Cause of pregnancy-dependent mammary tumour-induced suppression of lactation in GR/A mice.

To clarify the cause of pregnancy-dependent mammary tumour (PDMT)-induced suppression of lactation due to the retardation of mammary gland growth, we studied the following: (I) mammary gland growth at the end of pregnancy in mice with PDMT [PDMT(+)] in comparison with mice without PDMT [PDMT(-)] and (II) effects of PDMT removal at the end of pregnancy on mammary gland growth and function. In Experiment I, little difference was observed between PDMT(-) and PDMT(+) groups in mammary DNA content and plasma level of lactogenic hormone or progesterone at the end of pregnancy, indicating that the retardation of mammary gland growth by PDMT does not occur during pregnancy. In Experiment II, the surgical removal of PDMT 1 or 2 days before parturition resulted in a complete restoration of mammary gland growth and lactation. All results strongly suggest that PDMT has a deleterious effect on mammary gland growth during their abrupt regression after parturition, perhaps by secreting some mammary growth inhibitory factor(s).