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

Abstract

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.

How does salt retention raise blood pressure?

“…The Paul laboratory, which previously reported that the ␣ 2 Na ϩ pump couples to contractility in the aorta (106), also reported that reduced ␣ 2 expression (␣ 2 ϩ/Ϫ ) accelerated the induction of DOCA-salt hypertension (107).…”

“…In striking contrast, at least in some cells types, including vascular smooth muscle myocytes, neurons, and glia, ␣ 2 and ␣ 3 are confined to PM microdomains closely juxtaposed to the underlying sarco(endo)plasmic reticulum (SER) (56,57,106). Indeed, preliminary results suggest that ␣ 1 may be excluded from these junctional microdomains (65).…”

“…The half-maximally effective concentration of ouabain (EC 50 ) was 1.3 nM (120). This ouabain concentration should inhibit just the ␣ 2 Na ϩ pumps in mouse artery myocytes, which express only ␣ 1 and ␣ 2 Na ϩ pumps (106,120).…”

“…But the NCX and ␣ 2 Na ϩ pumps are colocalized in PM microdomains at PM-SER junctions (Fig. 2) (56,106) where these two transport systems appear to function cooperatively (4,30 (20), and this increases myogenic tone (50,120) by a direct effect on the contractile machinery.…”

Local subplasma membrane Ca 2+ signals detected by a tethered Ca 2+ sensor

“…There are four ␣-subunit isoforms (18). Na ϩ pumps with an ␣1 and those with an ␣2 are both expressed in astrocytes and in arterial myocytes (19)(20)(21). In both cell types, ␣2 Na ϩ pumps are confined to PM microdomains that overlie the jS͞ER, whereas ␣1 Na ϩ pumps are more uniformly distributed in the PM (22)(23)(24).…”