Effect of Antiporcine Relaxin Treatment on Parturition in Pigs

and Implications Antibody against porcine relaxin (antipRLX540; 1:950,000) was produced in sheep and used to determine the effect on relaxin and progesterone secretion, and on parturition in late-pregnant pigs. In group 1, Yorkshire gilts with normal estrous cycles were bred on the second observed estrus and fitted with an indwelling jugular cannula and an intraperitoneal cannula on day 100 of pregnancy. Gilts were infused at 6-hour intervals with antipRLX540 or phosphate buffer saline (PBS) beginning on day 103 until parturition. From days 103 to 120, daily blood samples were collected for radioimmunoassay of relaxin, progesterone, and prolactin. In group 2, bred gilts were randomly assigned to antipRLX540, relaxin, and PBS treatment on days 111, 113, and 115. Blood was collected twice daily from day 108 to 120, and every 20 minutes on days 111, 113, and 115 beginning 60 minutes before treatment and continuing 180 minutes. Parturition in gilts given antipRLX540 occurred on day 112.7 compared with day 114.0 in relaxin-treated gilts and day 114.3 in PBS controls (P<.05). Duration of delivery from first to last piglet was greatly delayed in antipRLX540 gilts (240 minutes) compared with PBS controls ([117 minutes] P<.005). Average number of stillborns was greater in antipRLX540than in PBS-treated controls (2.4 vs. 1.0; P<.05). Plasma progesterone concentrations were similar in antipRLX540and PBS-treated gilts throughout the period of the study. By day 113, progesterone decreased in antipRLX540-treated gilts compared with relaxinand PBStreated gilts. Prolactin levels were similar in both antipRLX540and PBS-treated gilts; however, from 1 to 3 days postpartum the antipRLX540 group had higher prolactin concentration (P<.05). The results indicate that antipRLX540 decreased circulating plasma concentrations of unbound or free relaxin during the last 10 days of pregnancy. AntipRLX540 markedly increased both the duration of delivery of piglets and the average number of stillbirths in this litter-bearing species compared with PBS-treated controls. This study provides strong evidence that increasing circulating concentrations of relaxin during late pregnancy is crucial for unimpaired parturition in the pig. Introduction Relaxin and progesterone are produced by ovarian corpora lutea of pigs during pregnancy and after hysterectomy. During normal pregnancy of about 114 days, the concentrations of circulating progesterone peak by day 8 and remain high until they decrease just before parturition. Relaxin accumulates in luteal tissue, increases gradually during pregnancy, and is released into the blood in peak quantities just before parturition. Relaxin stimulates growth and softening of the cervix in pigs (1). A primary function of relaxin in mammals during late pregnancy involves the preparation of the reproductive system for delivery of the newborn. This study was designed to (1) determine whether antiserum to porcine relaxin produced in sheep would affect parturition in late pregnant pigs; (2) assess the duration of delivery from first to last piglet, average number of stillborns, and newborn survival rates; (3) compare concentrations of circulating progesterone, relaxin, and PRL in gilts injected with antipRLX540 or PBS continuously from days 103 to 114; and (4) compare concentrations of circulating progesterone and relaxin in gilts injected high doses of antipRLX540, relaxin, or PBS on days 111, 113, and 115.