Role of Na+-K+ ATPase Alterations in the Development of Heart Failure.

Na⁺-K⁺ ATPase is an integral component of cardiac sarcolemma and consists of three major subunits, namely the α-subunit with three isoforms (α₁, α₂, and α₃), β-subunit with two isoforms (β₁ and β₂) and γ-subunit (phospholemman). This enzyme has been demonstrated to transport three Na and two K ions to generate a trans-membrane gradient, maintain cation homeostasis in cardiomyocytes and participate in regulating contractile force development. Na⁺-K⁺ ATPase serves as a receptor for both exogenous and endogenous cardiotonic glycosides and steroids, and a signal transducer for modifying myocardial metabolism as well as cellular survival and death. In addition, Na⁺-K⁺ ATPase is regulated by different hormones through the phosphorylation/dephosphorylation of phospholemman, which is tightly bound to this enzyme. The activity of Na⁺-K⁺ ATPase has been reported to be increased, unaltered and depressed in failing hearts depending upon the type and stage of heart failure as well as the association/disassociation of phospholemman and binding with endogenous cardiotonic steroids, namely endogenous ouabain and marinobufagenin. Increased Na⁺-K⁺ ATPase activity in association with a depressed level of intracellular Na⁺ in failing hearts is considered to decrease intracellular Ca²⁺ and serve as an adaptive mechanism for maintaining cardiac function. The slight to moderate depression of Na⁺-K⁺ ATPase by cardiac glycosides in association with an increased level of Na⁺ in cardiomyocytes is known to produce beneficial effects in failing hearts. On the other hand, markedly reduced Na⁺-K⁺ ATPase activity associated with an increased level of intracellular Na⁺ in failing hearts has been demonstrated to result in an intracellular Ca²⁺ overload, the occurrence of cardiac arrhythmias and depression in cardiac function during the development of heart failure. Furthermore, the status of Na⁺-K⁺ ATPase activity in heart failure is determined by changes in isoform subunits of the enzyme, the development of oxidative stress, intracellular Ca²⁺-overload, protease activation, the activity of inflammatory cytokines and sarcolemmal lipid composition. Evidence has been presented to show that marked alterations in myocardial cations cannot be explained exclusively on the basis of sarcolemma alterations, as other Ca²⁺ channels, cation transporters and exchangers may be involved in this event. A marked reduction in Na⁺-K⁺ ATPase activity due to a shift in its isoform subunits in association with intracellular Ca²⁺-overload, cardiac energy depletion, increased membrane permeability, Ca²⁺-handling abnormalities and damage to myocardial ultrastructure appear to be involved in the progression of heart failure.