Change in responsiveness of rabbit corpus luteum to prostaglandin F-2\g=a\during pregnancy and pseudopregnancy

Summary. Previous studies have suggested that prostaglandin F-2\g=a\(PGF-2\g=a\)may have a role in luteolysis in rabbits. Rabbits (4\p=n-\6/group) were given a single injection of saline, or 100, 500 or 2500 \g=m\gPGF-2\g=a\(i.m.) on Day 7, 9, 12 or 15 of pregnancy or pseudopregnancy. Daily blood samples were taken via the marginal ear vein before and for 3 days after the PGF-2\g=a\injection. Concentrations of serum progesterone were determined by radioimmunoassay in pseudopregnant rabbits. There were no signifi-cant differences between PGF-2\g=a\-treated and control rabbits on Days 7 or 9. On Day 12 of pseudopregnancy, progesterone concentration was significantly (P < 0\m=.\05)lower in treated than in control rabbits, the effect being dose dependent. On Day 15 of pseudopregnancy, it was not possible to distinguish between controls and treated groups because luteolysis occurred in all rabbits. In contrast, on Days 7 and 9 of pregnancy, the concentration of progesterone in treated groups was lower than in the control groups (P < 0.05), the effect being dose dependent. This difference was maintained throughout the sampling period and resulted in termination of pregnancy. By Day 12 of pregnancy, the response to PGF-2\g=a\ was transient, with a significant decline in progesterone for only 2 days, followed by a return to control concentrations and normal delivery of litters. On Day 15 of preg-nancy, no treatment with PGF-2\g=a \ significantly altered progesterone concentration. Our results indicate that (i) there is a change in responsiveness to PGF-2\g=a \ over time during pregnancy and pseudopregnancy and (ii) there are differences between pregnancy and pseudopregnancy in the pattern of responsiveness