Na⁺ pump α₂-isoform specifically couples to contractility in vascular smooth muscle: Evidence from gene-targeted neonatal mice

The relative expression of α₁- and α ₂-Na⁺/K⁺-ATPase isoforms found in vascular smooth muscle is developmentally regulated and under hormonal and neurogenic control. The physiological roles of these isoforms in vascular function are not known. It has been postulated that the α₁-isoform serves a "housekeeping" role, whereas the α₂-isoform localizes to a subsarcolemmal compartment and modulates contractility. To test this hypothesis, isoform-specific gene-targeted mice in which the mRNA for either the α₁- or the α₂-Na⁺/K ⁺-ATPase isoform was ablated were utilized. Both of these knockouts, α₁^-/- and α₂^-/-, are lethal; the latter dies at birth, which allows this neonatal aorta to be studied. Isometric force in α₂^-/--aorta was more sensitive to contractile agonists and less sensitive to the vasodilators forskolin and sodium nitroprusside (SNP) than wild-type (WT) aorta; α ₂^+/--aortas had intermediate values. In contrast, neonatal α₁^+/--aorta was similar to WT. Western blot analysis indicated a population of 70% α₁- and 30% α₂-isoforms in the WT. Thus in terms of the total Na ⁺/K⁺-ATPase protein, the α₂ ^-/--aorta (at 70%) would be similar to the α₁ ^+/--aorta (at 65%) but with a dramatically different phenotype. These data suggest that individual α-isoforms of the Na⁺/K ⁺-ATPase differ functionally and that the α ₂-isoform couples more strongly to activation-relaxation pathways. Three-dimensional image-acquisition and deconvolution analyses suggest that the α₂-isoform is distributed differently than the α ₁-isoform. Importantly, these isoforms do not localize to the same regions.