The secretion of prolactin in intact and lutectomized pregnant ewes. Effect of the anti-progesterone steroid RU 486

To study the role, if any, of luteal factors in the control of prolactin se cretion during the last two thirds of pregnancy in the ewe, we examined: a) the effect of RU 486 administration on prolactin secretion on days 97, 112 and 131 of pregnancy in five intact ewes and in five ewes from which the c orpus luteum (CL) was removed on day 78 of pregnancy; and b) the secretory patterns of prolactin on days 60, 80, 100 and 120 of pregnancy in five inta ct ewes and in five ewes from which the CL was removed on day 70 of pregnan cy. In a pilot experiment, we showed that daily i.v. injections (from day 9 1 to day 105 of pregnancy) of RU 486 at a dose of 50 mg caused a marked rel ease of prolactin, without any effect on the secretion of progesterone and progression of pregnancy. In experiment 1, a single i.v. injection of 50 mg of RU 486 resulted in a significant (P < 0.01) increase in plasma prolacti n concentrations on any day of pregnancy examined in the intact and lutecto mized ewes. The prolactin responses (the maximum concentrations, the time t o maximum concentrations and the area under the response curves) were not d ifferent between the two groups in any stage of pregnancy examined. In the two groups, spontaneous parturition occurred at term with alive lambs. Ther e was no difference between the two groups in gestation length and lamb bir th weight. In experiment 2, we showed that plasma concentrations of prolact in fluctuated in a pulsatile manner during the last two-thirds of pregnancy . The mean prolactin concentrations, the frequency and the amplitude df pro lactin pulses were not significantly different between the intact and the l utectomized ewes in any stage of pregnancy examined. In conclusion, these e xperiments demonstrated that the ovine CL of pregnancy is not involved in t he control of prolactin secretion in the ewe. The stimulation of prolactin secretion by the RU 486 is probably due to its anti-progesterone action exe rted at the level of the receptor. The placental progesterone plays a centr al role in the control of prolactin secretion during the last two-thirds of pregnancy. (C) Academie des Sciences / Elsevier, Paris.