Differentiated micro domains on the luminal surface of the capillary endothelium

Abstract

The distribution of anionic sites on the luminal surface of the fenestrated endothelium of intestinal and pancreatic blood capillaries was investigated using cationized ferritin (CF), pI 8.4, as a probe. CF was administered in vivo by i.v. injection, or added to perfusates in in situ experiments. Anionic sites were found distributed in differentiated micro domains clearly associated with structural elements involved in capillary permeability. The affinity and intensity of CF labelling decreased in this order: 1- fenestral diaphragms, 2- coated pits and vesicles, 3- plasmalemma proper. Neither the membranes, nor the associated stomatal diaphragms of plasmalemmal vesicles and transendothelial channels were labelled by CF.

All anionic sites could be removed from the luminal surface of the endothelium by perfusion with protases of broad specificity (pronase, papain). The anionic sites of the fenestral diaphragms were preferentially removed by perfusion with heparinase, an indication that they are contributed by heparan sulfates. The implications of these findings are discussed in relation to capillary permeability to charged macromolecules, primarily plasma proteins.