Regulation of mouse placental lactogen secretion by factors secreted from the pituitary in vitro.

The effect of factors secreted from the pituitary on mouse placental lactogen I (mPL-I) and mPL-II secretion in vitro was examined. Co-culture of mouse placental cells from day 7 of pregnancy with the pituitary cells of the mother significantly stimulated mPL-I secretion but did not regulate mPL-II secretion. The effect on mPL-I secretion was dependent on the number of pituitary cells. The conditioned medium of pituitary cells also significantly stimulated mPL-I secretion but did not regulate mPL-II secretion. The stimulatory effect of mPL-I secretion was dependent on the volume of the conditioned medium. The number of cells containing mPL-I assessed by immunocytochemistry was increased by the co-culture in a cell number-dependent manner. Northern blot analysis for mPL-I indicated that treatment of placental cells with the pituitary-conditioned medium results in an increase of mPL-I gene expression. These findings suggest that factors secreted from the pituitary directly regulate mPL-I secretion, but not mPL-II secretion, before midpregnancy in vivo.