Relationship between histological structure and lactate dehydrogenase isoenzyme pattern of rat mammary tumors induced by dimethylbenz(a)-anthracene

Mammary gland tumors induced by 7,12-dimethylbenz(a)-anthracene (DMBA) in female Wistar rats could be divided into secreting and nonsecreting ones. Histological, histochemical, cytophotometric, and electron microscopic examinations yielded indirect evidence for the ab ovo malignant nature of the nonsecreting tumors but not for the secreting ones. Nonsecreting tumors showed high total lactate dehydrogenase (LDH) activity and a definite shift of the isoenzyme pattern toward isoenzymes containing M subunits compared to those of lactating mammary glands. Secreting tumors could be characterized by low total LDH activity and a moderate increase of M/H ratio. These biochemical results support the hypothesis that nonsecreting mammary tumors induced in rats--independently of metastatization and differentiation--have to be considered carcinomas while the secreting ones benign adenomas.