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

Abstract

Objective: The presence of both α1- and α2-isoforms of the Na+/K+-ATPase (NKA) in cardiomyocytes indicates different functions. We hypothesized that preferential localization of the α2-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 (Ipump and INCX) were measured in control and detubulated cardiomyocytes. Intracellular Na+ concentration [Na+]i was assessed with the fluorescent dye SBFI. Results: The α2-isoform abundance was higher in the t-tubules than in the surface sarcolemma. We established that 0.3 μM ouabain specifically blocked the α2-isoform in isolated rat cardiomyocytes. This low concentration blocked 10.7 ± 0.6% of Ipump in control, but only 6.0 ± 0.5% in detubulated cardiomyocytes. Moreover, measured and calculated α1-specific and α2-specific Ipump 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 α2-isoform, but only 9.5% of the α1-isoform, were localized to the t-tubules. Despite the small abundance of the α2-isoform (∼ 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 α2-isoform increased INCX indicating local subsarcolemmal accumulation of Na+ near NCX. Conclusions: The α2-isoform of the NKA is functionally coupled to the NCX and can regulate Ca2+ handling without changing global [Na+]i. © 2007 European Society of Cardiology.