The Role of the Ovary for the Maintenance of Pregnancy in Cats

Plasma progesterone (P(4)) concentrations are maintained in pregnant cats until parturition, but become low in pseudopregnant cats 40-45 days after infertile mating. This difference in P(4) concentrations is considered to be due to P(4) secretion by the placenta of pregnant cats. Therefore, to clarify these points, we performed ovariectomy (OVX) at various stages of pregnancy, examined the pregnancy status and measured LH and P(4) concentrations in peripheral, ovarian and uterine venous blood. After OVX, abortion occurred in 100% (5/5), 80% (4/5), 40% (2/5) and 60% (3/5) of Groups I (Day 35), II (Day 40), III (Day 45) and IV (Day 50) cats, respectively. In the remaining cats, normal delivery took place on days 63-69 [mean, 66.1 +/- 1.1 (SE)] of pregnancy. The time to abortion after OVX was 4-8 (mean, 5.6 +/- 0.8), 3-17 (mean, 8.0 +/- 3.6), 10 and 11, and 2-4 (mean, 3.0 +/- 0.7) days in Groups I, II, III and IV, respectively. The plasma P(4) concentrations were 1-2 ng/ml in all groups on the day after OVX, decreasing to less than 1 ng/ml from the 2nd day onwards. The concentrations of P(4) in ovarian venous blood at the time of OVX decreased with the stage of pregnancy, but were clearly higher than those in peripheral blood. The plasma P(4) concentrations in uterine venous blood were similar to those in peripheral blood. These results suggest that peripheral P(4) in pregnant cats is the result of P(4) secretion secreted only by the ovarian corpus luteum, not by the placenta, but indicate that either P(4) is not essential for the maintenance of pregnancy in cats from day 40-45 of pregnancy onwards, or that the placenta provides a local source of P(4) that does not appear in measurable amounts in the peripheral circulation.