Studies on mechanism of action of gonadotrophins of steroidogenesis in the ovary of the immature rat

Abstract

Experiments were designed to examine the effects of treating immature rats with pregnant mares' serum gonadotrophin (PMSG) on several important parameters of ovarian development. PMSG caused an increase in ovarian steroid secretion both in vivo and in vitro. Plasma oestradiol levels in the PMSG-primed rats reached a maximum in the animals injected 48 h previously (analogues to pro-oestrus) and declined to a minimum in the animals injected with PMSG 72 h previously. Testosterone levels, like those of oestradiol reached maximum in the animals injected with PMSG 48 h previously but unlike oestradiol did not decline abruptly 24 h later. Progesterone levels were maximal in the animals injected with PMSG 72 h previously (i.e. after the expected LH/FSH surge and after ovulation had occurred, as indicated by the presence of ova in the oviducts of the animals). Human Choronic Gonadotrophin (HCG) added to incubates of the ovaries of animals injected with PMSG 48 h previously caused a marked increase in oestradiol and testosterone secretion whereas the post ovulatory ovaries secreted predominantly progesterone. The oestradiol secreted under these conditions might have been principally due to an enhanced synthesis of testosterone and the production of increased amounts of substrate for aromatization. Evidence is produced to support this hypothesis. The presence of cycloheximide suppressed the HCG-induced steroid secretion, from which it was inferred that protein synthesis is required for the HCG-induced steroidogenesis. In addition, experiments were carried out to investigate the effect of 5a-reduced androgens on oestradiol secretion from granulosa cells isolated from pre-ovulatory follicles of PMSG primed immature rats. Our results demonstrate that the major 5a-reduced androgens found in the ovary of immature rat do suppress the aromatization of exogenous testosterone. These results are discussed in the context of the control of follicular development and of the onset of puberty in the immature rat.

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