Capillary permeability to sulphate‐substituted and neutral dextran fractions in the rat hindquarter vascular bed

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Abstract
In order to investigate the impact of molecular charge on capillary permeability to macro‐molecules, capillary reflection coefficients (a) for various concentrations of two neutral dextran fractions (M̄w 70 000 and 139 000) and one negatively charged, sulphate‐substituted dextran fraction (M̄w 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 a than the two neutral ones (D 70 and D 139) and, further, whereas o for D 70 and D 139 decreased with increasing dextran concentration, o 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, s̀ 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 poly anions.