Prostaglandin induced parturition in the rabbit; quantitative aspects of the route and duration of administration

Prostaglandin F(2α) (PGF(2α)) will induce parturition when infused intraaortically into the cranial end of the abdominal aorta of the pregnant rabbit continuously from day 21 of gestation. At a dose of 2.25 μg/h, delivery occurs in 35.8±4.4 hr (mean±S.E.M.; n = 5). When the same dose of PGF(2α) is infused continuously via the inferior vena cava the latency to delivery is much more variable. Out of 6 animals infused intravenously, live litters were delivered in 2 animals 240 and 280 hr after starting the infusion. The remaining 4 rabbits delivered after 39, 69.5, 69.5 and 77.5 hr. The mean and S.E.M. for this group of 4 was 63.9±8.5 hr and that for all 6 intravenously infused animals was 129.25±42.01 hr. When the intravenous groups are compared with those receiving an intraarterial infusion of PGF(2α) the latency to delivery is significantly (P < 0.05) longer in each case. When the PGF(2α) infusion was made intraaortically for a short period (2 to 8 hr) and then stopped, the mean latency to delivery was 93.1±16.7 hr (n = 18). This is significantly (P < 0.01) longer than when the PGF(2α) was infused continuously. This group of 18 animals can be subdivided into those infused for 4 to 8 hr in which delivery occurred in 52.3±3.9 hr (n = 10) and those infused for 2 or 3 hr, in which delivery occurred in 144.1±28.7 hr (n = 8). The difference between these 2 groups is significant (P < 0.01). 2 rabbits infused intraaortically for 1.5 hr with 2.25 μg PGF(2α)/h delivered live litters at term. Preliminary results on 3 animals submitted to different infusion regimes suggest that delivery is accompanied by a fall in plasma progesterone concentrations from 18.9 to 2.6 ng/ml.