Prostaglandin Release by Zygotes and Endometria of Pregnant Rabbits 1

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Abstract
When 4-day rabbit zygotes were incubated for 1 h at 37.degree. C in vitro, very little prostaglandin (PG) was released into the medium, and the concentration of PC in the zygotes after incubation was also low. The release of PGE and PGF into the medium, and their concentration in the zygotes after incubation, increased sharply on days 6 and 7 of pregnancy, reaching, by day 7, values close to 200 ng of each PG released in 1 h per mg of protein. By constrast, endometrial samples on days 4 and 5 of pregnancy released more PGF and less PGE than the zygotes of the same ages on a per mg of protein basis, and on days 6 and 7, less of both PG. Endometrial concentrations of PG after incubation, except for PGF on Day 4, were always lower than values for zygotes. Endometrial concentrations of PG on day 6 were lower before than after incubation. Although there was a slight upward trend in PG release by endometrial samples with increasing length of pregnancy, the changes were minimal and, in the case of PGE, none of the mean values exceeded 1 ng per mg of protein. In 7-day blastocysts, high levels of both PGF and PGE were found in the blastocoelic fluid, and these did not change during the 1-h incubation. The release of PGF and PGE during in vitro incubation of ruptured and washed day 6 blastocysts was stimulated by arachidonic acid, and that of PGF, but not PGE, inhibited by indomethacin. The release of PGE, but of PGF, from day 6 blastocysts was inhibited by low temperature, and the same conditions inhibited release of both PGF and PGE from endometrial cell suspensions. Both blastocysts and endometria have the capability to synthesize PG, the blastocysts being particularly active in this regard on days 6 and 7 of pregnancy. In vivo, day 6 and 7 blastocysts release large quantities of PG which trigger some of the local endometrial changes associated with pregnancy.