The acute effect of placentectomy and hysterectomy on peripheral plasma progesterone levels and ovarian progesterone secretion in the pregnant rat.

The role of placental luteotrophins in modulating plasma progesterone concentrations and ovarian progesterone secretion was examined in 16-day pregnant rats. In an initial experiment rats were placentectomized and their plasma progesterone concentrations monitored for 24 h; the rats were conscious within 30 min of placentectomy. Relative to control values, progesterone concentrations fell significantly within 0.5 h. A venous outflow technique was then used to monitor rates of progesterone secretion from ovaries of hysterectomized and control rats maintained under anaesthesia. Hysterectomy had no apparent effect on either progesterone secretion or plasma progesterone concentrations for at least 2 h. A final experiment was carried out to compare the effects of hysterectomy on plasma progesterone concentrations in conscious rats with those of placentectomized rats of the first experiment. Progesterone concentrations did not change significantly in hysterectomized rats for 4 h but fell to very low values by 24 h. These results suggest that placental luteotrophins do not have an acute, direct role in the control of plasma progesterone levels but are needed to maintain progesterone secretion in the longer term and possibly inhibit uterine luteolysin release.