In bovine luteal cells, progesterone can directly affect its own synthesis by increasing the activity of 3β-HSD. The effect of progesterone on its own secretion coincides with increased expression of the genes for 3β-HSD, StAR, and cytochrome P450scc. Therefore, progesterone regulates its own synthesis by affecting the activity of the enzymes that take part in luteal steroidogenesis, and also by affecting the expression of the genes for these enzymes. The aims of this study were: (a) to determine whether progesterone affects the expression of the gene for its own receptor, thereby affecting its own synthesis; and (b) to determine whether oxytocin and noradrenaline affect the expression of the genes for the oxytocin receptor (OT-R), the progesterone receptor (P4-R), and the β<sub>2</sub> receptor (β<sub>2</sub>-R), thereby regulating luteal steroidogenesis. Two populations of luteal cells were used in the present study: from 6<sup>th</sup>–10<sup>th</sup> and 11<sup>th</sup>–16<sup>th</sup> days of the estrous cycle, which were isolated from <i>corpus luteum</i> (CL) from slaughtered cows. The luteal cells were treated for six hours with one of the following hormones: luteinizing hormone (LH; 100 ng/ml); progesterone (P<sub>4</sub>; 10<sup>–5</sup>M); progesterone antagonist (aP<sub>4</sub>; 10<sup>–5</sup>M); noradrenaline (NA; 10<sup>–5M</sup>); or actinomycin D (ActD; 500 ng/ml). After treatment, the medium was collected for the determination of progesterone concentration. With LH, the P<sub>4</sub> concentration in the medium increased with both 6<sup>th</sup>–10<sup><sup>th</sup> and 11<sup>th</sup>–16<sup>th</sup> days. None of the other treatments affected the progesterone concentration of the medium. The level of expression of the genes for OT-R, P<sub>4</sub>-R and β<sub>2</sub>-R were determined. Total RNA was extracted from cells, treated with DNase, and subjected to reverse transcription. Treatment with luteinizing hormone was the only treatment that increased the level of expression of the gene for P<sub>4</sub>-R in both 6<sup>th</sup>–10<sup>th</sup> and 11<sup>th</sup>–16<sup>th</sup> days of the estrous cycle. Both treatment with luteinizing hormone and treatment with progesterone increased the level of expression of the gene for OT-R in 6<sup>th</sup>–10<sup>th</sup> days. The basal level of expression of the gene for OT-R was higher in 6<sup>th</sup>–10<sup>th</sup> days than in 11<sup>th</sup>–16<sup>th</sup> days. This suggests that there is positive feedback between progesterone and oxytocin, with both playing a role as a local, intra-ovarian factor that enhances the function of the <i>corpus luteum</i>.
Determination of progesterone concentration during the estrous cycle in dairy cows using a chemiluminescence assay