The Na⁺-K⁺ ATPase (NKA) is a transmembrane protein that sets and maintains the electrochemical gradient by extruding three Na⁺ ions in exchange for two K⁺ ions. An important physiological role proposed for vascular smooth muscle NKA is the regulation of blood pressure via modulation of vascular smooth muscle contractility (5). To investigate the relations between the level of NKA in smooth muscle and blood pressure, we developed mice carrying a transgene for either the NKA 1- or 2-isoform (1_sm+ or 2_sm+ mice) driven by the smooth muscle specific -actin promoter, SMP8. Interestingly, both -isoforms, the one contained in the transgene as well as the one not contained, were increased to a similar degree at both protein and mRNA levels. The total -isoform protein was increased 1.5-fold (1_sm+ mice) to 7-fold (2_sm+ mice). The increase in total NKA -isoform protein was accompanied by a 2.5-fold increase in NKA activity in 2_sm+ gastric antrum. Immunocytochemistry of the 1- and 2-isoforms in 2_sm+ aortic smooth muscle cells indicated -isoform distributions were similar to that in wild-type cells. 2_sm+ mice (high expression) were hypotensive (109.9 ± 1.6 vs. 121.3 ± 1.4 mmHg, n=13-11), whereas 1_sm+ mice (low expression) were normotensive (122.7 ± 2.5 vs. 117.4 ± 2.3, n=11-12). 2_sm+, but not 1_sm+, aorta relaxed faster from a KCl-induced contraction than wild-type aorta. Our results show that smooth muscle displays unique coordinate expression of the -isoforms. Increasing smooth muscle NKA decreases blood pressure, and is dependent on the degree of increased -isoform expression.