Regulation of Na ⁺ ‐K ⁺ ATPase (NKA) α‐isoforms and contractility in smooth muscle: evidence from transgenic mice

NKA is an enzyme that maintains Na+ and K+ gradients, and coupled to Na+-Ca2+ exchange (NCX), modulates Ca2+ homeostasis. It has been hypothesized that the α1-isoform has a “housekeeping” role, whereas the α2-isoform regulates SR Ca2+ loading and hence contractility. We developed mice to express either the α1- or α2-isoform (α1sm+, α2sm+ mice) in smooth muscle using the α-actin promoter, SMP8. Western blot data of aorta and antrum showed that the α1-isoform was elevated by 1.4-fold in α1sm+ mice and the α2-isoform by 4.7-7.4-fold in α2sm+ mice. Interestingly, in both transgenic lines coordinate upregulation of the opposite isoform was observed at the mRNA and protein levels. Immunomicrographs showed that the α-isoforms in α2sm+ aortic smooth muscle cells were distributed similar to WT. PMCA and NCX were elevated ~5-fold in α2sm+ mice, but not actin or myosin light chains. The regulatory β-subunit, thought to be expressed in abundance, was elevated with the β1- and β2-isoforms increased by 1.3- and 1.4-fold. The α2sm+ mice showed altered vascular phenotypes including faster rates of relaxation (95.2 ± 8.6 s vs. 119.4 ± 8.2 s, P = 0.06) and lower systolic blood pressure (109.9 ± 1.6 mmHg vs. 121.3 ± 1.4 mmHg, P<0.05). Thus, preliminary evidence suggests that the NKA α-isoforms, NCX, and PMCA may be regulated as a unit in mouse smooth muscle to coordinately regulate Ca2+ and contractility. Supported by NIH HL66044 (RJP, RML).