LUTEOLYTIC EFFECTS OF OESTRADIOL AFTER HYPOPHYSECTOMY OR PITUITARY STALK SECTION IN CYCLING SHEEP

Experiments were carried out with a view to determining the relative importance of the pituitary gland and the uterus for the luteolytic action of oestradiol in the cycling ewe.The influence of the pituitary was modified by stalk section or hypophysectomy and that of the uterus by hysterectomy. Oestradiol (0.5 mg twice daily) caused an increase in ghe life-span of the corpus luteum (CL) if treatment began on day 3 of the cycle, and a decrease when injections commenced on days 9 or 11.Pituitary stalk sections performed on days 3 or 11 caused little change in CL weight on day 14. However, in either case, oestradiol administration from day 11 resulted in a large fall in CL weight, indicating that intact pituitary connections to the hypothalamus were not essential for the luteolytic action of oestradiol. Luteal regression following hypophysectomy on day 11 was accelerated by oestradiol benzoate injected from day 11. On the other hand, hysterectomy on day 11 always resulted in maintenance of the CL to day 20, whether or not stalk section or oestradiol administration had taken place. This indicated that the presence of the uterus was essential for the luteolytic effects of oestradiol.These experiments show that the luteolytic properties of oestradiol reflect mainly a direct action of this hormone on the uterus.