Luteinizing hormone receptors on granulosa cells from preovulatory follicles and luteal cells throughout the oestrous cycle of pigs

We used granulosa cells isolated from porcine preovulatory follicles and luteal cells isolated without enzyme dissociation to study the dynamic of changes in human chorionic gonadotropin (hCG) binding parameters (maximum binding capacity B and binding affinity expressed by the equilibrium dissociation constant KD) through the entire luteal phase, paying particular attention to the period between the end of the follicular phase and the beginning of the luteal phase. The oestrous cycle phase was determined by morphological criteria. Five stages of the oestrous cycle were distinguished: (1) preovulatory phase, (2) 1–3 days after ovulation, (3) 5–8 days after ovulation, (4) 8–10 days after ovulation and (5) 12–14 days after ovulation. Highly purified hCG was iodinated and used in radioreceptor assays and autoradiographic studies. Thirty-one autoradiograms from the five distinguished stages of the oestrous cycle were analysed and their optical densities were compared. To establish binding parameters in the five types of cells, 15 Scatchard plots were analysed. There were no statistically significant differences between KD values among all luteal cells (approximately 1.59 ± 0.39 × 10−10 mol l−1); however, mean KD in luteal cells was significantly (p < 0.001) lower than KD in granulosa cells (3.38 ± 0.53 × 10−10 mol l−1). The number of available LH/CG receptors per cell varied with the stage of the luteal phase. There were about 7300 available LH/CG receptors on a single luteal cell isolated from a CL not later than 3 days after ovulation. When the corpora lutea matured, the number of LH/CG receptors per cell increased, reaching approximately 28 400 receptors maximum per cell between the 8th and the 10th day of the luteal phase. Later, when the corpora lutea regressed, the number of receptors fell to 17 400. There were much more LH/CG receptors on granulosa cells than on luteal cells, i.e. 76 600 receptors per granulosa cell. The large number of LH/CG receptors on granulosa cells isolated from preovulatory follicles confirms the key role of LH in transformation of follicles into corpora lutea. The distinctly lower number of these receptors on all types of luteal cells might reflect decreased sensitivity of porcine luteal cells to LH.