Morphological and functional evidence for an important role of the endothelial cell glycocalyx in the glomerular barrier

Abstract

In this study, we pursued the somewhat controversial issue whether the glycosaminoglycans (GAG) in the endothelial cell glycocalyx are important for glomerular size and charge selectivity. In isoflurane-anesthetized mice, Intralipid droplets were used as indirect markers of the glomerular endothelial cell-surface layer, i.e., the glycocalyx. The mice were given intravenous injections of GAG-degrading enzymes, which due to their high molecular weight remained and acted intravascularly. Flow-arrested kidneys were fixed and prepared for electron microscopy, and the distance between glomerular endothelial cells and the luminal Intralipid droplets was measured. The relative frequency of Intralipid droplets was calculated for each 50-nm increment zone up to 500 nm from the endothelial cell membrane surface as were the mean distances. Glomerular size and charge selectivity were estimated from the clearance data for neutral Ficolls (molecular radii of 12–72 Å), and albumin in isolated kidneys was perfused at 8°C. In enzyme-treated animals (hyaluronidase, heparinase, and chondroitinase), the relative Intralipid droplet frequency in the zone closest to the endothelial cells, i.e., 0–50 nm, was increased ∼2.5 times compared with controls. Also, the mean distance between the Intralipid droplets and the endothelium was decreased from 176 to 115–122 nm by enzyme treatment. These changes were accompanied by an increase in the fractional clearance for albumin. In conclusion, both morphological and functional measurements suggest the endothelial cell glycocalyx to be an important component of the glomerular barrier.