Ribonuclease activity in the rat mammary gland during pregnancy, lactation and mammary involution.

The rat mammary gland has a high content of ribonucleic acid during late pregnancy when the gland is functionally inactive (Greenbaum & Slater, 1957b). Slater (1956) suggested that this high level of ribonucleic acid might result from a very low activity of nucleic acid-degrading enzymes (ribonucleases) during late pregnancy. Rat mammary gland contains high concentrations of heavymetal ions, e.g. copper and zinc, which inhibit crystalline pancreatic ribonuclease in low concentration (cf. McDonald, 1955). The activity of ribonucleases in the rat mammary gland over the lactation cycle was therefore investigated. Further, since a correlation has been reported for the concentration of ribonucleic acid and the activity of ribonucleases in normal and malignant tissues (Ledoux, Piler, Vanderhaeghe & Brandli, 1957), it was decided to investigate the changes in ribonuclease activity in the rat mammary gland in relation to the very abrupt increase in ribonucleic acid concentration which occurs during lactation (Greenbaum & Slater, 1957b; Slater & Planterose, 1958). METHODS