Continuous infusion of relaxin on periparturient progesterone, oxytocin and relaxin plasma concentrations and time of parturition in beef heifers

These studies were designed to determine whether continuous i.v. infusion of increasing dosages of porcine relaxin during late pregnancy in beef heifers would influence circulating blood concentrations of relaxin, progesterone and oxytocin, and time of onset of parturition. Beef heifers were bred by artificial insemination and, on Day 277, fitted with indwelling jugular cannulas for hormone infusion and blood sampling from Day 277 to Day 286. Intravenous infusion of purified porcine relaxin (pRLX, 3000 U mg−1) was started in heifers (n = 8) at increasing dosages (200 U h−1 on Days 277 and 278, 300 U h−1 on Days 279 and 280, 500 U h−1 on Day 281, 600 U h−1 on Day 282, and 700 U h−1 on Days 283–286). Phosphate-buffered saline (PBS, 10 ml h−1) was infused during these same times to control animals (n = 6). Relaxin treatment steadily increased the circulating plasma concentration of immunoreactive relaxin to more than 120 ng ml−1 compared with less than 0.5 ng ml−1 in PBS-treated controls. Relaxin infusion in increasing dosages over the treatment time was associated with a significant decrease (P < 0.01) in plasma progesterone concentration compared with the PBS controls. The rate of change in progesterone levels between pRLX and PBS groups differed (P < 0.05) at 300 U h−1, 600 U h−1 and 700 U h−1 dosage intervals, respectively. Plasma levels of oxytocin at 4h intervals remained similar (P > 0.05) during the pretreatment period and throughout continuous infusion of pRLX and PBS. Mean concentrations of oxytocin in PBS control heifers peaked at 0.95 pg ml−1 during the corresponding infusion of 700 U h−1 pRLX, which peaked at 0.77 pg ml−1. Although continuous i.v. infusion of relaxin resulted in a decrease in circulating blood levels of progesterone, it did not significantly reduce the interval between the beginning of pRLX treatment and parturition compared with the PBS-infused control heifers. These results indicate that continuous i.v. infusion of high levels of porcine relaxin resulted in a decrease in progesterone secretion in late pregnant beef heifers.