Sodium selective erythrocyte glycocalyx and salt sensitivity in man

Negatively charged surfaces of erythrocytes (RBC) reflect properties of the endothelial glycocalyx. Plasma electrolytes counteract these charges and thus control the repulsive forces between RBC and endothelium. Although Na+ is supposed to exert a rather high affinity to the RBC surface, a direct comparison between Na+ and K+ in counteracting the RBC surface has been never made. Therefore, we measured Na+/K+ selectivity of the RBC surface in 20 healthy volunteers applying the previously published salt blood test (SBT). It turned out that the Na+/K+ selectivity ratio of the RBC glycocalyx is on average 6.1 +/- 0.39 (ranging from 3 to 9 in different individuals). Considering standard plasma Na+ and K+ concentrations, binding probability of Na+/K+ at the RBC surface is about 180:1. The SBT reveals that plasma K+ counteracts only about 7 % of the negative charges in the RBC glycocalyx. As an in vivo proof of principle, a volunteer's blood was continuously tested over 6 months while applying a glycocalyx protective polyphenol-rich natural compound (hawthorn extract). It turned out that RBC Na+ sensitivity (the inverse of Na+ buffer capacity) decreased significantly by about 25 % while Na+/K+ selectivity of the RBC glycocalyx declined only slightly by about 8 %. Taken together, (i) plasma Na+ selectively buffers the negative charges of the RBC glycocalyx, (ii) the contribution of K+ in counteracting these negative surface charges is small, and (iii) natural polyphenols applied in vivo increase RBC surface negativity. In conclusion, low plasma Na+ is supposed to favor frictionless RBC-slipping through blood vessels.