Progesterone has two major regulatory functions. First, progesterone controls the release of LH. High concentrations of progesterone reduce frequency of secretion of pulses of gonadotropin releasing hormone (GnRH) from the hypothalamus. Frequency of secretion of pulses of LH from the anterior pituitary gland is thus reduced, and the surge of LH and subsequent ovulation are prevented. In the cow, 37% of the variance in frequency of pulses of LH and 38% of the variance in concentrations of estradiol-17p are accounted for by concentrations of progesterone.
Second, progesterone establishes the capacity of the endometrium to secrete prostaglandin F2a (PGF2a) and regulates the timing of initial increases in secretion of PGF2a for luteolysis. Increases usually begin around day 11 of the cycle in the ewe and around day 14 in the doe and cow (Fig. 1). Progesterone also modulates episodic secretion of PGF2a, keeping it at midrange values until luteal regression has begun. With decreasing concentrations of progesterone, greater secretion of PGF2a by the uterus and the corpus luteum completes luteal regression. Maximal secretion of PGF2a occurs after luteal secretion of progesterone has ceased (Fig. 1).
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