LIGAND-INDUCED RAPID DESENSITIZATION CAUSES STRUCTURAL ALTERATION OF RAT OVARIAN LH HCG RECEPTOR/

The role of the physical state of ovarian membranes was studied in the process of the early desensitization of the LH/hCG receptor. Thirty m in after injection of a desensitizing dose of hCG to rats, the hCG-res ponsive adenylylcyclase activity was reduced, whereas hCG binding to o varian membranes was still normal. Early desensitization decreased rig idity of membrane lipids, determined by fluorescence polarization of D PH. Possible structure-functional properties of the LH/hCG receptor we re analyzed by thermal perturbation technique. Desensitization decreas ed thermal stability of the LH/hCG receptor in membranes and in proteo liposomes. Desensitization modified the quenching of protein fluoresce nce and intrinsic fluorescence spectral properties of membranes. The S tern-Volmer constants for control and desensitized membranes were foun d to be 4.3 and 5.5, respectively, indicating that desensitization ele vated the accessibility of fluorophores for acrylamide. The changes of the physical properties of membranes resulting from desensitization w as exhibited solely in the treatment with hCG in vivo, but not in vitr o. The results suggest that the hCG-induced alteration of the physical state of luteal membranes may be a requirement for the induction of c hanges that lead to desensitization.