Throughout a period of pseudopregnancy the peripheral blood levels of progesterone, oestradiol-17 beta, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), as well as the size-distribution of ovarian antral follicles were estimated in the rat. The progesterone concentrations, as measured by a competitive protein-binding technique, exceeded metoestrous values (25 ng/ml plasma) from day 3 of pseudopregnancy onwards. The highest levels were found on days 6 and 8 (91 ng/ml). From day 8 onwards the levels decreased gradually but were still above metoestrous values on the day of pro-oestrus after pseudopregnancy. Concentrations of oestradiol-17 beta, as measured by radioimmunoassay, were within the range of those at metoestrus (about 5 pg/ml plasma) until day 10. Thereafter levels increased to a value of 57 pg/ml. Concentrations of FSH, measured by radioimmunoassay, were within the range of metoestrous values until day 10 (about 100 ngNIAMD-rat-FSH RP-1/ml serum), but declined to a level of 33 ng/ml on day 12. Concentrations of LH, measured by radioimmunoassay, were generally within the wide range of metoestrous values (9-60 ng NIAMD-rat-LH RP-1/ml serum), but concentrations found on days 4, 8 and 10 were significantly lower than those found on preceding or subsequent days. Histological determination of the number of follicles present in various volume-classes, showed an increase in antral follicles on days 1 and 2, comparable to the increase observed during metoestrus and dioestrus 1 of the normal cycle. There was no change in the follicles between days 3 and 10 and they resembled those of early dioestrus. Preovulatory growth had occurred by day 12. Injection of human chorionic gonadotrophin (HCG) on days 2, 4 or 6 showed that ovulation could be induced only in some of the larger follicles. On the basis of these results it is suggested that during pseudopregnancy the high progesterone levels present result in a decreased plasma LH level which is insufficient to cause full maturation of the follicles and to stimulate oestrogen secretion to the levels required for induction of an ovulatory surge of LH release.
PIP: Peripheral levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol-17beta, and progesterone, and follicular growth in the pseudopregnant rat were studied. Progesterone, measured by competitive protein-binding, exceeded metestrus values (25 ng/ml plasma) from Day 3 of pseudopregnancy. Days 6 and 8 revealed the highest values (91 ng/ml). Estradiol-17beta, measured by radioimmunoassay, was within the metestrus range (5 pg/ml) until Day 10, when the levels began increasing to a value of 57 pg/ml. FSH was in the metestrus range (about 100 ng/ml) until Day 10 but then declined to 33 ng/ml on Day 12. LH levels were generally within the range of metestrus levels (9-60 ng/ml) but were markedly lower on Days 4, 8, and 10 than on preceeding or subsequent days. The number of follicles present in various volume-classes, showed an increase in antral follicles on Days 1 and 2, comparable to the increase observed during metestrus and diestrus 1 of the normal cycle. Preovulatory growth occurred by Day 12. The injection of 15 IU of human chorionic gonadotropin on Days 2, 4, or 5 showed that ovulation could be induced only in some of the larger follicles. It is suggested that during pseudopregnancy the high progesterone levels present result in a decreased LH level sufficient to cause full maturation of the follicles and to stimulate estrogen secretion to the levels required for induction of an ovulatory surge of LH release.