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An explanation is proposed for the fact that luteinizing hormone (lh) injected into intact pseudopregnant rabbits acts in a luteolytic manner while in hypophysectomized rabbits it acts like a luteotrophin. Recent evidence shows that oestrogen is both luteotrophic and steroidogenic in rabbits. It is pointed out that injection of lh into intact rabbits causes ovulation of follicles which presumably serve as the endogenous source of oestrogen. In the absence of endogenous oestrogen, corpora lutea degenerate. In hypophysectomized rabbits, follicles do not ovulate in response to lh, and the latter is assumed to cause the unovulated follicles to secrete oestrogen which in turn acts in a luteotrophic manner on the corpora lutea.

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S. J. Seiner, W. Schramm, and P. L. Keyes

Summary. The potential role of macrophages and T lymphocytes in the destruction of the corpus luteum at the end of the luteal phase was investigated by treating pseudopregnant rabbits with the immunosuppressant glucocorticoid methylprednisolone. Eleven specific pathogen-free New Zealand White rabbits were injected with pregnant mares' serum gonadotrophin (40 iu, i.m.), followed 2 days later by human chorionic gonadotrophin (40 iu, i.v.) to stimulate ovulation. The following day (day 1 of pseudopregnancy) all animals had an oestradiol-filled Silastic capsule implanted s.c., to ensure that oestradiol, the luteotrophic hormone in this species, would not be limiting. From day 10 of pseudopregnancy, three animals were injected with a low dose of methylprednisolone (2 mg kg−1 per day) until day 20. Three other animals were injected with a higher dose of methylprednisolone (20 mg kg−1 per day) from day 13 of pseudopregnancy until day 19. Five animals served as control, vehicle-injected animals. Blood samples were taken at intervals and assayed for progesterone. Immunofluorescence was used to stain luteal tissue for macrophages, T lymphocytes and class II antigens, and positive cells were counted under high-power magnification. Methylprednisolone treatment reduced (by about 70%), but did not eliminate, the macrophages in the regressing corpora lutea. In contrast, the high dose of methylprednisolone essentially eliminated T lymphocytes, and reduced (by about 90%) the number of cells expressing class II antigen in the luteal tissue. Despite the effects of methylprednisolone on these cells, serum progesterone profiles were not altered by treatment with methylprednisolone, and pseudopregnancy was of normal duration. These results indicate that the methylprednisolone-induced reduction in numbers of luteal macrophages and T lymphocytes is compatible with a luteal phase of normal duration in rabbits.

Keywords: methylprednisolone; pseudopregnancy; macrophages; T lymphocytes; rabbit

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P. L. Keyes, R. M. Possley, and K.-C. M. Yuh

Summary. On Day 10 of pseudopregnancy, rabbits were given an i.v. injection of hCG (10–20 i.u.) that was sufficient to cause new ovulations and the loss of follicular oestradiol secretion. There was an immediate 3–4-fold rise in serum progesterone which returned to near prestimulation values (∼ 27 ng/ml) within 12 h in the presence of an implant containing oestradiol-17β. In the absence of oestradiol, serum progesterone continued to decline to reach low values ( ∼ 4 ng/ml) within 24 h and the original corpora lutea subsequently regressed. The administration of oestradiol 24 h after injection of hCG, when progesterone secretion was low, arrested any further decline in progesterone and then restored serum progesterone to normal values. This steroidogenic effect of oestradiol in vivo was a function of enhanced luteal steroidogenesis; corpora lutea removed and incubated for 12 h produced progesterone at high, linear rates, whereas the corpora lutea from animals that did not receive oestradiol produced low or insignificant quantities of progesterone in vitro. We conclude that hCG at these doses is compatible with continued responsiveness of the corpora lutea to oestrogen and that hCG produces its luteolytic effect primarily by ovulating follicles, thus stopping the secretion of the luteotrophic hormone, oestradiol.

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M. C. Wiltbank, R. C. Dysko, K. P. Gallagher, and P. L. Keyes

Summary. Blood flow in the corpus luteum of the pseudopregnant rabbit was measured with tracer-labelled microspheres before and at 1 and 3 h after saline treatment (N = 8) or after inhibition of progesterone synthesis with aminoglutethimide (N = 10). Before treatment luteal blood flow (29·5 ± 3·9 ml/min·g−1 (mean ± s.e.m.)) was much higher than blood flow to other tissues (ovarian stroma = 2·9 ± 0·6; uterus = 0·5 ± 0·1; adrenal gland = 2·6 ± 0·2 ml/min·g−1). Aminoglutethimide reduced serum progesterone by 60% within 1 h but luteal blood flow was unchanged (26·2 ± 3·5 ml/min·g− 1). At 3 h after aminoglutethimide, serum progesterone remained low and luteal blood flow was slightly reduced to 22·5 ± 3·4 ml/min·g−1. This reduction was associated with a significant decline in mean arterial blood pressure which resulted in luteal vascular resistance being unaltered by aminoglutethimide treatment. Further analysis of these data indicated that serum progesterone concentration was not significantly correlated with blood flow to the corpora lutea or with blood flow to other tissues. In contrast, mean arterial blood pressure was highly correlated with blood flow to the corpus luteum (r = 0·80; P < 0·001) but not to the ovarian stroma (r = 0·04), or adrenal gland (r = 0·06). These results indicate that luteal blood flow is not acutely responsive to changes in luteal progesterone production and suggest that luteal blood flow changes passively with changes in arterial blood pressure.

Keywords: corpus luteum; ovary; blood flow; progesterone; aminoglutethimide; rabbit

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K.-C. M. Yuh, R. M. Possley, R. K. Brabec, and P. L. Keyes

Summary. On the day after ovulation, the thecal tissue and associated mural granulosa lutein cells of the rabbit corpus luteum were separated from the granulosa lutein 'core' by dissection and these tissues were cultured separately or together (whole corpus luteum) in defined medium for 10 days on stainless-steel grids. The medium was changed completely every 24 h. Replicate tissues were cultured with testosterone (10 ng/ml), but no other hormones were added to the medium. Progesterone production increased during the first 2 days of culture for whole corpus luteum, granulosa lutein cells and the thecal compartment which also included granulosa lutein cells. After 3 days, the production of progesterone declined gradually, but was still detectable on Day 10. The production of the metabolite, 20α-dihydroprogesterone, by whole corpus luteum was equal to or greater than that of progesterone. Without the addition of testosterone, the granulosa lutein cells produced little (10 pg/culture) oestradiol during 1 day of culture, but the thecal compartment and whole corpus luteum each produced about 100 pg/culture on Day 1 and declining quantities over the next 2 days. In the presence of testosterone added to the medium, the formation of oestradiol was greatly increased for all tissues for 5–6 days of culture, after which time oestradiol was no longer detectable with or without testosterone in medium. Transmission electron microscopy of cells after 10–12 days of culture revealed fine structure that is characteristic of luteal cells, including abundant smooth endoplasmic reticulum, lipid droplets, and junctions between the luteal cells. The corpus luteum in culture resembles the corpus luteum in situ in that steroidogenesis and differentiation can proceed for a period after ovulation without extrinsic hormonal stimulation.