Pseudopregnancy-dependent changes in rat ovarian LH/hCG receptors in relation to membrane lipid fluidity

The specific binding of [125I] hCG to ovarian membrane preparations as well as membrane fluidity have been investigated in immature rats during hormonally-induced pseudopregnancy. Membrane fluidity was monitored either by fluorescence polarization analysis of 1,6-diphenyl-1,3,5-hexatriene or by electron spin resonance of 16-, 12-, 5-doxyl stearic acid and CAT 16. A significant positive correlation was found between membrane lipid rigidity and the number of LH/hCG receptors. Luteinization of the ovary induced mobility of molecules in the hydrophobic membrane part at about the C16 carbon level. The changes in rigidity of membrane lipid were the apparent result of alterations in the cholesterol to phospholipids ratio. The results suggest that the increased rigidity of membrane lipid during pseudopregnancy may maximally expose ovarian LH/hCG receptors maintained in a cryptic form.