The Na+/K+-ATPase α2-isoform regulates cardiac contractility in rat cardiomyocytes

Objective: The presence of both alpha(1)- and alpha(2)-isoforms of the Na+/K+-ATPase (NKA) in cardiomyocytes indicates different functions. We hypothesized that preferential localization of the alpha(2)-isoform to the t-tubules, locally controlling the Na+/Ca2+-exchanger (NCX), underlies a specific role in Ca2+ handling. Methods: We studied NKA isoform distribution in isolated cardiomyocytes from Wistar rats using immunocytochemistry. NKA pump and NCX currents (I-pump and I-NCX) were measured in control and detubulated cardiomyocytes. Intracellular Na+ concentration [Na+](i) was assessed with the fluorescent dye SBFI. Results: The alpha(2)-isoform abundance was higher in the t-tubules than in the surface sarcolemma. We established that 0.3 PM ouabain specifically blocked the alpha(2)-isoform in isolated rat cardiomyocytes. This low concentration blocked 10.7 +/- 0.6% of I-pump in control, but only 6.0 +/- 0.5% in detubulated cardiomyocytes. Moreover, measured and calculated alpha(1)-specific and alpha(2)-specific I-pump in control (547 +/- 29 pA and 66 pA, respectively) and in detubulated cells (495 +/- 30 pA and 31 pA, respectively) showed that 53% of the alpha(2)-isoform, but only 9.5% of the alpha(1)-isoform, were localized to the t-tubules. Despite the small abundance of the alpha(2)-isoform (similar to 11% of total NKA), selective inhibition of this isoform induced a 40% increase in contractility in field stimulated cardiomyocytes, but no increase in global [Na+](i). However, inhibition of the alpha(2)-isoform increased I-NCX indicating local subsarcolemmal accumulation of Na+ near NCX. Conclusions: The alpha(2)-isoform of the NKA is functionally coupled to the NCX and can regulate Ca2+ handling without changing global [Na+](i). (c) 2007 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.