Suppression of lactation by pregnancy-dependent mammary tumors in GR/A mice.

Pregnancy-dependent mammary tumors (PDMT) in GR/A mice appear during pregnancy, disappear soon after parturition, and appear again during subsequent pregnancies. The retardation of pup growth, an indication of the level of milk production, was also observed with the advance of lactation numbers in this strain. This study was performed to elucidate the relationship between PDMT and lactational performance. At the end of the second pregnancy, mice were divided into two groups according to the presence of PDMT [PDMT(-) and PDMT(+) groups]. Although all PDMT disappeared within a day after parturition, the weight and growth of pups on Day 12 of lactation were significantly less in the PDMT(+) group than in the PDMT(-) group. Associated with this, the DNA and RNA contents of the mammary glands were apparently lower in the former than in the latter, although the differences were not statistically significant. There was little difference in mammary RNA/DNA ratio between groups. No difference was also observed between groups in endocrine organ weights, mother body weights, morphology of the mammary glands, adrenals and ovaries and plasma prolactin and progesterone levels. These results suggest that PDMT suppression of lactation is principally due to the retardation of mammary gland growth. Furthermore, no significant correlations were obtained between the size of PDMT and the parameters for mammary gland function. The data suggest that the development of PDMT per se is important for the retarded mammary gland growth.