The Rat Estrous Cycle Associated with Functional Corpora Lutea and the Mechanism of Its Recurrence

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Pseudopregnancy (PSP)-like cycles characterized by a fully functional luteal phase and comparable in length to the period of pseudopregnancy have been induced in normally cycling rats by the isografting of anterior pituitary (AP) implants beneath the kidney capsule. Hormonal factors associated with the induction and maintenance of these spontaneously recurring PSP-like cycles were studied by measuring blood levels of prolactin, LH, FSH, estradiol-17β, progesterone and 20α-dihydroprogesterone. Transplantation of isografts comprising 1/4 anterior pituitary extended the length of the diestrous period approximately to that of normal pseudopregnancy. However, over a range of tissue mass extending from 1/4 to 2 glands, a significant correlation was observed between the duration of diestrus and the quantity of anterior pituitary tissue employed. More than 2 glands did not further extend the length of the luteal period. The secretory patterns of progesterone and 20α-dihydroprogesterone in ovarian vein blood during the PSP-like cycle were similar to those of normal pseudopregnancy. Within limits, the progesterone concentration varied directly with the quantity of transplanted pituitary tissue. Prolactin levels measured either in serum or in the in situ pituitary did not fluctuate during the luteal phase of the PSP-like cycle; however, the magnitude of the serum prolactin concentration was directly correlated with the quantity of pituitary tissue implanted possibly reflecting a direct contribution of the implant to the circulating hormone level. It is assumed that the prolactin supply from the isograft was responsible for induction and maintenance of the PSP-like cycle, since premature termination resulted from early ergocornine injection and/or removal of the grafted pituitary tissue. However, completion of the PSP-like cycle following removal of the AP-graft on or after day 5 demonstrating the capacity of the in situ pituitary to secrete adequate prolactin for maintenance. When animals were hypophysectomized during the first half of the PSP-like cycle, the isografted pituitary being left in place, progesterone secretion increased significantly providing evidence of a suppressive action on ovarian progesterone secretion by the in situ pituitary. Hypophysectomy performed during the second half of the PSP-like cycle resulted in a decreased progesterone secretion suggesting a change in luteal responsiveness to luteotrophic stimulation taking place at midcycle. The initiator of this qualitative change in luteal function probably resides in the in situ pituitary. The circulating levels of FSH and LH did not fluctuate from basal levels; however, ovarian vein estradiol-17β increased gradually during the second half of the PSP-like cycle. It is supposed that the rising estradiol level eventually triggered gonadotrophin release on the subsequent proestrus which was required for recurrence of the cycle. Finally, a statistically significant prolongation of the PSP-like cycle was induced by bilateral hysterectomy, this prolongation being of greater length when hysterectomy was performed prior to the 9th day.
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The Rat Estrous Cycle Associated with Functional Corpora Lutea and the Mechanism of Its Recurrence

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