| Summary: | The corpus luteum plays a fundamental role in regulating reproduction via progesterone production. Still, there is little data on factors regulating the maintenance of luteal function during early pregnancy in gilts. Previous studies emphasize the role of hypoxia and HIF-1 in the regulation of steroidogenic and angiogenic genes expression and progesterone production by ovarian cells. Using the corpus luteum of cyclic and early pregnant gilts we analyzed: (1) the in vitro effects of reduced oxygen tension on progesterone production and mRNA expression of <i>HIF1A</i> and luteal function regulators, <i>STAR</i> and <i>VEGFA</i>; (2) the ex vivo profiles of mRNA and protein expression of HIF-1α, <i>STAR</i>, <i>VEGFA</i> and transcription factors-cJUN and CREB, regulating <i>STAR</i> expression, in the corpus luteum of cyclic and pregnant gilts. The synthesis of progesterone was gradually inhibited in cyclic or pregnant gilt luteal tissue (on day 13 of cycle or pregnancy) incubated in a decreasing concentration−20%, 10%, and 3% of oxygen (O<sub>2</sub>). Luteal tissues of pregnant gilts produced trace amounts of progesterone in 10% O<sub>2</sub>, which was similar to cyclic gilts in 3% O<sub>2</sub>. <i>HIF1A</i> expression increased after 24 h of incubation in tissues of cyclic gilts in 3% vs. 20% O<sub>2</sub> (<i>p</i> < 0.01), whereas levels of <i>STAR</i> and <i>VEGFA</i> increased significantly in cyclic and pregnant gilt tissues incubated in 10% and 3% vs. 20% O<sub>2</sub>. The ex vivo mRNA expression of <i>HIF1A</i> and <i>VEGFA</i> was elevated (<i>p</i> < 0.001) on day 14 vs. day 12 of pregnancy. The protein expression of HIF-1 and <i>VEGFA</i> increased (<i>p</i> < 0.001), whereas the level of <i>STAR</i> (mRNA and protein) and progesterone dropped (<i>p</i> < 0.001) on day 14 of the estrous cycle vs. a parallel day of pregnancy and/or day 12 of the estrous cycle. The content of phosphorylated cJUN and CREB was elevated (<i>p</i> < 0.01) in the luteal tissue on day 12 or 14 of pregnancy vs. parallel days of the estrous cycle. These increases of phosphorylated cJUN and CREB may be involved in <i>STAR</i> expression in the luteal tissue during early pregnancy in gilts.
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