Abstract
Sheep grazing the oestrogenic varieties of subterranean clover (<i>Trifolium subterraneum </i>L.) exhibit an irreversible loss in fertility that has been associated with changes in the mechanisms responsible for ovulation, gamete transport and fertilization. The integrity of the mechanism whereby oestradiol-17β(E<sub>2</sub>) evokes the preovulatory release of luteinizing hormone (LH) from the pituitary has been tested. Intravenous injections or infusions of 40 <i>µ</i>g E<sub>2</sub> failed to evoke a preovulatory-type surge of LH in 4 out of 5 of the permanently infertile, ovariectomized animals that had been off oestrogenic pastures for at least 3 years. In 4 control animals, E<sub>2</sub> treatment caused a release of LH similar to that seen at oestrus. A second series of experiments tested whether this failure was due to a clover-induced refractory state of the pituitary itself. When treated with a synthetic gonadotrophin-releasing hormone (2.5 <i>µ</i>g/h for 3 h), all animals showed a rapid release of LH, similar to the preovulatory release. This suggested that the pituitaries of these animals were capable of a normal discharge of LH and implied that the failure of E<sub>2</sub> to evoke this discharge was due to long-term interference with the hypothalamic control mechanism.
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