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

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.

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 (I_NKA and I_NCX) were measured and α₂-isoform current (I_NKA,α2) was separated from total I_NKA 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. I_NKA,α2 density was reduced by 78% in CHF, in agreement with decreased protein expression (74%). When I_NKA,α2 was blocked in Sham cardiomyocytes, contractile parameters converged with those observed in CHF. In Sham, abrupt activation of I_NKA led to a decrease in I_NCX, 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.

Downregulation of the α₂-isoform causes attenuated control of NCX activity in CHF, reducing its capability to extrude Ca²⁺ from cardiomyocytes.