The effect of hypophysectomy on serum placental lactogen and progesterone in the mouse

Hypophysectomy of pregnant mice on days 10, 12, and 16 of gestation resulted in a large and rapid (12-24 h postoperatively) increase in serum placental lactogen (mPL) concentrations, as measured by homologous RIA. Serum progesterone (P4) concentrations decreased compared to those in intact and sham-operated controls. Significantly, the increase in serum mPL in mice hypophysectomized on days 10 and 12 preceded any decrease in serum P4 concentrations by at least 24 h, suggesting that two indirectly related mechanisms may be involved in the control of the serum concentrations of these hormones. The weight of the uterus, including the feto-placental units, placental weight, and placental content of mPL did not differ between intact, sham-operated, and hypophysectomized groups. The gestational profiles of serum GH and PRL concentrations were similar in the sham-operated and intact controls. Although serum P4 concentrations decreased after hypophysectomy on days 10 and 12 of gestation, the maintenance of the pregnancy was not affected; thus, the reduced concentrations of serum P4 (approximately 50% of control by day 16) were adequate to allow the pregnancy to continue. Hypophysectomy on day 16 of gestation resulted in spontaneous abortion 48 h postoperatively in all but one animal. We conclude that the absence of some pituitary factor(s) results in a rapid increase in serum mPL, while a concomitant, albeit gradual, decrease in serum P4 begins 48 h postoperatively. Therefore, when the pituitary no longer contributes to the luteotropic complex of the midpregnant mouse, an increase in serum mPL alone cannot maintain serum P4 concentrations at control values.