Regulation of Na + ,K + -ATPase α-Subunit Expression by Mechanical Strain in Aortic Smooth Muscle Cells

Abstract

We have previously demonstrated that vascular sodium pump activity is stimulated in several rat models of hypertension. In addition, others have reported an upregulation of mRNA for the Na ⁺ ,K ⁺ -ATPase α ₁ -subunit in hypertension. To test the effect of sustained, cyclic, stretch-relaxation stimuli on the expression of α ₁ - and α ₂ -subunits of Na ⁺ ,K ⁺ -ATPase in vascular smooth muscle cells, we used the Flexercell strain unit to stretch rat aortic smooth muscle cells for several days on a collagen-coated silicone elastomer substratum. Six-second cycles of stretch-relaxation were applied to obtain 10% average surface elongation (22% maximum) for 4 days. Control cells were not stretched but were grown on a similar surface. The effect of Gd ³⁺ , a blocker of stretch-activated channels, was also investigated. At the end of 4 days, protein expression of α ₁ - and α ₂ -subunits was determined by Western blot analysis. Intensity of the bands for α ₁ - and α ₂ -subunits was quantified with the use of a computerized image analyzer. In the stretched cells, both the α ₁ - and the α ₂ -subunit protein-band intensities were significantly increased compared with those of the nonstretched cells. Treatment with 50 μmol/L Gd ³⁺ during the application of stretch prevented the upregulation of α ₂ -expression but not that of α ₁ -expression. Sodium pump activity, the functional counterpart of Na ⁺ ,K ⁺ -ATPase, was inhibited as a result of stretch; Gd ³⁺ had no effect on this variable. Our results suggest that in vascular smooth muscle, stretch may be a signal for the upregulation of both the α ₁ - and α ₂ -isoforms. However, a differential response of the two isoforms to the blocker of stretch-activated channels implies involvement of different mechanisms. This alteration in protein expression is not reflected in the function of the enzyme.