Interaction of Estradiol and a Nonsteroidal Follicular Fluid Substance in the Regulation of Gonadotropin Secretion in the Mare

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Abstract
During the estrous cycle of the mare and several other species, the time of maximum secretion of estradiol and maximum follicular growth coincides with the minimum secretion of FSH. A factorial experiment was designed to examine the interaction of estradiol with a nonsteroidal follicular fluid (FF) substance on the concentrations of circulating gonadotropins in the ovariectomized mare. Estradiol treatment alone (1 mg/3 ml cottonseed oil, i.m.) induced synchronous decreases in the concentrations of both FSH and LH immediately (3–8 h) post-treatment, followed by a significant increase above the pretreatment concentration at 12 h and another significant decline at 48 h. Charcoal-extracted equine FF (30 ml, i.v.) induced a significant decrease in concentration of FSH from 8–24 h post-treatment. This decrease in FSH was not accompanied by a decrease in LH. However, at 24 h post-treatment the concentration of LH was significantly increased relative to pretreatment concentrations. When charcoal-extracted FF treatment was given in combination with estradiol, both FSH and LH were initially decreased through 8 h post-treatment. By 24 h, concentrations of LH increased to a level which was significantly greater than pretreatment, while concentrations of FSH remained significantly decreased for the remainder of the experiment. This decrease in FSH was greater than that induced by either treatment alone, indicating that there was an interaction of estradiol with FF in FSH suppressing activity.