The authors address themselves to the extremely important question of whether the level of myometrial activity is determined by the rate of PG synthesis in the myometrium itself and to what extent it is influenced by PGs synthesized in the endometrium (decidua) or membranes. Furthermore, they consider whether the factors controlling PG synthesis in the endometrium and myometrium are the same and exert the same action on both tissues. They present evidence to indicate that the endometrium (decidua) and placenta are major sites of uterine PG synthesis and that PGs produced in the endometrium act directly on the myometrium (oxytocic effect) and in some species are released into the uterine vein to cause luteal regression (luteolytic effect). It seems that the PGs measured in the uterine vein are mainly of endometrial origin and do not provide information on the level of PG synthesis in the myometrium. Oxytocin can stimulate PG synthesis in the endometrium. This effect is determined in part by the number of oxytocin receptors, which in turn is determined by the concentration of estradiol-17β (and perhaps progesterone). The authors explore the question of the action of oxytocin on the myometrium, particularly to ask whether it is a direct action or is mediated by PG production. Finally, we present evidence favoring the view that the conceptus, and later the fetus, suppresses the level of PG synthetase activity in the endometrium (decidua) and membranes in early pregnancy, thus protecting the myometrium from PGs of endometrial origin and preserving the CL. In this way, the fetus would exert an inhibitory control on the level of myometrial activity, supplementing the inhibitory action of progesterone on the myometrium and oxytocin release from the pituitary. As term approaches, the level of suppression decreases, permitting increased PG synthetase activity in the decidua and membranes. The final surge in PG synthesis and release is probably influenced by a number of factors including an increase in the levels of oxytocin and estrogen and a decrease in the levels of progesterone. The dependence on any one of these individual events may vary in different species. The mechanism by which the conceptus (fetus) suppresses PG synthesis is not known but the authors present the hypothesis that it is probably hormonal and that the factor may be produced by the chorion.
