Reduction in Pituitary Desensitization and Prolongation of Gonadotropin Release by Estrogen during Continuous Administration of Gonadotropin-Releasing Hormone in Women: Its Antagonism by Progesterone*

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Abstract
The present study was designed to elucidate gonadal steroid influences on gonadotropin release and subsequent pituitary desensitization to GnRH. Sixteen women, 10 of whom were normal and 6 of whom had hypogonadism, were infused with GnRH at rates ranging from 0.313–10 εg/h via an indwelling iv catheter for 66 h. Blood samples obtained throughout the GnRH infusion were analyzed for LH, FSH, estradiol, and progesterone. A prompt and substantial release of gonadotropin occurred in women with ovarian failure or during the luteal phase in normal women compared with that during the follicular phaseof the menstrual cycle. Thereafter, a gradual decrease in gonadotropin secretion occurred due to pituitary desensitization, which was slower in the follicular phase than in other groups. A dose-related increase in integrated LH release occurred during GnRH infusion, but this response tapered off with administration of large doses of GnRH to women with ovarian failure or during the luteal phase. In contrast, it increased linearly up to the maximum dose of GnRH in the follicular phase. These data suggest that 1) basal levels of estrogen suppress the early rapid release of gonadotropin in response to GnRH and reduce subsequent pituitary desensitization, resulting in the prolonged release of LH; 2) estrogen widens the range of doserelated increases in gonadotropin in response to GnRH; and 3) these effects of estrogen are antagonized by progesterone.

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