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

The relative expression of alpha(1)- and alpha(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 alpha(1)-isoform serves a "housekeeping" role, whereas the alpha(2)-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 alpha(1)- or the alpha(2)-Na+/ K+-ATPase isoform was ablated were utilized. Both of these knockouts, alpha(1)(-/-) and alpha(2)(-/-), are lethal; the latter dies at birth, which allows this neonatal aorta to be studied. Isometric force in alpha(2)(-/-)- aorta was more sensitive to contractile agonists and less sensitive to the vasodilators forskolin and sodium nitroprusside (SNP) than wild-type (WT) aorta; alpha(2)(+/-)- aortas had intermediate values. In contrast, neonatal alpha(1)(+/-)- aorta was similar to WT. Western blot analysis indicated a population of 70% alpha(1)- and 30% alpha(2)-isoforms in the WT. Thus in terms of the total Na+/ K+-ATPase protein, the alpha(2)(-/-)- aorta (at 70%) would be similar to the alpha(1)(+/-)- aorta (at 65%) but with a dramatically different phenotype. These data suggest that individual alpha-isoforms of the Na+/K+- ATPase differ functionally and that the alpha(2)-isoform couples more strongly to activation-relaxation pathways. Three-dimensional image-acquisition and deconvolution analyses suggest that the alpha(2)-isoform is distributed differently than the alpha(1)-isoform. Importantly, these isoforms do not localize to the same regions.