Endocrine changes after foetal gonadectomy and during normal and induced parturition in the mare

Abstract

There is a lack of knowledge of the relative influences of the mother and foetus in the initiation of parturition in the horse; environmental as well as endocrine factors appear to be important. The equine foetal gonads are the source of the C-19 precursors utilized by the placenta for the synthesis of oestrogens and foetal gonadectomy results in an immediate and sustained fall in maternal plasma oestrogen, but not progestagen, concentrations. Placental oestrogens are not essential for the maintenance of pregnancy, initiation of lactogenesis or the onset of parturition but they are important for normal growth of the foetus and the synthesis of prostaglandin F. Concentrations of cortisol in amniotic fluid, but not those in maternal or foetal blood, increase during the second half of pregnancy but cortisol does not appear to constitute a trigger for parturition in the mare. An explosive rise in maternal plasma concentrations of 13, 14 dihydro 15-oxo prostaglandin F-2α (PGFM) occurs during labour. Parturition can be readily and rapidly induced in the mare at term by administration of oxytocin or the prostaglandin analogue, fluprostenol. However, glucocorticoids and natural PGF-2∝ are unreliable in this regard.