Abstract
Negative charges in the glycocalyx of red blood cells (RBC) and vascular endothelial cells (EC) facilitate frictionless blood flow through blood vessels. Na+ selectively shields these charges controlling surface electronegativity. The question was addressed whether the ambient Na+ concentration controls RBC-EC interaction. Using atomic force microscopy (AFM) adhesion forces between RBC and endothelial glycocalyx were quantified. A single RBC, mounted on an AFM cantilever, was brought in physical contact with the endothelial surface and then pulled off. Adhesion forces were quantified (i) after enzymatic removal of negative charges in the glycocalyx, (ii) under different ambient Na+ and (iii) after applying the intracellular aldosterone receptor antagonist spironolactone. Removal of negative surface charges increases RBC-EC interaction forces. A stepwise increase of ambient Na+ from 133 to 140 mM does not affect them. However, beyond 140 mM Na+ adhesion forces increase sharply (10% increase of adhesion force per 1 mM increase of Na+). Spironolactone prevents this response. It is concluded that negative charges reduce adhesion between RBC and EC. Ambient Na+ concentration determines the availability of free negative charges. Na+ concentrations in the low physiological range (below 140 mM) allow sufficient amounts of vacant negative charges so that adhesion of RBC to the endothelial surface is small. In contrast, Na+ in the high physiological range (beyond 140 mM) saturates the remaining negative surface charges thus increasing adhesion. Aldosterone receptor blockade by spironolactone prevents Na+ induced RBC adhesion to the endothelial glycocalyx. Extrapolation of in vitro experiments to in vivo conditions leads to the hypothesis that high sodium intake is likely to increase the incidence of thrombotic events.
Highlights
Van der Waals forces comprise the attractive and repulsive forces between molecules other than those due to covalent bonds
A significantly larger force for separating red blood cells (RBC) from the endothelial cell surface is necessary after polymeric heparan sulfate residues of the GC have been degraded by heparinase treatment. This is consistent with the view that after enzymatic removal of negative charges located in the endothelial glycocalyx the attractive forces dominate, i.e., RBC-endothelial cells (EC) adhesion forces are pronounced
When cells were incubated for 4 days in low Na+ culture medium in absence of aldosterone and acutely exposed to high Na+ RBC-EC interaction forces do not change
Summary
Van der Waals forces comprise the attractive and repulsive forces between molecules other than those due to covalent bonds. They include the electrostatic interaction of ions with charged molecules. In the vascular system, such interactions occur selectively between plasma Na+ and the negatively charged glycocalyx (GC) of red blood cells (RBC) and vascular endothelial cells (EC). Since the physiology of blood flow in the vascular system strongly depends on the availability of sufficient amounts of negative surface charges (Vink et al, 1995) we quantified the forces between RBC and EC in different ambient Na+ concentrations. The focus was put on the quantification of forces that occur between RBC and EC in relation to Na+ homeostasis
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