The autonomy of the rabbit corpus luteum

The rabbit corpus luteum possesses LH receptors that are coupled to adenylyl cyclase, but paradoxically it does not require LH as a luteotrophic factor for the maintenance of progesterone secretion. This suggests that rabbit luteal cells may not respond physiologically to LH. Therefore, the present study was undertaken to investigate the responsiveness of the rabbit corpus luteum of pseudopregnancy to human chorionic gonadotrophin (hCG) which acts on the same receptor as LH. Pseudopregnancy was induced by injection of 40 IU pregnant mare serum gonadotrophin followed 50 h later by an injection of 40 IU hCG (day 0). On days 7 and 11 of pseudopregnancy, corpora lutea were obtained and incubated for 2 or 5 h in the presence of either 0·1 or 1 μg/ml hCG or 1 mm monobutyryl cyclic AMP (bcAMP). Neither hCG nor bcAMP stimulated progesterone production by the isolated corpus luteum, despite a sustained high rate of progesterone production by the tissue throughout the incubation period. By contrast, Graafian follicles removed from the same ovaries and incubated under the same conditions responded both to hCG and bcAMP with large increases in progesterone production. To determine whether the cyclic AMP content of the corpus luteum was altered by in vitro exposure to hCG, day 7 and day 11 corpora lutea were incubated for 5 or 15 min with various concentrations of hCG, and cyclic AMP in the tissue was then measured. Even at the highest concentration of hCG tested (10 μg/ml), the cyclic AMP content of the corpus luteum was unaltered. Given this result, the acute effects of various concentrations of hCG on the adenylyl cyclase activity of homogenates of day 11 corpora lutea were examined. Consistent with previous reports of others, adenylyl cyclase activity was stimulated, but only at a high concentration of hCG (1 μg/ml), and the degree of stimulation of the enzyme (∼75%) was quite modest. By contrast, the adenylyl cyclase activity of homogenates of rabbit Graafian follicles was stimulated by even the lowest concentration of hCG tested (0·01 μg/ml). Thus, the adenylyl cyclase of the rabbit follicle is much more sensitive to hCG stimulation than the luteal form of the enzyme. Given the poor responsiveness of luteal adenylyl cyclase to hCG, the possibility was considered that cyclic AMP production in response to hCG might be obscured by luteal cell phosphodiesterase. When day 11 corpora lutea were incubated with hCG in the presence of the phosphodiesterase inhibitor, isobutyl methylxanthine (5 mm), there was a marked increase in the cyclic AMP content of the tissue. Despite this large increase in endogenous cyclic AMP, progesterone production by the corpora lutea was again unaffected. Thus, the rabbit corpus luteum is insensitive to stimulation in at least two major respects. The adenylyl cyclase coupled to the LH receptor is resistant to stimulation by LH (hCG), and steroidogenesis, as reflected by progesterone production in vitro, is not stimulated acutely by cyclic AMP. These results suggest that progesterone biosynthesis in the rabbit corpus luteum is not a regulated process, but rather a process that becomes autonomous as a result of the differentiation of granulosa cells into luteal cells. Therefore, the role of a luteotrophin, such as oestrogen in the rabbit, is to maintain the health and viability of the luteal cells, which have, as an intrinsic property, the capacity to produce progesterone at a high rate. Journal of Endocrinology (1994) 143, 423–431