The Na+-K+-ATPase α2-subunit isoform modulates contractility in the perinatal mouse diaphragm

Abstract

This study uses genetically altered mice to examine the contribution of the Na⁺-K⁺-ATPase α₂ catalytic subunit to resting potential, excitability, and contractility of the perinatal diaphragm. The α₂ protein is reduced by 38% in α₂-heterozygous and absent in α₂-knockout mice, and α₁-isoform is upregulated 1.9-fold in α₂-knockout. Resting potentials are depolarized by 0.8–4.0 mV in heterozygous and knockout mice. Action potential threshold, overshoot, and duration are normal. Spontaneous firing, a developmental function, is impaired in knockout diaphragm, but this does not compromise its ability to fire evoked action potential trains, the dominant mode of activation near birth. Maximum tetanic force, rate of activation, force-frequency and force-voltage relationships, and onset and magnitude of fatigue are not changed. The major phenotypic consequence of reduced α₂ content is that relaxation from contraction is 1.7-fold faster. This finding reveals a distinct cellular role of the α₂-isoform at a step after membrane excitation, which cannot be restored simply by increasing α₁ content. Na⁺/Ca²⁺ exchanger expression decreases in parallel with α₂-isoform, suggesting that Ca²⁺ extrusion is affected by the altered α₂ genotype. There are no major compensatory changes in expression of sarcoplasmic reticulum Ca²⁺-ATPase, phospholamban, or plasma membrane Ca²⁺-ATPase. These results demonstrate that the Na⁺-K⁺-ATPase α₁-isoform alone is able to maintain equilibrium K⁺ and Na⁺ gradients and to substitute for α₂-isoform in most cellular functions related to excitability and force. They further indicate that the α₂-isoform contributes significantly less at rest than expected from its proportional content but can modulate contractility during muscle contraction.