LUTEOTROPHIC FACTORS IN HYPERSTIMULATED PSEUDOPREGNANT RABBIT .2. HIGH-SENSITIVITY TO HCG OF LUTEAL TISSUE AND SMALL LUTEAL CELLS

Previous studies on rabbit corpus luteum (CL) led to the conclusion th at the luteotrophic complex, in rabbit, may include LH as well as oest radiol for normal luteal function. However, the requirement for LH is controversial. We have recently demonstrated the existence of a human chorionic gonadotrophin (hCG)-stimulated aromatase activity in culture d corpora lutea from a hyperstimulated pseudopregnant rabbit model, wh ich develops a large number of corpora lutea, with only a few or no fo llicles in the ovaries. The present study was undertaken to investigat e the in vitro responsiveness to hCG, dibutyryl cAMP (dbcAMP) and oest radiol of those corpora lutea. Pseudopregnancy (PP) was induced in rab bits by i.m. injection of 200 IU equine chorionic gonadotrophin daily for 2 days followed on day 4 by i.m. injection of 200 IU hCG (day 0 of PP). Luteal tissue and small and large luteal cells obtained at days 5 and 9 of PP were cultured for 24 h or during several days respective ly with or without hCG, dbcAMP or oestradiol. Basal progesterone secre tion was 3.6- and 22-fold higher in large cells compared with small on es at day 5 and 9 of PP respectively. When stimulated by small doses o f hCG, luteal tissue responded by a 5-fold increase in progesterone se cretion. Small cells produced four times higher amounts of progesteron e than controls in the presence of 1 mIU/ml hCG and more than ten time s in the presence of 0.1 IU/ml hCG, whereas large cells were insensiti ve to hCG stimulation. dbcAMP mimicked the effect of hCG on progestero ne secretion by luteal tissue and luteal cells and oestradiol stimulat ed basal progesterone secretion in both small and large luteal cells. Given the large contribution of non stimulated large cells to luteal p rogesterone production and the remarkably high sensitivity of luteal t issue to gonadotrophin in vitro it seems that interactions between the two types of cells might occur during LH stimulation. Our results sug gest that LH could participate in the luteotrophic complex at least in part through the stimulation of endogenous oestradiol production by l uteal cells.