Relationship between timing of ovariectomy and maintenance of pregnancy in the guinea-pig.

The effect of ovariectomy (OVX) and OVX + progesterone (P) treatment on the regulatory profile and uterine function of the guinea-pig are described. Before day 23 of gestation in the intact pregnant guinea-pig, the placental contribution to P-content is small in comparison with the increasing ovarian contribution. After day 30, the ovarian P-content starts to decrease and the placental P-content exceeds the ovarian contribution, indicating the "luteo-placental shift" (LPS) in P-biosynthesis. Thus, when 14 guinea-pigs were ovariectomized around day 21 of gestation all started aborting within 53 hours of OVX. A gradual increase in intrauterine pressure (IUP), a decrease in P-levels in heart and uterine vein plasma and in the uterine and placental tissues and an increase in the levels of PGF in uterine vein plasma and uterine tissue were observed in these animals. However, if the OVX was delayed until after day 30 of gestation, to examine the biological consequences of advanced LPS in P-biosynthesis, there was no increase in IUP and the animals did not abort in the next 5 days. Furthermore, P-therapy following OVX in the day 21 pregnant guinea-pigs prevented the increase in IUP and the animals did not abort. These observations establish for the guinea-pig a correlation between the success in pregnancy maintenance and the degree of the LPS in P-biosynthesis. These studies therefore emphasize the indispensable role of progesterone in pregnancy maintenance.