Luteal Responses to Gonadotropin-Releasing Hormone during the Luteal Phase: Relation to the Age of Corpus Luteum

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Abstract
The pituitary and luteal responsiveness of GnRH were studied in 20 normal women at different stages of the luteal phase (LP). Blood samples were collected every 15 min for 180 min before and 120 min after the iv injection of 25 μg GnRH. The studies were performed in the early LP (ELP; days 2–3 of LP; n = 5), mid-LP (MLP: days 4–8 of LP; n = 11), late LP (LLP; days 9–12 of LP; n = 13), and premenstrual phase (PMP; days 13–14 of LP; n = 3). Plasma LH, FSH, progesterone (P), and estradiol (E) levels were assayed by RIA. The data were analyzed as integrated secretory area before (ISAb) and after GnRH stimulation (ISAs) and in terms of their percent increase with respect to the basal value. In all studies, GnRH elicited increases in plasma LH and FSH (P < 0.001). On the other hand, in the ELP, GnRH did not alter steroid ISAs compared to their ISAb, while significant increases in plasma P and E levels were found in the MLP (P, P < 0.01; E, P < 0.02) and LLP (P and E, P < 0.01). In the PMP, two women had no increase in steroid secretion; in the remainder of the subjects, both P and E ISAs markedly increased. This different pattern was not related to basal steroid levels. All women who had a blunted steroid response in the ELP or PMP had a normal secretory response of both P and E when studied at the other LP stages of the same cycle. Furthermore, there was a positive linear correlation between plasma P and E for the ISAb and ISAs values, while the secretory patterns of gonadotropins and steroids were not related to each other. In conclusion, the corpus luteum is able to respond to GnRH at a well identified period of the LP. This pattern indicates variable dependence of the corpus luteum on the functional activity of the hypothalamic-pituitary axis.

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