MECHANISM OF ACTION OF OXYTOCIN IN RETARDING MAMMARY INVOLUTION: STUDY IN HYPOPHYSECTOMIZED RATS

Abstract

SUMMARY: On the 4th day after parturition lactating rats were hypophysectomized and their litters removed. Control rats were injected only with physiological saline for the next 10 days, and showed pronounced regression of the mammary parenchyma and no secretion. When oxytocin was injected together with prolactin and adrenocorticotrophic hormone (ACTH) for 10 days, mammary secretion, often with duct engorgement, was observed in twenty-two out of twenty-four rats, whereas only three out of fourteen rats given prolactin and ACTH showed slight secretory activity. Lobule-alveolar structure was significantly better preserved in the former rats. The combination of oxytocin, prolactin and cortisol acetate was also more effective than prolactin and cortisol acetate in maintaining secretory function and in retarding mammary involution. These results in hypophysectomized rats, as well as similar findings previously reported by us in intact rats, demonstrate that the favourable effects of oxytocin in maintaining secretory activity and retarding mammary involution are exerted directly on the mammary gland.