained on day 119 (±0.4 SE) of gestation, which was later than controls (P < 0.001). The control sows exhibited maximal levels of relaxin on day 114.3 (±0.5 SE) of gestation. Levels of progesterone in plasma samples obtained within 3 days of the relaxin surge are also presented. In all five treatment groups, there was a simultaneous occurrence of the relaxin surge and a rapid drop in progesterone levels. This study provides further evidence that the prepartum surge of relaxin level s is associated with PGs and termination of luteal function in the pig. © 1979 by The Endocrine Society.