Feedforward activation of endothelial ENaC by high sodium.

Kidney epithelial sodium channels (ENaCs) are known to be inactivated by high sodium concentrations (feedback inhibition). Recently, the endothelial sodium channel (EnNaC) was identified to control the nanomechanical properties of the endothelium. EnNaC-dependent endothelial stiffening reduces the release of nitric oxide, the hallmark of endothelial dysfunction. To study the regulatory impact of sodium on EnNaC, endothelial cells (EA.hy926 and ex vivo mouse endothelium) were incubated in aldosterone-free solutions containing either low (130 mM) or high (150 mM) sodium concentrations. By applying atomic force microscopy-based nanoindentation, an unexpected positive correlation between increasing sodium concentrations and cortical endothelial stiffness was observed, which can be attributed to functional EnNaC. In particular, an acute rise in sodium concentration (+20 mM) was sufficient to increase EnNaC membrane abundance by 90% and stiffening of the endothelial cortex by 18%. Despite the absence of exogenous aldosterone, these effects were prevented by the aldosterone synthase inhibitor FAD286 (100 nM) or the mineralocorticoid receptor (MR)-antagonist spironolactone (100 nM), indicating endogenous aldosterone synthesis and MR-dependent signaling. Interestingly, in the presence of high-sodium concentrations, FAD286 increased the transcription of the MR by 69%. Taken together, a novel feedforward activation of EnNaC by sodium is proposed that contrasts ENaC feedback inhibition in kidney.

Firewall function of the endothelial glycocalyx in the regulation of sodium homeostasis.

Plasma sodium, slightly above normal and in presence of aldosterone, stiffens vascular endothelium and reduces nitric oxide release with the consequence of endothelial dysfunction. This process is mediated by epithelial sodium channels (ENaC) and, most likely, the endothelial Na(+)/K(+)-ATPase. Both, ENaC and Na(+)/K(+)-ATPase, are located in the plasma membrane of endothelial cells and embedded in the endothelial glycocalyx (eGC). This negatively charged biopolymer is directly exposed to the blood stream and selectively buffers sodium ions. We hypothesize that the glycocalyx could interfere with endothelial sodium transport when extracellular sodium varies in the physiological range. Therefore, we modeled the endothelial cell as a pump-leak system measuring changes of intracellular sodium in cultured human endothelial cells. Experiments were performed under low/high extracellular sodium conditions before and after enzymatic eGC removal, and with inhibition of Na(+)/K(+)-ATPase and ENaC, respectively. Three major observations were made: (1) eGC removal by heparinase treatment facilitates sodium to enter/exit the endothelial cells. (2) The direction of net sodium movement across the endothelial plasma membrane depends on the concentration of extracellular sodium which regulates both the Na(+)/K(+)-ATPase and ENaC activity. (3) Removal of eGC and inhibition of sodium transport modify the electrical resistance of endothelial cells. We conclude that the eGC serves as a potential "firewall" preventing uncontrolled access of sodium to the pump-leak system of the endothelial cell. After eGC removal, sodium access to the system is facilitated. Thus the pump-leak system could be regulated by ambient sodium and control vascular permeability in pathophysiological conditions.

Salt overload damages the glycocalyx sodium barrier of vascular endothelium.

Sodium overload stiffens vascular endothelial cells in vitro and promotes arterial hypertension in vivo. The hypothesis was tested that the endothelial glycocalyx (eGC), a mesh of anionic biopolymers covering the surface of the endothelium, participates in the stiffening process. By using a mechanical nanosensor, mounted on an atomic force microscope, height (∼400 nm) and stiffness (∼0.25 pN/nm) of the eGC on the luminal endothelial surface of split-open human umbilical arteries were quantified. In presence of aldosterone, the increase of extracellular sodium concentration from 135 to 150 mM over 5 days (sodium overload) led the eGC shrink by ∼50% and stiffening by ∼130%. Quantitative eGC analyses reveal that sodium overload caused a reduction of heparan sulphate residues by 68% which lead to destabilization and collapse of the eGC. Sodium overload transformed the endothelial cells from a sodium release into a sodium-absorbing state. Spironolactone, a specific aldosterone antagonist, prevented these changes. We conclude that the endothelial glycocalyx serves as an effective buffer barrier for sodium. Damaged eGC facilitates sodium entry into the endothelial cells. This could explain endothelial dysfunction and arterial hypertension observed in sodium abuse.