A new method for studying pituitary responsiveness in vivo using pulses of LH-RH analogue in ewes passively immunized against native LH-RH.

In two experiments with ovariectomized ewes, the plasma levels of LH and FSH were measured following passive immunization against LH-RH. The pulsatile secretion of LH was inhibited and the levels of the hormone fell quickly and were undetectable within 24 h. The levels of FSH fell more slowly and were always detectable. In the second of the experiments, pulses of LH of normal amplitude were re-established in the immunized ewes by hourly intravenous injections of 200 ng of Des-Gly10 (Pro9)-LH-RH Ethylamide, a synthetic analogue of LH-RH that does not cross-react with the antiserum yet has similar biological activity to native LH-RH. The analogue was injected over 6 h and had no detectable effect on the levels of FSH. When the ewes were injected with 50 micrograms of oestradiol benzoate, the LH responses to the analogue were completely inhibited for 4-5 h then slowly recovered to normal levels. The oestrogen injection also depressed the levels of FSH for varying amounts of time. On the basis of the normal LH patterns and responses to oestrogen, we conclude that combining passive immunization against LH-RH with injections of an LH-RH analogue provides a valuable model for testing factors that affect pituitary responsiveness to LH-RH.