The effect of blocking progesterone receptors on the mechanism of ovulation in superovulated ewes
1999
Goncalves, S.C. | Horta, A.E.M.
The aim of this work was to check the effect of blocking progesterone receptors on the mechanism of ovulation, in superovulated ewes. The oestrous cycle of 10 ewes was synchronised with vaginal sponges containing 40 mg of fluorgestone acetate (FGA, "Chronogest", Intervet) during 9 days. The day before sponge withdrawal, all animals received 7.5 mg of PGF2 alpha and 1500 u.i. of PMSG, intramuscularly. The control group received only the treatment above (C, n=5). To the remaining animals (T, n=5) it was further administered a progesterone antagonist (onapristone, ZK 98.299, Schering AG) every 12 h from sponge withdrawal to 24 h after the onset of oestrous (1 mg Kg-1 b.w., intravenously). In both groups, peripheral concentrations of progesterone and LH were respectively measured by radioimmunoassay and ELISA during the periovulatory period. All animals in both groups showed oestrous behaviour, LH surge and ovulated. Animals receiving the progesterone antagonist started oestrous behaviour significantly sooner than controls (24 h vs. 31.2 h, respectively, P0.05). Intervals from sponge withdrawal and onset of oestrous to LH surge were not different between groups. The average number of ovulations per animal was not significantly different between groups (5.4 vs. 10, respectively for groups T and C; P0.05). Peripheral progesterone concentrations were found basal in both groups during the studied period. Progesterone blockade during the follicular phase of the oestrous cycle in the ewe did not prevent ovulations to occur, suggesting a non essential role of the intrafollicular progesterone on the mechanism of ovulation in this species. Oestrous behaviour on the antiprogesterone treated animals occurred significantly sooner and in a more synchronised way than in controls, suggesting that intrafollicular progesterone may interfere locally delaying follicular growth
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