Abstract
Adenosine triphosphatase (ATPase) activity was localized at an ultrastructural level in the resting mammary glands of female BALB/c mice. A Mg++ dependent ATPase was localized in the plasma membranes of both the epithelial and myoepithelial cells of the mammary tubules. A second type of ATPase activity that was not Mg++-dependent but that was Na+ and K+ dependent was localized primarily in the plasma membranes of the myoepithelial cells. Preincubation with either ouabain or N-ethylmaleimide decreased the quantity of reaction product, indicating that both types of ATPase activity were sensitive to these inhibitors. Control media, containing adenosine triphosphate and Pb(NO3)2 without cations, demonstrated that the amount of nonezymatic hydrolysis was negligible. These differences in the cationic requirements for plasma membrane ATPase activity can be used to distinguish histochemically the epithelial from myoepithelial cells in mammary tissue.