Progesterone metabolism in bovine endometrial cells and the effect of metabolites on the responsiveness of the cells to OT-stimulation of PGF2α

Oxytocin receptor (OTR) expression is suppressed by progesterone (P4) during the luteal phase of the estrous cycle and then it increases at the time of luteolysis, but its regulation is still not completely understood. The objective of this work was to characterize P4 metabolism by endometrial cells in vitro and determine if metabolites were able to modify prostaglandin secretion in response to oxytocin (OT). Endometrial epithelial and stromal cells were incubated with 3H-P4 or 3H-pregnenolone (P5) for 6 or 24 h. Metabolites in the medium were separated by HPLC. The results showed that P4 and P5 were converted to two major polar metabolites and a less polar metabolite that was identified as 5α- or 5β-pregnanedione by LC/MS. Progesterone metabolism was similar in both stromal and epithelial cells. To determine if 5α- or 5β-pregnanedione were able to modify PGF2α synthesis, cells were cultured with P4, 5α- or 5β-pregnanedione (100 ng ml−1) for 48 h and then each group of cells was incubated for a further 4–6 h with or without OT (200 ng ml−1). Results showed that only P4 caused significant (P < 0.001) increase in basal, but not OT-stimulated, PGF2α synthesis. OT binding assays showed no significant effect of progesterone or its metabolites on OTR concentration. In conclusion, bovine endometrial cells are able to metabolize pregnenolone and progesterone but neither 5α- nor 5β-pregnanedione altered prostaglandin synthesis or OTR number in endometrial epithelial cells. These data suggest that 5-pregnanediones do not play a role in the regulation OT-stimulated PGF2α secretion during the bovine estrous cycle.