Relaxin activity in foaling mares

Plasma relaxin concentrations were measured hourly by radio immunoassay in 4 pregnant mares from 11 days before until 4 days after natural foaling. Pre-partum levels ranged from 4 to 7 ng/ml without any surge until the second stage of labour when they increased rapidly to about 11 ng/ml. In 3 of these mares, relaxin activity declined immediately after the expulsion of the placenta and was below detectable levels within 36 h. In the other mare relaxin activity did not fall until after the mechanical removal of the placenta 7 h after foaling. Eight mares were induced to foal by the administration of oxytocin. In 5 out of 6 mares induced with a single i.v. injection of 40 i.u. plasma relaxin increased following injection and peaked at 17 ng/ml shortly after foaling and expulsion of placenta. Activity then rapidly declined. In the 6th mare, which retained the placenta, relaxin activity did not decrease after foaling. Administration of more oxytocin to this mare after foaling resulted in a further increase in plasma relaxin activity, but oxytocin treatment of other mares after placental delivery failed to elicit a similar increase. In the 7th mare 2 i.v. injections of 40 i.u. oxytocin induced a relaxin concentration of 34 ng/ml whereas in the remaining mare one i.m. injection of 40 i.u. oxytocin led to a relaxin profile similar to that in naturally foaling mares. Oxytocin may therefore be responsible for the increased secretion rate of relaxin at foaling. These data support the concept that the placenta is the sole significant source of relaxin in the pregnant mare.