Hormone receptor control of pulsatile secretion of PGF 2α from the ovine uterus during luteolysis and its abrogation in early pregnancy

Abstract

Studies on the control of PGF 2α secretion by hormone receptors in the ovine uterus based on in vivo infusion experiments in the autotransplanted uterus are summarized. In addition, direct measurement of hormone receptor dynamics for oestradiol-17β, progesterone and oxytocin in the ovine uterus under experimental conditions are reported. Investigation of the antiluteolytic effect of early pregnancy in sheep was approached in two ways: (a) studies on the antiprostaglandin secreting effect of the embryo and (b) studies on a putative luteoprotective effect of the embryo. In the former study, the measurement of PGF 2α in samples of uterine venous plasma collected hourly from 5 sheep on days 12 to 17 postmating failed to reveal the presence of any major peaks of PGF 2α secretion. In the latter approach, several putative luteoprotective substances were examined for their ability to block the luteolytic effect of PGF 2α given as 5 separate hour-long pulses over 25 h into the arterial supply of the autotransplanted ovary. These substances included: day 14–15 uterine venous plasma from pregnant sheep, PGE 2, ovine prolactin, ovine LH, ovine placental lactogen and porcine relaxin. In no case was a convincing luteoprotective effect demonstrated suggesting that the antiprostaglandin secreting effect of the embryo may be the more important of these two antiluteolytic mechanisms. Experiments were performed to investigate circulating levels of endogenous oxytocin by monitoring intramyometrial pressure on a continuous basis in conscious sheep. These studies revealed the presence of hour-long periods of intense uterine activity at intervals of 5.5 to 6.5 hours on days 14 to 16 in the cyclic animal, but not in the early pregnant animal. Based on these results and on recent reports on the production of oxytocin by the corpus luteum, a hypothesis is presented to explain the hormone receptor control of episodic pulsatile secretion of PGF 2α in the cyclic sheep, including a possible mechanism for its abrogation during early pregnancy.