Introduction
The relative expression of A1- and A2- Na-K ATPase isoforms, found in VSM, is developmentally regulated and under hormonal & neurogenic control. The physiological role of these isoforms is largely unknown. It is postulated that A1 serves a “housekeeping” role, whereas A2 is reported to be localized to a subsarcolemmal compartment and modulates contractility.
Methods
To test this hypothesis, isoform specific, gene-targeted mice were developed, in which the mRNA for either the A1 or A2 Na-K ATPase isoform has been ablated. Vascular contractility and metabolism were studied in the aorta from these mice.
Results
A1-/- and A2-/- are lethal, the latter dies at birth but neonatal aorta could be studied. Isometric force in A2-/- aorta was more sensitive to contractile agonists and less sensitive to the vasodilator, sodium nitroprusside, than wild type (WT); A2+/- were intermediate. In contrast, neonatal A1+/- were similar to WT. We also studied metabolism, Jlac, JO2, and contractility in the adult +/- mice. Western analysis indicates that total Na-K ATPase protein in +/- animals is reduced compared to WT. In the absence of glucose (but with B-hydroxybutyrate as substrate), force to phenylephrine was inhibited by 40% in A2+/- and WT; but strikingly A1+/- were unaffected. Jlac was ouabain-sensitive and showed isoform specific differences consistent with their ouabain affinities. JO2 and force were measured under basal and KCl, phenylephrine, and ouabain stimulated conditions. Basal JO2 is significantly higher in the A2+/- compared to WT or A1+/- (P< 0.05). Contractile economy is significantly increased in the A1+/- under stimulated conditions.
Conclusions
Taken together, these data suggest that individual A-isoforms of the Na-K ATPase differ both functionally and with regard to their energetic requirements. The A2-isoform appears to be more strongly coupled to activation pathways. Supported by APS fellowship (DAS).
References