Signal-transducing function of Na+-K+ -ATPase is essential for ouabain's effect on [Ca2+](i) in rat cardiac myocytes

We showed before that Na+-K+-ATPase is also a signal transducer in neonatal rat cardiac myocytes. Binding of ouabain to the enzyme activates multiple signal pathways that regulate cell growth. The aims of this work were to ex tend such studies to adult cardiac myocytes and to determine whether the si gnal-transducing function of Na+/K+-ATPase regulates the well-known effects of ouabain on intracellular Ca2+ concentration ([Ca2+](i)). In adult myocy tes, ouabain activated protein tyrosine phosphorylation and p42/44 mitogen- activated protein kinases (MAPKs), increased production of reactive oxygen species (ROS), and raised both systolic and diastolic [Ca2+](i). Pretreatme nt of myocytes with several Src kinase inhibitors, or overexpression of a d ominant negative Ras, antagonized ouabain-induced activation of MAPKs and i ncreases in [Ca2+](i). Treatment with PD-98059 (a MAPK kinase inhibitor) or overexpression of a dominant negative MAPK kinase 1 also ablated the effec t of ouabain on MAPKs and [Ca2+](i). N-acetyl-cysteine, which blocks the ef fect of ouabain on ROS, did not prevent the ouabain-induced rise in [Ca2+]( i). Clearly, the activation of the Ras/MAPK cascade, but not ROS generation , is necessary for ouabain-induced increases in [Ca2+](i) in rat cardiac my ocytes.