Autoradiographic localization of radioactivity from [3-H]oxytocin in the rat mammary gland and oviduct.

[3-H]Oxytocin was incubated in vitro with pieces of oviduct and mammary gland and the tissue were subjected to autoradiography. Radioactivity was localized only in smooth muscle cells of the oviduct and in regions of mammary tissue where myoepithelial cells are found. In contrast, radioactivity was not concentrated in any region of skeletal muscle, a nontarget tissue for oxytocin. The localization of radioactivity in oviduct and mammary cells was absent when the tissues were incubated with [3-H]oxytocin and an excess of unlabeled oxytocin. These findings provide evidence for the presence of specific and high affinity receptors for oxytocin in its target tissues.