Feedback regulation of pulsatile LH secretion in the ewe: stimulation of frequency by estradiol

This study tested the hypothesis that estradiol can enhance LH pulse frequency in the ewe by an action which does not depend on other ovarian hormones. Long-term ovariectomized ewes were treated with a small subcutaneous estradiol implant at a time equivalent to the early breeding season (October), and frequent blood samples (6-min intervals) were obtained during sequential 3-hour periods over the next 84 h. All ewes responded with an increase in frequency of LH pulses, a response evident by 60 h and maintained at 84 h after initiation of the estradiol stimulus. Mean (+/- SE) pretreatment frequency was one pulse every 41 +/- 2 min; that at the height of the response was one pulse every 34 +/- 2 min (p less than 0.01). This increased rate was equivalent to the annual mid-winter maximum observed in ovariectomized ewes not treated with estradiol. These findings are consistent with the hypothesis that estradiol can enhance LH pulse frequency by an action which does not depend on other ovarian steroids. It is suggested that this action contributes to the heightened pace of LH pulses during the follicular phase of the estrous cycle.