Indeed, levels of PGF2a remain elevated during early pregnancy or in cultured conceptus, just like the expression of COX-2, and luteolysis is nonetheless prevented. It is well known that the pul-satility in PGF2a production, rather than the absolute quantity, is the major factor that triggers luteolysis. Because this pulsatility is itself regulated by pulses of OT and that OTRs are undetectable during early pregnancy, high quantities of endometrial COX-2 and PGF2a do not seem to be a critical issue at this time. On the other hand, PGE2 production is also stimulated during early pregnancy and PGE2 secretion in bovine conceptus from d10P to d19P is increased in vitro.
The high COX-2 staining in the endometrium and the conceptus between d18P and d24P supports an increase in PGs during early pregnancy. Enzymes downstream of COX-2, like PGE synthase and PGF synthase, could also be regulated to favor the synthesis of one type of PG, because there is a correlation between the up-regulated COX-2 and the increase in PGE2, not PGF2a, in the endometrium of the ewe. Indeed, COX-2 and PGE synthase exhibit similar patterns of expression during the cycle in the bovine endometrium, and there is a correlation between the expression of COX-2 and PGE synthase and the production of PGE2 in vitro.