Endocrine Abstracts

The ovarian enzyme 20alpha-hydroxy steroid dehydrogenase (20HSD) plays a central role in the termination of pregnancy in rodents. The enzyme which is expressed mainly in the corpus luteum, catalyzes the reduction of progesterone to 20alpha hydropregn-4en-3-one which is biologically inactive and cannot maintain pregnancy. During pregnancy ovarian 20HSD expression is inhibited, activity peaked in the CL of pregnancy about 24 hrs before parturition at the same time that progesterone blood concentrations decreased. 20HSD is also highly expressed in the zona reticularis of the adrenal cortex gland in female mice. The specific activity of the adrenal enzyme is higher than that found in the ovary. Since the expression of the adrenal enzyme may interfere with the homeostasis of progesterone blood concentration during pregnancy, we studied the expression and regulation of the adrenal 20HSD during this period. We found the adrenal 20HSD activity decreased during pregnancy in parallel to the ovarian enzyme. However, in contrast to the ovarian enzyme that was induced and expressed 24 hrs before parturition the adrenal enzyme activity remains low for 3 weeks post partum. PGF2alpha (or its synthetic analog cloprostenol) which induces the expression of the ovarian 20aHSD during pregnancy had no effect on the expression of the adrenal enzyme. The adrenal activity remained low and did not depend on the presence or absence of suckling pups. Prolactin that down regulates ovarian 20aHSD expression had no effect on the adrenal enzyme. We conclude that the mechanism that regulates 20HSD expression differs from that that regulates its expression in the ovary, this mechanism is still unknown. The adrenal 20HSD expression is down regulated like the ovarian one probably to maintain the high progesterone blood concentration.