Na⁺/K⁺-ATPase α2-isoform preferentially modulates Ca²⁺ transients and sarcoplasmic reticulum Ca²⁺ release in cardiac myocytes

Aims Na⁺/K⁺-ATPase (NKA) is essential in regulating [Na⁺]i, and thus cardiac myocyte Ca²⁺ and contractility via Na⁺/Ca²⁺ exchange. Different NKA-α subunit isoforms are present in the heart and may differ functionally, depending on specific membrane localization. In smooth muscle and astrocytes, NKA-α2 is located at the junctions with the endo(sarco)plasmic reticulum, where they could regulate local [Na⁺], and indirectly junctional cleft [Ca ²⁺]. Whether this model holds for cardiac myocytes is unclear. Methods and resultsThe ouabain-resistant NKA-α1 cannot be selectively blocked to assess its effect. To overcome this, we used mice in which NKA-α1 is ouabain sensitive and NKA-α2 is ouabain resistant (SWAP mice). We measured the effect of ouabain at low concentration on [Na ⁺]i, Ca²⁺ transients, and the fractional sarcoplasmic reticulum (SR) Ca²⁺ release in cardiac myocytes from wild-type (WT; NKA-α2 inhibition) and SWAP mice (selective NKA-α1 block). At baseline, Na⁺ and Ca²⁺ regulations are similar in WT and SWAP mice. For equal levels of total NKA inhibition (∼25%), ouabain significantly increased Ca²⁺ transients (from ΔF/F0 1.5 ± 0.1 to 1.8 ± 0.1), and fractional SR Ca²⁺ release (from 24 ± 3 to 29 ± 3) in WT (NKA-α2 block) but not in SWAP myocytes (NKA-α1 block). This occurred despite a similar and modest increase in [Na⁺]i (∼2 mM) in both groups. The effect in WT mice was mediated specifically by NKA-α2 inhibition because at a similar concentration ouabain had no effect in transgenic mice where both NKA-α1 and NKA-α2 are ouabain resistant. ConclusionNKA-α2 has a more prominent role (vs. NKA-α1) in modulating cardiac myocyte SR Ca ²⁺ release.