Altered Na+/Ca2+-exchanger activity due to downregulation of Na+/K+-ATPase α2-isoform in heart failure

Aims The Na+/K+-ATPase (NKA) α2-isoform is preferentially located in the t-tubules of cardiomyocytes and is functionally coupled to the Na+/Ca2+-exchanger (NCX) and Ca2+ regulation through intracellular Na+ concentration ([Na+]i). We hypothesized that downregulation of the NKA α2-isoform during congestive heart failure (CHF) disturbs the link between Na+ and Ca2+, and thus the control of cardiomyocyte contraction. Methods and results NKA isoform and t-tubule distributions were studied using immunocytochemistry, confocal and electron microscopy in a post-infarction rat model of CHF. Sham-operated rats served as controls. NKA and NCX currents (INKA and INCX) were measured and α2-isoform current (INKA,α2) was separated from total INKA using 0.3 µM ouabain. Detubulation of cardiomyocytes was performed to assess the presence of α2-isoforms in the t-tubules. In CHF, the t-tubule network had a disorganized appearance in both isolated cardiomyocytes and fixed tissue. This was associated with altered expression patterns of NKA α1- and α2-isoforms. INKA,α2 density was reduced by 78% in CHF, in agreement with decreased protein expression (74%). When INKA,α2 was blocked in Sham cardiomyocytes, contractile parameters converged with those observed in CHF. In Sham, abrupt activation of INKA led to a decrease in INCX, presumably due to local depletion of [Na+]i in the vicinity of NCX. This decrease was smaller when the α2-isoform was downregulated (CHF) or inhibited (ouabain), indicating that the α2-isoform is necessary to modulate local [Na+]i close to NCX. Conclusion Downregulation of the α2-isoform causes attenuated control of NCX activity in CHF, reducing its capability to extrude Ca2+ from cardiomyocytes.