A new perspective on the mechanism of corpus luteum regression.

Wide angle x-ray diffraction has been used to examine the phase behavior of microsomal membranes from regressing corpora lutea of prepubertal pseudopregnant rats. During periods of optimal progesterone secretion, all of the membrane lipid was in the liquid-crystalline phase at physiological temperature and, therefore, was fluid. However, mixtures of liquid-crystalline and gel phase lipid were observed under identical conditions in microsomal membrane preparations from animals undergoing spontaneous or prostaglandin F2 alpha-induced regression. This was accompanied by a parallel rise in the lipid phase transition temperature. In addition, the proportion of lipid in the gel phase increased with time after prostaglandin F2 alpha treatment. These results indicate that the mechanism of corpus luteum regression may involve phase changes in the phospholipid bilayer of cellular membranes. The resulting presence of gel phase lipid in the membrane matrices could contribute to the loss of tissue function.