Relaxin enhances synchronization of parturition induced with prostaglandin F2 alpha in swine.

To investigate the effects of relaxin on induction of parturition in swine, 59 sows and gilts were randomly allotted within parity to 4 treatment groups. Control animals were injected with saline while the other 3 groups received prostaglandin F2 alpha (PGF, 10 mg i.m.) at 0800 h on Day 112 of gestation to induce luteolysis and parturition. Relaxin (600 U i.m.) was administered either on Day 111 or on both Days 110 and 111 to 2 groups of sows receiving PGF. All farrowings were attended and the time of delivery of each piglet was recorded. PGF injection on Day 112 of gestation induced parturition in 43 of 44 treated animals. The interval (mean +/- SD) from Day 112 to parturition in control animals was 70 +/- 30 h and this was decreased to 30 +/- 9 h with PGF alone. When relaxin was administered prior to PGF on Day 111 or on both Days 110 and 111, parturition occurred after 26 +/- 4 or 27 +/- 3 h, respectively. Thus, relaxin treatment had little effect on reducing the interval to parturition, but improved the synchrony of onset of farrowing as indicated by a decrease (P less than 0.01) in variation. Relaxin pretreatment prevented any instances of delayed parturition (after 35 h) which occurred in 1/3 of the cases in response to PGF alone. This variation in response appeared unrelated to parity of sow. The mechanism by which relaxin enhanced the synchronous onset of parturition after PGF treatment in swine is unknown, but the most likely suggestions are through sensitization of the uterus to the withdrawal of progesterone and the well-known effects of relaxin on cervical dilatation.