Effects of pituitary graft and 2-bromo-alpha-ergocryptine on mammary DNA synthesis in mice in relation to mammary tumorigenesis.

3H-thymidine incorporation into mammary DNA as an index of mammary DNA synthesis in C3H/He virgin females was significantly higher at proestrus than at estrus and diestrus. The rise in incorporation at proestrus was completely blocked by the administration of 2-bromo-alpha-ergocryptine (Bromocriptin) 1 day before proestrus. The incorporation was maintained at a higher level than that at proestrus by pituitary grafts. The results supported the view that mammary DNA synthesis, which is stimulated by prolactin, is an important factor for mammary tumorigenesis.