Ouabain Interaction with Cardiac Na1/K1-ATPase Initiates Signal Cascades Independent of Changes in Intracellular Na1

We have shown previously that partial inhibition of the cardiac myocyte Na1/K1-ATPase activates signal pathways that regulate myocyte growth and growth-related genes and that increases in intracellular Ca21 concentration ([Ca21]i) and reactive oxygen species (ROS) are two essential second messengers within these pathways. The aim of this work was to explore the rela-tion between [Ca21]i and ROS. When myocytes were in a Ca21-free medium, ouabain caused no change in [Ca21]i, but it increased ROS as it did when the cells were in a Ca21-containing medium. Ouabain-induced increase in ROS also occurred under conditions where there was little or no change in [Na1]i. Exposure of myocytes in Ca21-free medium to monensin did not increase ROS. Increase in protein tyrosine phosphorylation, an earl