Secretion of estradiol-17β by porcine endometrium and myometrium during early pregnancy and luteolysis

Past studies of the source of estrogens secreted during maternal recognition of pregnancy in pigs have focused on embryonic rather than uterine origin of these steroids. The present study documents: (1) the expression of the gene CYP 17, encoding cytochrome P450 17α-hydroxylase/C17–20 lyase and (2) the synthesis and secretion of estradiol-17 β (E2) in endometrial and myometrial tissues in gilts. The expression of CYP 17 gene was shown in porcine endometrium and myometrium. Basal endometrial secretion of E2 was higher in pregnant gilts than in cyclic gilts (days 14–16). The myometrium secreted more E2 during the expected time of luteolysis compared to early pregnancy. Basal secretion of E2 during pregnancy was higher from the endometrium than from the myometrium. Conversely, during luteolysis E2 secretion was higher from the myometrium and lower from the endometrium. In pregnant and cyclic gilts (days 14–16), progesterone (P4, 10−5 M) in vitro significantly increased E2 secretion regardless of reproductive status. Oxytocin (OT, 10−7 M) had no influence on E2 secretion and did not change the stimulatory effect of P4 in both tissues examined. In conclusions: (1) the CYP 17 gene transcript is present in porcine endometrium and myometrium; (2) porcine endometrium and myometrium release E2 in vitro; (3) the endometrium releases more E2 than the myometrium during early pregnancy; (4) the myometrium releases E2 mainly during luteolysis; (5) the endometrium and myometrium can increase E2 release in vitro if substrate (P4) is provided during early pregnancy and luteolysis. These data suggest active estrogen production by the myometrium and endometrium as an alternative source for this signal for recognition of pregnancy in the pig.