Cellular distribution of prostacyclin synthase and specific prostacyclin binding sites in bovine corpora lutea of pregnancy.

The cellular distribution of prostacyclin (PGI2) synthase and specific PGI2 binding sites was investigated by light microscope immunocytochemistry and quantitative autoradiography. The immunostaining for the enzyme was found in small (15-18 microns) and large (18-45 microns) cells as well as in non-luteal cells, arteriole smooth muscle and endothelium. There was no consistent difference between small and large luteal cells and luteal vs. non-luteal cells in the immunostaining. Vascular smooth muscle and endothelial cells, on the other hand, contained less immunostaining than the other cells. Contrary to wide-spread distribution of PGI2 synthase, specific PGI2 binding sites were only found in the luteal cells. On a per cell basis, large luteal cells contained more sites than small luteal cells and vice versa when expressed per unit cell area. The [3H]PGI2 binding to the luteal cells was time and temperature dependent and was inhibited by excess unlabeled PGI2 but not by its metabolite, 6-keto-PGF1 alpha or other eicosanoids which bind to their own receptors. In conclusion, while PGI2 synthase is widely distributed among different cell types in bovine corpora lutea, specific binding sites which may mediate luteotropic actions of PGI2 are only present in small and large luteal cells.