58 STAGE–SPECIFIC EFFECTS OF THE PROGESTERONE RECEPTOR ANTAGONIST, RU486, ON EARLY BOVINE EMBRYO DEVELOPMENT
Authors: O'Meara C. M. , Fair T. , Lonergan P. 
Published: December 2011

Progesterone plays a key role in the reproductive events associated with pregnancy establishment and maintenance. High concentrations of circulating progesterone in the immediate post-conception period are associated with an advancement of conceptus elongation, an associated increase in interferon-tau production and higher pregnancy rates in cattle. Progesterone-induced changes in the uterine environment are thought to be responsible for the reported advancement in conceptus elongation; however, the function of the progesterone receptor in embryos is not known. Therefore, the aim of the current study was to examine the effect of adding a progesterone receptor antagonist (mifepristone, RU486) at various stages of early embryonic development and at varying concentrations to examine the effects on subsequent embryo development in vitro. Bovine zygotes (n = 2902), 2-cell (n = 1991) and 8-cell (n = 1244) embryos, derived by in vitro maturation and fertilization, were cultured in synthetic oviduct fluid medium in the absence or presence of RU486 at concentrations ranging from 0.0004 to 20 μg mL–1. Cleavage rate (of zygotes), 8-cell development rate (of 2-cell embryos) and development to the blastocyst stage (for all cell stages) were recorded at Day 2, 3 and 8 post-insemination (day of IVF = Day 0), respectively. Cultures of zygotes in the presence of RU486 at concentrations of 0.004 and 0.04 μg mL–1 resulted in a decline in cleavage rate (62.5 ± 2.55% and 48.8 ± 5.07% for respective treatments vs controls without RU486 81.9 ± 5.97%; P ≤ 0.05). These same concentrations resulted in a significant decline in blastocyst development on Day 8 (18.8 ± 1.82% and 17.4 ± 4.85% for respective treatments compared to controls 35.1 ± 4.89%; P ≤ 0.05). Cultures at concentrations of 0.4 μg mL–1 resulted in a 10-fold decrease in blastocyst development (3.3 ± 1.3%; P ≤ 0.05) and concentrations in excess of 10 μg mL–1 completely ablated blastocyst development (P ≤ 0.05). Cultures of 2-cell embryos with RU486 at concentrations below 8 μg mL–1 had no effect on 8-cell rate or blastocyst development. However, cultures with RU486 at 10 μg mL–1 resulted in a significant decline in the proportion reaching the 8-cell stage (59.1 ± 4.59% vs 38.1 ± 2.13% for control and treated, respectively) and developing to the blastocyst stage (32.8 ± 4.68% vs 17.8 ± 3.77% for control and treated, respectively; P ≤ 0.05). Cultures with RU486 at a concentration of 20 μg mL–1 resulted in a dramatic effect in 8-cell rate (16.3 ± 2.55%; P ≤ 0.05) and prevented blastocyst development. Similarly, cultures of 8-cell embryos with RU486 at concentrations at or below 10 μg mL–1 had no effect on blastocyst development. However, cultures at concentrations of 15 or 20 μg mL–1 resulted in no blastocyst development. In conclusion, addition of the progesterone and glucocorticoid receptor antagonist RU486 to culture media has a clear stage-specific and concentration-dependent effect on bovine embryo development, which is more pronounced at earlier developmental stages.
Supported by Science Foundation Ireland (07/SRC/B1156).