Pulsatile Secretion of LH and Ovarian Estradiol During the Follicular Phase of the Sheep Estrous Cycle

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Abstract
The concentration of LH [luteinizing hormone] in jugular venous plasma and the ovarian secretion of estradiol was measured every 10 min for periods of 60 or 120 min on 28 occasions in 13 ewes with utero-ovarian or ovarian autotransplants. Blood samples were collected at various times during the follicular phase; 2 days prior to estrus (estrus -2), estrus -1 and on the day of estrus and the results compared to those obtained during the luteal phase. During the luteal phase, episodic pulses of LH occurred at intervals of approximately 3 h 20 min and each pulse was followed by a significant rise in the secretion of estradiol. During the follicular phase, the pulses of LH occurred with increased frequency (1/h) but with decreased amplitude. Although each pulse of LH was smaller, the peak of secretion of estradiol following each pulse was significantly greater during the follicular than during the luteal phase. By estrus, the secretion of estradiol declined in spite of LH values in the range of the preovulatory LH surge. The increased secretion of estradiol from the preovulatory Graafian follicle may be due to repeated stimulation by episodic pulses of LH which occur with increasing frequency.