Unusually high concentrations of sialic acids on the surface of vascular endothelia

Sialic acids present on luminal surfaces of vascular endothelium were determined by perfusing neuraminidase free of proteolytic activity through carotid arteries, iliac arteries and jugular veins of anaesthetized rabbits and guinea-pigs and through human umbilical veins. Total sialic acids released in I h from arteries and veins, determined fluorimetrically, were 24-51 X 10(6) molecules/micron 2 endothelial surface; this was more, by up to two orders of magnitude, than sialic acids releasable by neuraminidase from other types of cells, i.e. from 0.15 X 10(6) for human erythrocytes to 15 X 10(6) for human platelets. Sialic acids therefore provide extraordinarily high negative charge densities on vascular surfaces exposed to the flowing blood. As all circulating cells are also negatively charged, strong electrostatic repulsion must exist between them and normal vessel walls. These observations can therefore account for the general property of non-adherence of circulating cells in normal blood vessels of which the so-called 'non-thrombogenicity', meaning the non-adherence of platelets to normal vascular endothelium, is one example. It is suggested that a major biological function of these extraordinarily high negative charge concentrations is the mutual repulsion between endothelial surfaces and blood cells which promotes their unimpeded circulation.