Capillary permeability of sulphate-substituted and neutral dextran fractions in the rat hindquarter vascular bed.

In order to investigate the impact of molecular charge on capillary permeability to macromolecules, capillary reflection coefficients (sigma) for various concentrations of two neutral dextran fractions (Mw 70 000 and 139 000) and one negatively charged, sulphate-substituted dextran fraction (Mw 109 000) were determined in the isolated perfused rat hindquarter preparation, during stable haemodynamic conditions. The capillary reflection coefficient was calculated as the ratio between the estimated colloid osmotic pressure in vivo, determined by an osmotic transient technique, and that in vitro, measured with an osmometer. Despite its intermediate size the sulphated dextran (D 109-S) showed a markedly higher sigma than the two neutral ones (D 70 and D 139) and, further, whereas sigma for D 70 and D 139 decreased with increasing dextran concentration, sigma for D 109-S remained constant over the whole range of concentrations investigated. At dextran concentrations equivalent to an in vivo colloid osmotic pressure of 23.5 mmHg, sigma averaged 0.901 for D 109-S, compared with 0.547 and 0.693 for D 70 and D 139, respectively, and with 0.87 for albumin in normal plasma. It is suggested that capillary macromolecular permeability is due to both structural and electrophysical properties of the microvascular barrier, where negative charges of the endothelial glycocalyx and of wall-adsorbed plasma proteins cause repulsion of circulating large polyanions.