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. infus ion of increasing dosages of porcine relaxin during late pregnancy in beef heifers would influence circulating blood concentrations of relax in, progesterone and oxytocin, and time of onset of parturition. Beef heifers were bred by artificial insemination and, on Day 277, fitted w ith indwelling jugular cannulas for hormone infusion and blood samplin g from Day 277 to Day 286, Intravenous infusion of purified porcine re laxin (pRLX, 3000 U mg(-1)) was started in heifers (n = 8) at increasi ng dosages (200 U h(-1) on Days 277 and 278, 300 U h(-1) on Days 279 a nd 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 i nfused during these same times to control animals (n = 6). Relaxin tre atment steadily increased the circulating plasma concentration of immu noreactive 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 decr ease (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 4 h intervals remained similar (P > 0.05) during the pretreatment peri od and throughout continuous infusion of pRLX and PBS. Mean concentrat ions of oxytocin in PBS control heifers peaked at 0.95 pg ml(-1) durin g 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 sign ificantly reduce the interval between the beginning of pRLX treatment and parturition compared with the PBS-infused control heifers. These r esults indicate that continuous i.v. infusion of high levels of porcin e relaxin resulted in a decrease in progesterone secretion in late pre gnant beef heifers. (C) 1997 Elsevier Science B.V.