Immunocytochemical localization of receptor-bound human chorionic gonadotropin in pseudopregnant rat ovaries

Localization of receptor-bound human chorionic gonadotropin (hCG) in pseudopregnant rat ovarian cells was studied by the peroxidase-antiperoxidase (PAP) complex method. The animals were injected, 2,6,12, and 24 hr prior to killing with a single intravenous dose of hCG. The hormone localized to the periphery of the luteal and interstitial glandular cells. The luteal cells seemed to bind different amounts of the hormone, which suggests that the number of receptor varies from one cell to another or that the receptors are not similarly accessible to the circulating hormone in all cells. The hormone seemed to disappear unequally from the luteal cells, possibly due to the variable amounts of primarily bound hormone or to different removal mechanisms. The hormone was distributed unevenly at the luteal cell periphery. This observation supports previous findings that hCG preferentially binds to the cell surface regions directed towards the capillary areas. Compatible results were obtained with anti-hCG serum and with antisera against the hCG subunits. Our results support the previous autoradiographic evidence of the plasma membrane localization of luteinizing hormone receptor in the luteal cells, and are compatible with the concept that only a small fraction of the receptor-hormone complexes may be internalized.