Effects of growth hormone releasing hormone on rat ovarian steroidogenesis

During the last decade, it has been shown that each part of the somatotrophic axis can influence granulosa cell function. Growth hormone releasing hormone (GHRH) may be effective through the release of hypophyseal growth hormone (GH) and the subsequent increase of insulin-like growth factors (IGF). There is also some evidence that GHRH could act directly on ovarian function. The aim of this study was to determine the mechanism through which GHRH affects granulosa cell steroidogenesis in the ovary. Granulosa cells were obtained from immature, oestrogen- treated rats supplemented with or without follicle stimulating hormone (FSH) in vivo and were cultured for 48 h to evaluate steroid production. GHRH was administered either in vivo at the same time as FSH, or in vitro in the presence or absence of testosterone and FSH. Our results show that co-treatment with GHRH and FSH in vivo induced significant increases in plasma IGF-I concentrations and steroid production by cultured granulosa cells. The addition of GHRH to culture medium did not significantly alter steroid production by either non-differentiated (no FSH in vivo) or differentiated (FSH in vivo granulosa cells. In contrast, treatment in vitro with IGF-I significantly increased steroidogenesis in both cases. Our results suggest that any physiologically significant effect of GHRH on ovarian function is probably to be exerted via activation of the somatotrophic axis and the subsequent amplification of ovarian FSH responsiveness by IGF-I.