Effects of the uterus and mid-pregnancy conceptus on luteal lifespan of the rat.

Hysterectomy on or after Day 10 resulted in a quick return of pseudopregnant rats to oestrus but the interval to oestrus in pregnant rats was significantly longer (P less than 0.01), indicating that the gravid uterine horn interfered with uterine-induced luteolysis by Day 10. When pseudopregnant rats were injected with extracts of conceptuses or pregnant rat serum (PRS), samples collected on Days 10 or 12 of pregnancy delayed the return to oestrus (P less than 0.01 for conceptus extracts, P less than 0.05 for PRS) and serum progesterone concentrations on Day 13 of pseudopregnancy were higher (P less than 0.01) than in controls. Bioassays utilizing mammary development or inhibition of oestrus in virgin cyclic rats as endpoints failed to detect lactogenic activity in Day 10 conceptus extracts or PRS, and radioreceptor assays using bovine prolactin (NIH-B5) as a standard showed that Day 10 extracts had only about 2--3% of the lactogenic activity found in Day 12 extracts. Injections of dilutions of Day 12 extracts with 2 or 4 times the lactogenic activity as Day 10 extracts did not extend pseudopregnancy. These results suggest that by Day 10 the conceptus produces a factor capable of interfering with luteolysis and that this factor is not placental lactogen.