A Role for Estradiol in Enhancing Luteinizing Hormone Pulse Frequency during the Follicular Phase of the Estrous Cycle of Sheep*

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Abstract
Experiments were conducted to test the hypothesis that the increased frequency of LH [luteinizing hormone] pulses during the follicular phase of the sheep estrous cycle can be explained by the withdrawal of progesterone. This steroid imposes a profound inhibition of LH pulse frequency in the luteal phase. Experimental ewes were ovariectomized in the late luteal phase of the estrous cycle and divided into 3 groups: no estradiol provided basal estradiol maintained at 1-2 pg/ml by small s.c. Silastic estradiol implants; and peak estradiol of 5-6 pg/ml provided by larger estradiol implants. Control ewes had intact ovaries; their follicular phases were synchronized by insertion and subsequent removal of progesterone implants. LH pulses were monitored beginning 24 h after ovariectomy of experimental ewes or progesterone implant removal from intact control ewes. In the follicular phase controls. LH Pulse frequency increased 3- to 4-fold after progesterone withdrawal, reaching up to 2 pulses per h. When estradiol was not provided after ovariectomy of experimental ewes, LH pulse frequency also increased, but not to the extent seen in the follicular phase control. This high frequency was achieved, however, in experimental ewes treated with either basal or peak estradiol. Both estradiol treatments also reduced LH pulse amplitude. The high frequency of LH pulses in the follicular phase evidently is not solely a consequence of progesterone withdrawal at luteolysis. Estradiol (but not necessarily rising estradiol) apparently also contributes to the high frequency pulses of LH that occur in the ewe at this time.