Sodium binding in the arterial wall

There are two fractions of sodium in the arterial wall which can be selectively and reversibly exchanged for other ions. These fractions are believed to be bound to the acid mucopolysaccharide – protein component of the paracellular matrix. The first fraction seems to be loosely bound by weakly acidic groups, probably the carboxyl groups of the hexuronic acid moieties. It can be exchanged for hydrogen even in neutral solutions by reducing the concentration of sodium ions in the medium. In the rat tail artery, the average size of this fraction did not exceed 0.5 meq/100 g of dry fat-free tissue. The second fraction behaves as if it is more tightly bound, apparently by strongly acidic groups like the sulfo groups of the mucopolysaccharides. It amounts to 2.21 ± 0.20 meq/100 g of dry fat-free tissue if a simplified system of sodium and calcium chloride solutions is used. In the presence of normal plasma concentrations of potassium and magnesium, the sum of both sodium fractions is increased substantially and is equal to approximately 5 meq/100 g of dry fat-free tissue. This bound sodium is increased in arteries of immature animals and animals with established DOCA hypertension, both of which have a higher content of the acid mucopolysaccharides.