Changes in physical properties of ovarian membranes after hCG-induced desensitization in rats

Using pseudopregnant rat ovaries, the possibility was examined whether hCG-induced early desensitization of the LH/hCG receptor was accompanied by changes in the physical state of membranes. Thirty min after a single s.c. injection of 75 IU hCG, the hCG-responsive adenylylcyclase activity was reduced, whereas hCG binding to ovarian membranes was still normal. Membrane lipid rigidity, as determined by fluorescence polarization of DPH, decreased as early as 30 min after injection of a desensitizing dose of hCG. There was no difference in membrane rigidity when ovarian membranes were incubated 0.5 or 2 h with hCG or LH. The decrease of membrane lipid rigidity in the process of rapid desensitization of rat luteal tissue does not appear to be associated with protein synthesis. Desensitization also modified the differential scanning calorimetric profile. The results indicate that hCG-induced changes in the physical state of rat ovary membranes are preceded by the process of desensitization.