Effects of human chorionic gonadotropin and cyclic adenosine monophosphate on progesterone secretion by the ovine...

Progesterone production by the ovine placenta was investigated between d 80 and 115 of gestation. Serum progesterone concentrations in ewes ovariectomized (ovx) on d 75 of gestation were measured throughout the remainder of gestation, and after the ewes were injected with human chorionic gonadotropin (hCG) or saline on d 80 or 115. In addition, cotyledonary tissue was collected from intact ewes sacrificed on d 80 or 115 and progesterone accumulation was determined during 2 h incubation with or without pregnenolone supplementation and in the presence or absence of hCG or dibutyryl cyclic adenosine monophosphate (dbcAMP). Serum concentrations of progesterone in ovx ewes increased from 3.5 +/- .4 ng/ml on d 80 to 16.4 +/- 2.1 ng/ml on d 115 (P less than .05). That increase was coincident with a 1.5-to 4.5-fold increase in progesterone output by placental tissue in vitro. Addition of pregnenolone enhanced progesterone accumulation in all tissue incubations. Addition of dbcAMP increased progesterone accumulation in the incubation medium only when supplemented with pregnenolone. Human chorionic gonadotropin did not increase placental progesterone secretion in vivo or in vitro. The results confirm the enhanced secretion of progesterone by the ovine placenta between d 80 and 115 of gestation and indicate that the increase results primarily from increased secretory capability per unit of placenta. The tropic mechanism controlling the placental secretion of progesterone remains unclear, but the mechanism may involve elevation of intracellular cAMP.