Prostaglandin F2 alpha administered in vivo induces Ca2+-dependent protein phosphorylation in rat luteal tissue

The present study was performed in order to further elucidate the mechanism of action of PGF2 alpha in luteolysis in the rat ovary. Seven days after priming with superovulatory doses of pregnant mare serum gonadotropin and human chorionic gonadotropin to induce luteal tissue formation, the rats were injected with a luteolytic dose of the prostaglandin F2 alpha analogue cloprostenol. The ovaries were then homogenized, a 30,000 x g supernatant and pellet were prepared, whereafter aliquots of the preparations were incubated in the presence of [gamma-32P]ATP with or without Ca2+. The phosphorylated proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and localized by autoradiography. The presence of Ca2+ caused an increased phosphorylation of a 45 kDa protein band in the particulate, but not in the cytosol, fraction. Furthermore, PGF2 alpha rapidly increased the 32P incorporation into the same protein band of 45 kDa. Thus, the PGF2 alpha-stimulated 32P incorporation was Ca2+-dependent and seen only in the particulate fraction. These results suggest that PGF2 alpha in its role as a luteolytic agent stimulates a Ca2+-dependent phosphorylation of a specific protein in luteal membranes of the rat ovary.