The Na(+)-K(+)-ATPase alpha2-subunit isoform modulates contractility in the perinatal mouse diaphragm

This study uses genetically altered mice to examine the contribution of the Na(+)-K(+)-ATPase alpha2 catalytic subunit to resting potential, excitability, and contractility of the perinatal diaphragm. The alpha2 protein is reduced by 38% in alpha2-heterozygous and absent in alpha2-knockout mice, and alpha1-isoform is upregulated 1.9-fold in alpha2-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 alpha2 content is that relaxation from contraction is 1.7-fold faster. This finding reveals a distinct cellular role of the alpha2-isoform at a step after membrane excitation, which cannot be restored simply by increasing alpha1 content. Na+/Ca2+ exchanger expression decreases in parallel with alpha2-isoform, suggesting that Ca2+ extrusion is affected by the altered alpha2 genotype. There are no major compensatory changes in expression of sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, or plasma membrane Ca(2+)-ATPase. These results demonstrate that the Na(+)-K(+)-ATPase alpha1-isoform alone is able to maintain equilibrium K+ and Na+ gradients and to substitute for alpha2-isoform in most cellular functions related to excitability and force. They further indicate that the alpha2-isoform contributes significantly less at rest than expected from its proportional content but can modulate contractility during muscle contraction.