Plasma and ovarian oestradiol and the variability in the LH surge induced in ewes by the ram effect

The proportion of anoestrous ewes ovulating after exposure to a sexually active ram is variable mainly due to whether an LH surge is induced. The aim of this study was to determine the role of oestradiol (E-2) in the ram-induced LH surge. In one study, we measured the plasma concentrations of E-2 in ewes of different breeds before and after the 'ram effect' and related these patterns to the presence and latency of the LH surge, while another compared ovarian responses with the 'ram effect' following exposure to rams for 2 or 12 h. In all ewes, the concentration of E-2 increased 2-4 h after rams were introduced and remained elevated for 14.5 +/- 0.86 h. The quantity of E-2 secreted before the LH surge varied among breeds as did the mean concentration of E-2. The granulosa cells of IF ewes collected after 12 h exposure to rams secreted more E-2 and progesterone and had higher levels of StAR than the 2 h group but in MV ewes there was no differences between these groups for any of these parameters. These results demonstrate that the LH surge induced by the rams is a result of increased E-2 secretion associated with increased levels of STAR in granulosa cells and that these responses varied among breeds. The results suggest that the variable occurrence of a LH surge and ovulation may be the result of variable ovarian responses to the 'ram effect' and insensitivity of the hypothalamus to the E-2-positive feedback signal. Free French abstract: A French translation of this abstract is freely available at http://www.reproduction-online.org/content/149/5/511/ suppl/DC1.