DOES THE OVINE PLACENTA SECRETE ACTH UNDER NORMOXIC OR HYPOXIC CONDITIONS

In the sheep, maternal plasma cortisol is increased in late pregnancy, and fetal plasma cortisol and adrenocorticotropic hormone (ACTH) rise precipitously in late gestation. In many species, the placenta contains ACTH. These experiments were designed to test whether the ovine placenta contains ACTH and whether there is net secretion of ACTH by the uteroplacental unit into either the maternal or fetal circulation. Pregnant ewes and their fetuses were prepared with maternal and fetal arterial and uterine and umbilical venous catheters. Arterial and venous samples were taken from both sides of the placenta before and during hypoxia induced by the ewe breathing 9-11% O-2, and arteriovenous (a-v) differences in ACTH, Po-2, PCO2, and progesterone were analyzed. A positive a-v difference in Po-2 (48.2 +/- 3.4 mmHg) and negative a-v differences in PCO2 and progesterone (-3.5 +/- 0.7 mmHg and -25 +/- 5 ng/ml, respectively) were found across the placenta in the ewe, and a positive a-v difference in PCO2 (4.8 +/- 0.9 mmHg) and negative a-v differences in Po-2 and progesterone (-8.1 +/- 1.5 mmHg and -13 +/- 3 ng/ml, respectively) were found across the placenta in the fetus, indicating that the umbilical and uterine venous catheters were properly placed. Hypoxia decreased fetal and maternal arterial Po-2 from 22.8 +/- 1.3 to 13.8 +/- 0.7 and from 98.8 +/- 3.3 to 37.0 +/- 2.6 mmHg, respectively, and increased fetal and maternal arterial ACTH immunoreactivity from 95 +/- 60 to 2,676 +/- 795 and from 149 +/- 21 to 275 +/- 88 pg/ml, respectively. There was, however, no a-v difference in immunoreactive ACTH concentration across the placenta in either the mothers or the fetuses under either resting or hypoxemic conditions (ewe: -5 +/- 3 pg/ml at 0 min, 82 +/- 114 pg/ml at 30 min; fetus: 20 +/- 21 pg/ml at 0 min, 473 +/- 282 pg/ml at 30 min). The results indicate that the ovine placenta is not a significant source of plasma ACTH in either the pregnant ewe or the fetus in late gestation under either basal or hypoxemic conditions.