However, prolactin levels were correlated with prolactin receptor expression and were of sufficient magnitude to reactivate the CL, as indicated by the increase in progesterone secretion on March 27 and by the termination of embryonic diapause. The abundance both of prolactin receptor mRNA and of the number of prolactin binding sites in the ovary increased substantially after March 29, and highest levels were during implantation and early-postimplantation gestation, the time when progesterone production is maximal. This is in accordance with findings in the ovary of the pregnant mouse, where the long form of the prolactin receptor is most abundant during the luteal phase. The present result differs from findings in the rat, where the level of ovarian expression of both short and long forms of prolactin receptor mRNA was constant through most of gestation, declining only as parturition approached. proventil inhaler
Treatment of animals with bromocriptine did not affect basal levels of prolactin receptor mRNA in whole ovaries; however, it prevented the preimplantation increase in serum prolactin and prolactin receptor mRNA and thus abrogated the activation of the mink CL. This effect is consistent with findings from earlier studies showing that transient bromocriptine treatment resulted in low progesterone levels and extended embryonic diapause. The effects of disruption of prolactin secretion and its receptor are in keeping with the results of transgenic deletion of the prolactin receptor in the mouse, which has multiple negative consequences on reproduction, including luteal defects and complete failure of embryo implantation. The current investigation reveals that prolactin receptor mRNA and receptor binding are coupled and associated with serum prolactin levels, not only during normal gestation, but also in animals treated with dopamine agonists to block prolactin secretion. These correlations provide evidence in support of the conclusion that prolactin up-regulates its own receptor in the mink ovary, as has been shown in a variety of tissues and species. This is consistent with recent findings in which administration of prolactin to hy-pophysectomized rats, and incubation of luteinized rat granulosa cells in vitro with prolactin, increased the abundance of the prolactin receptor mRNA.