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

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 .