Plasminogen activators in the mouse mammary gland. Decreased expression during lactation

The enzyme content and mRNA level for both urokinase-type and tissue-type plasminogen activators have been explored during the life cycle of the adult mouse mammary gland. Both enzymes were detected, and urokinase-type plasminogen activator was the predominant form. A marked decrease in enzyme content occurred in late gestation and was maintained throughout lactation; upon weaning, the enzyme content returned to the levels found in virgin mice. These effects were entirely accounted for by changes in the respective mRNA concentrations, which were determined with respect to both total tissue RNA and poly(A+) mRNA. Thus, plasminogen activator-catalyzed proteolysis may occur at high levels throughout the life cycle of the mouse mammary gland, except during lactation.