Na(+)/K)+)-ATPase α2-isoform preferentially modulates Ca2(+) transients and sarcoplasmic reticulum Ca2(+) release in cardiac myocytes.
OBJECTIVE
Na(+)/K(+)-ATPase (NKA) is essential in regulating [Na(+)](i), and thus cardiac myocyte Ca(2+) and contractility via Na(+)/Ca(2+) 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.


RESULTS
The 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), Ca(2+) transients, and the fractional sarcoplasmic reticulum (SR) Ca(2+) release in cardiac myocytes from wild-type (WT; NKA-α2 inhibition) and SWAP mice (selective NKA-α1 block). At baseline, Na(+) and Ca(2+) regulations are similar in WT and SWAP mice. For equal levels of total NKA inhibition (~25%), ouabain significantly increased Ca(2+) transients (from ΔF/F(0)= 1.5 ± 0.1 to 1.8 ± 0.1), and fractional SR Ca(2+) 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.


CONCLUSIONS
NKA-α2 has a more prominent role (vs. NKA-α1) in modulating cardiac myocyte SR Ca(2+) release.
Department of Pharmacology,  University of California at Davis, Genome Building Rm 3513, Davis, CA 95616-8636, USA.
Sanda
S
Despa
Jerry B
JB
Lingrel
Donald M
DM
Bers
D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007527:Isoenzymes; D008564:Membrane Potentials; D051379:Mice; D008822:Mice, Transgenic; D009154:Mutation; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000201:enzymology*; D010042:Ouabain / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology*; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*
D004791:Enzyme Inhibitors; D007527:Isoenzymes; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D012964:Sodium; C401838:Atp1a2 protein, mouse; C497344:Atp1a1 protein, mouse; D000254:Sodium-Potassium-Exchanging ATPase