Pituitary and Uterine Factors Controlling Regression of Corpora Lutea in Intact and Hypophysectomized Rabbits

In the pseudopregnant rabbit, the corpora lutea regress after approximately 17 days and the animal returns to estrus by 21 days. Progesterone levels in ovarian vein blood drop to trace amounts by Day 21 and reduction in luteal weight, cytological degeneration and accumulation of free and esterified cholesterol (mg/g) in the luteal tissue accompany this decrease in progesterone secretion. Concomitantly, secretion of 20α-hydroxypregn-4-en-3-one mounts rapidly, reflecting an accelerated rate of discharge of pituitary luteinizing hormone. Estradiol benzoate (EB), in doses of 2 μng twice daily sc, fails to prolong luteal function through Day 21 in intact rabbits unless the endometrium has been traumatized by threading the uterus on Day 4 or Day 8 of PSP. However, in hypophysectomized rabbits this 2 ng dose of EB is adequate to sustain luteal function even in the absence of uterine trauma. Exogenous LH (100 μg sc twice daily plus 100 μg iv once daily) administered on Days 16 through 20 reverses this luteotrophic response. It is suggested that during pregnancy or following uterine trauma the endometrium is altered so as to “spare” estrogen for luteal maintenance, whereas in nonfertile cycles diminishing progesterone secretion promotes LH discharge, which hastens luteolysis and permits a rapid return to estrus. (Endocrinology83: 291, 1968)