LUTEOTROPIC ACTIONS OF PLACENTAL LACTOGENS AT MIDPREGNANCY IN THE MOUSE

In this study the luteohopic activity of mouse placental lactogen I (m PL-I) at midpregnancy was assessed using in vivo and in vitro methodol ogies. Ovaries from 10-day pregnant mice were enzymatically dispersed and plated on fibronectin-coated wells in serum-free medium. The perce ntage of ovarian cells that stained for the presence of 3 beta-hydroxy steroid dehydrogenase activity was 24.4 +/- 2.7% at the time of platin g and remained constant (26.1 +/- 5.0%) after a 20-h attachment period . Two types of 3 beta-hydroxysteroid dehydrogenase-staining cells, wit h distinct differences in size and morphology, were present in the cul ture. Large luteal cells (26-45 mu m) were characterized by a small ro und nucleus and spherical shape with abundant cytoplasm. In contrast, small luteal cells (<20 mu m) were stellate, with little cytoplasm and a large oval nucleus. Basal progesterone secretion was maintained wit hout a change in cellular DNA content and cell number for 168 h of cul ture. Treatment of ovarian cells with mPL-I (0.05-10 mu g/ml) caused a dose-dependent increase in the progesterone concentration in the medi um. The magnitude and time course of mPL-I-stimulated progesterone acc umulation in culture were dependent on the time after plating that mPL -I treatment was initiated. The effects of mPL-II and mouse PRL (mPRL) on progesterone production were similar to those of mPL-I. The abilit y of sera from 10-, 14-, and 17-day pregnant mice to maintain progeste rone production in bromocryptine-treated hysterectomized mice was also examined. Mice were hysterectomized on day 9 of pregnancy, and serum progesterone, mPL-I, mPL-II, and mPRL concentrations were measured 72 h later. Twice daily injections of 0.5 ml day 10 pregnancy serum maint ained the circulating progesterone concentration at values not differe nt from those present at the time of hysterectomy. In contrast, serum progesterone concentrations were not maintained in mice treated with s erum of 14- or 17-day pregnant mice or with saline. Depletion of mPL-I from day 10 pregnancy serum by affinity chromatography on an anti-mPL -I column removed all luteotropic activity, as determined by the inabi lity of this modified serum to maintain the serum progesterone concent ration in bromocryptine-treated hysterectomized mice. A similar pool o f day 10 pregnancy serum chromatographed on a nonspecific IgG control column did maintain progesterone production, but at somewhat lower con centrations than those present at the time of surgery. These studies o ffer direct evidence that mPL-I and mPL-II are luteotropic and support progesterone production at midpregnancy in the mouse.