The involution of the mammary glands of lactating rats, which normally follows the cessation of suckling, was greatly retarded over a period of 9 days by administering oxytocin to the mothers, following removal of the litters on the 4th day of lactation. This effect was obtained with a commercial extract of the natural hormone, the same extract without preservative (benzethonium chloride), and with synthetic oxytocin. Vasopressin administered under the same conditions had a less well-marked effect. No retardation of mammary involution could be obtained with oxytocin in the absence of the anterior pituitary gland.
Similar results were obtained by administering prolactin to the mothers, but growth hormone (GH) had only a slight effect in maintaining the mammary glands. When both prolactin and GH were given, the maintenance of gland structure was particularly marked.
A majority of the animals receiving synthetic oxytocin showed vaginal mucification which is taken to be indicative of the presence of a luteotrophic hormone (prolactin).
These results are discussed in relation to the possible role of oxytocin in the release of prolactin and other lactogenic and galactopoietic hormones from the anterior lobe.