A role for estradiol in enhancing luteinizing hormone pulse frequency during the follicular phase of the estrous cycle of sheep.

Experiments were conducted to test the hypothesis that the increased frequency of LH 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 three groups: 1) no estradiol provided; 2) basal estradiol maintained at 1-2 pg/ml by small sc Silastic estradiol implants; and 3) 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 two 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. These results fail to support the hypothesis that the high frequency of LH pulses in the follicular phase is solely a consequence of progesterone withdrawal at luteolysis. Rather, they suggest that estradiol (but not necessarily rising estradiol) also contributes to the high frequency pulses of LH that occur in the ewe at this time.