Oestrogens regulate divergent effects of prolactin in the ovary

Prolactin exerts well established stimulatory effects both in vitro and in vivo in the rat ovary¹. However, in the ovary of higher mammals, prolactin action is not well characterised, and may differ from that observed in rodents². For example, in human clinical conditions of physiological or pathological prolactin excess, ovarian function is depressed³, and in isolated, human granulosa cells in vitro, prolactin seems to inhibit progesterone production⁴. To examine direct prolactin effects in the higher mammalian ovary, we have used an in vitro porcine granulosa-cell model⁵. In this system, prolactin action is bipotential, depending critically on the degree of granulosa-cell differentiation attained in vivo. Prolactin suppresses steroid production by cultured granulosa cells isolated from small (1-2 mm), immature follicles, but stimulates progesterone secretion by granulosa cells collected from large (>6 mm) mature follicles⁵. The present studies show that oestrogens may play an important part in regulating these divergent actions of prolactin in the ovary.