Antral follicle count (AFC) and anti-Müllerian hormone (AMH) concentrations are reflective for ovarian reserve and have been associated with improved reproductive performance in cattle. Key events for regulation of uterine receptivity are orchestrated by progesterone. As progesterone concentrations are greater in animals with high than low AFC, we tested the hypothesis, if the resulting improved uterine environment will lead to improved conceptus elongation and endometrial response to interferon tau. For four years, 10 heifers with lowest and highest AFC, respectively, were selected from 120 heifers. Reproductive tracts and blood samples for progesterone and AMH analysis were collected after synchronization and insemination. For a recovered conceptus, length was determined, and interferon tau (IFNT) transcript abundance was analyzed. Endometrial transcript abundance of interferon-stimulated gene 15 (ISG15) and oxytocin receptor (OXTR) were analyzed. Progesterone concentrations did not differ between low and high AFC groups (P = 0.1). A difference in conceptus length was not observed. Endometrial abundance of ISG15 did not differ between pregnant low and high AFC heifers. Abundance of OXTR was greater in open low AFC than open high AFC heifers (P < 0.01). Interaction of AMH and pregnancy status was determined, with greater AMH in pregnant than open high AFC heifers (P < 0.05). Improved uterine environment in high vs. low AFC heifers did not result in longer conceptuses or improved endometrial response. As the increase in OXTR transcript abundance was only detected in low AFC heifers, reported differences in reproductive performance might be associated with earlier initiation of luteolysis.
Summary Sentence
Up-regulation of endometrial oxytocin receptor transcription is a prerequisite for the initiation of luteolysis and occurs later in heifers with increased ovarian reserve indicating temporal differences in the initiation of luteolysis.
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