Summary. The outputs of prostaglandin (PG) F-2α and PGE-2, but not of 6-oxo-PGF-1α, from the guinea-pig uterus were significantly lower on Days 7 and 15 of pregnancy than on the corresponding days of the cycle. Uterine PGF-2α output increased 28-fold between Days 7 and 15 of the cycle but only 4- to 5-fold between these same days of pregnancy. Uterine PGE-2 and 6-oxo-PGF-1α outputs increased 2- to 3-fold between Days 7 and 15 of the cycle and of pregnancy. Endometrial PGF-2α synthesizing capacity was 60–70% lower on Days 7 and 15 of pregnancy than on the corresponding days of the cycle, although it increased 2-fold and 2·5-fold between these days of pregnancy and of the cycle, respectively. Endometrial PGE-2 and 6-oxo-PGF-1α synthesizing capacities showed no significant variation amongst Days 7 and 15 of the cycle and of pregnancy, except that endometrial PGE-2 synthesizing capacity was lower on Day 7 of the cycle.
Oestradiol treatment (10 μg s.c. daily from Days 10 to 14 of pregnancy) did not affect plasma progesterone concentrations, uterine 6-oxo-PGF-1α output, and endometrial PGF-2α, PGE-2 and 6-oxo-PGF-1α synthesizing capacities in 9/12 guinea-pigs when examined on Day 15. Uterine PGF-2α and PGE-2 outputs increased 3- and 1·5-fold, respectively, in these guinea-pigs, but were still much lower than the outputs from the Day-15 non-pregnant uterus. The pregnancies appeared unaffected in these oestradiol-treated guinea-pigs. In the other 3 oestradiol-treated animals, uterine PGF-2α output was 20- to 30-fold higher than in untreated, pregnant guinea-pigs on Day 15, and 2- to 3-fold higher than in Day-15 non-pregnant guinea-pigs. Uterine PGE-2 and 6-oxo-PGF-1α outputs also tended to be higher in these treated guinea-pigs. In these 3 guinea-pigs, endometrial PGF-2α, PGE-2 and 6-oxe-PGF-1α synthesizing capacities were 4·0-, 3·4- and 2·5-fold higher, respectively, than in untreated, pregnant guinea-pigs on Day 15, and tended to be higher than in Day-15 non-pregnant guinea-pigs. Plasma progesterone concentrations were much lower in these 3 animals than in the other 9 treated with oestradiol, and also much lower than in untreated, pregnant guinea-pigs on Day 15. However, only 1 of these 3 guinea-pigs was in the process of aborting, and this had the lowest progesterone concentration (0·5 ng/ml).
In the pregnant guinea-pig a large increase in output of PGF-2α from the uterus after Day 11 does not occur, uterine PGE-2 does not appear to be involved in luteal maintenance and general uterine PG production is not increased at the time of implantation.