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.
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