In the presence of aldosterone, high extracellular sodium concentrations increase the surface abundance of the epithelial Na⁺ channel (ENaC) and the stiffness of the cell cortex in vascular endothelial cells. Mechanical stiffening of the endothelial cell cortex, in turn, was found to reduce the release of the vasodilator nitric oxide (endothelial dysfunction). How changes in sodium concentration are translated into cellular processes is so far unknown. Here, the hypothesis was tested that high dietary salt concentrations increase endothelial cortical stiffness and ENaC surface abundance per se (i.e. in the absence of aldosterone), and thus determine endothelial function.

Therefore, experiments were performed on endothelial cells (ex vivo aorta preparations) of wild type (WT) and aldosterone synthase (Cyp11b2) deficient mice (AS-/-). Mechanical stiffness of the cell cortex was measured using atomic force microscopy. αENaC surface abundance was determined by quantum dot-based immunofluorescence.

We found that stiffness of the endothelial cortex and αENaC surface abundance were significantly reduced in AS-/- (-31% and -30%, respectively), compared to WT cells (ambient Na⁺-concentration of 135mM). Exposure to high ambient Na⁺ (150mM) caused an increase in endothelial cortical stiffness (WT: +26%; AS-/-: +41%) and ENaC surface density (WT: +55%; AS-/-: +42%), relative to the 135mM Na⁺ condition. Interestingly, the specific ENaC blocker benzamil decreased stiffness in both, AS-/- and WT cells, whereas the mineralocorticoid receptor antagonist spironolactone only reduced stiffness of WT cells.

We conclude that high extracellular sodium per se controls ENaC surface abundance and cortical stiffness of endothelial cells by an MR- and aldosterone-independent mechanism.