Release of Luteinizing Hormone by Oestradiol-17β and a Gonadotrophin-Releasing Hormone in Ewes Affected with Clover Disease

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Abstract
Sheep grazing the oestrogenic varieties of subterranean clover (Trifolium subterraneum 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(E2) evokes the preovulatory release of luteinizing hormone (LH) from the pituitary has been tested. Intravenous injections or infusions of 40 µg E2 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, E2 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 µg/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 E2 to evoke this discharge was due to long-term interference with the hypothalamic control mechanism.

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