Changes in the plasma membrane fluidity of granulosa cells during the process of luteinization

To elucidate the membrane fluidity of granulosa cells in luteinization process, porcine granulosa cells (GCs) were cultured in the serum free medium under the stimulation of human chorionic gonadotropin (hCG). Their cell membrane anisotropy, membrane lipid composition and progesterone synthesis were measured. The GCs plasma membrane anisotropy, which is reverse to fluidity, was measured with the steady-state spectrofluorometer using a hydrophobic fluorescent probe 1, 6-diphenyl-1, 3, 5-hexatriene (DPH). Since the membrane fluidity is generally affected by concentrations of cholesterol and phospholipids, phospholipid composition and the degree of unsaturation of acyl chain, concentrations of unesterified cholesterol and phospholipids, and phospholipid composition in the plasma membranes of GCs were determined. The results obtained were as follows. 1) The value of the anisotropy (membrane fluidity) of the GCs stimulated by hCG decreased (increased) significantly as compared to that without hCG addition. 2) The concentration of cholesterol in plasma membrane fraction decreased significantly by hCG stimulation. 3) In contrast, the concentration and composition of phospholipids showed no significant changes. 4) Progesterone concentration in culture medium was significantly increased by hCG stimulation at 30 mlU/ml and 300 mlU/ml throughout the time course of this study, compared with control. These results indicate the increase of membrane fluidity of the GCs luteinization induced by hCG is due to a decrease of cholesterol/phospholipids ratio. Because transfer of cholesterol into culture medium was not observed, it seems that the cholesterol of the GCs plasma membranes was utilized for progesterone synthesis at least in the serum free medium (GIT medium) during the process of luteinization.