ABSTRACT
Estrogens and progesterone, respectively, act through receptormediated processes to stimulate and suppress the synthesis of mRNA essential for production of cyclooxygenase in the endometrium (Part C). The local decrease in the effectiveness of progesterone associated with continued action of estrogen also increases myometrial oxytocin receptor synthesis as well as prostaglandin (PG) levels in uterine fluid. In addition, PG production is increased by oxytocin, which also acts via receptor binding to the endometrium. Prostaglandins and oxytocin then act directly on uterine musculature to simulate contractions. Activation of contractile processes requires an increase in myometrial Ca2+, which is achieved by decreased binding of Ca2+ to subcellular membranes and by influx into the cell.
