Autoradiographic localization of³H-ORG 2058, a synthetic progestin, in the mammary glands of the baboon

The uptake and retention of a radiolabeled synthetic progestin ORG 2058 by the mammary glands were examined in the baboon. Four estrogen-primed baboons were injected intravenously with 2.5 μg/kg body weight of³H-ORG 2058. One animal received an additional injection of 2.5 mg/kg body weight of unlabeled progesterone. One hour after the injections, the animals were killed and the mammary glands were removed and processed for autoradiography. Only 13.4% of the alveolar secretory cells studied demonstrated nuclear uptake and retention of the radiolabeled steroid, while 60% of the alveoli demonstrated at least 1 labeled cell per cross-section. The fibroblasts in the interlobular connective tissue and most of the cells lining the interlobular ducts were labeled; however, no cells in the intralobular connective tissue were labeled. These observations seem to indicate that all alveolar secretory cells do not contain progesterone receptors in the normal mammary gland.