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.
De Jong & Sharpe (1976) recently observed that bovine follicular fluid from which steroids had been removed reduced peripheral levels of FSH but not of LH when injected into newly castrated male rats. The effect was ascribed to a factor resembling testicular inhibin. In the present report, observations were extended to the effects of this inhibin-like factor on FSH and LH levels in female rats. Attention has also been paid to the question of whether the factor is present in species other than the cow and whether the factor is present in antral follicles of all sizes.
High concentrations of FSH occur in the serum of rats aged 10 to 20 days accompanied, around Day 15, by elevated, but variable concentrations of LH (Meijs-Roelofs, Uilenbroek, Osman & Welschen, 1973; Meijs-Roelofs, Uilenbroek, de Jong & Welschen, 1973). Rapid follicular growth occurs in the mouse ovary within the 1st week after birth (Pedersen, 1969), suggesting that serum FSH concentrations may be high during the infantile period in this species also. Changes in the relative abundance of follicles of different sizes between Days 16 and 20 of age suggest that LH is subsequently of increasing importance for follicular growth (Kent, 1972). The present work was undertaken to determine what changes in gonadotrophin levels occur during prepubertal development in the mouse.
Pituitary glands from immature female and male rats aged between 5 and 30 days were incubated in vitro and the effect of LH releasing hormone (RH) on the release of LH and FSH was studied. Pituitary gonadotrophin contents were also measured. Gonadotrophin release showed changes with age as well as sex differences: after LH-RH stimulation the female pattern of release of LH and FSH (expressed per mg pituitary tissue) showed a peak at day 15; the male pattern of LH release was characterized by a steady increase with age, whereas FSH release stayed more or less constant from day 10 onwards. In both sexes the LH:FSH ratio increased with age, both in pituitary gonadotrophin content and in the mixture of gonadotrophins released. It is discussed, that the prepubertal development of pituitary gonadotrophic function might be determined on the one hand by rather autonomous growth processes (more or less similar in female and male hypophyses) and on the other hand by modulating influences of sex steroid hormones, which are different in female and male animals.
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