Abstract
Summary
1. Since no convincing evidence exists that the action of oxytocin in maintaining mammary secretion in postpartum rats after litter removal is due to prolactin release, attempts were made to determine whether its effects were exerted directly on the mammary gland. After maintaining milk secretion in 25 such rats for 5 or 9 days by daily injections of prolactin or twice-daily injections of oxytocin, a mammary biopsy was taken from each rat. They were then given an intraperitoneal injection of 2 IU oxytocin or physiological saline, and 10 minutes later all except 5 rats were killed and the remainder of the previously biopsied mammary gland was removed. Mammary biopsies from the remaining 5 rats were taken after 10 minutes and also 1, 2, 4 and 8 hours later. 2. Histological examination revealed that prior to intraperitoneal oxytocin administration, most mammary alveoli contained considerable secretion and the ducts were relatively thin, whereas 10 minutes after oxytocin the alveoli appeared shrunken or collapsed with little or no secretion, and the ducts were widely distended with milk. No notable increase in alveolar filling followed until the 8th hour after oxytocin injection. Saline had no effect on mammary alveoli and they remained filled with secretion. 3. Ejection of milk from the alveoli into larger ducts by oxytocin is believed to remove the pressure on epithelial cells which secrete milk and on blood capillaries surrounding them, permitting synthesis of more milk by circulating prolactin and other hormones. Milk ejected into ducts is believed to be gradually reabsorbed into the circulation, and some may flow back into the alveoli. These local effects of oxytocin on the mammary gland are believed to account for its ability to maintain mammary secretion in postpartum rats after litter removal.
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