Effect of acute hyaluronidase treatment of the glycocalyx on tracer-based whole body vascular volume estimates in mice

The endothelial glycocalyx forms a hyaluronan-containing interface between the flowing blood and the endothelium throughout the body. By comparing the systemic distribution of a small glycocalyx-accessible tracer vs. a large circulating plasma tracer, the size-selective barrier properties of the glycocalyx have recently been utilized to estimate whole body glycocalyx volumes in humans and animals, but a comprehensive validation of this approach has been lacking at the moment. In the present study, we compared, in anesthetized, ventilated C57Bl/6 mice, the whole body distribution of small (40 kDa) dextrans (Texas Red labeled; Dex40) vs. that of intermediate (70 kDa) and large (500 kDa) dextrans (both FITC labeled; Dex70 and Dex500, respectively) using tracer dilution and vs. that of circulating plasma, as derived from the dilution of fluorescein-labeled red blood cells and large-vessel hematocrit. The contribution of the glycocalyx was evaluated by intravenous infusion of a bolus of the enzyme hyaluronidase. In saline-treated control mice, distribution volume (in ml) differed between tracers (P <0.05; ANOVA) in the following order: Dex40 (0.97 +/- 0.04) > Dex70 (0.90 +/- 0.04) > Dex500 (0.81 +/- 0.10) > plasma (0.71 +/- 0.02), resulting in an inaccessible vascular volume, i.e., compared with the distribution volume of Dex40, of 0.03 +/- 0.01, 0.15 +/- 0.04, and 0.31 +/- 0.05 ml for Dex70, Dex500, and plasma, respectively. In hyaluronidase-treated mice, Dex70 and Dex40 volumes were not different from each other, and inaccessible vascular volumes for Dex500 (0.03 +/- 0.03) and plasma (0.14 +/- 0.05) were smaller (P <0.05) than those in control animals. Clearance of Dex70 and Dex500 from the circulation was enhanced (P <0.05) in hyaluronidase-treated vs. control mice. These results indicate that the glycocalyx contributes to size-dependent differences in whole body vascular distribution of plasma solutes in mice. Whole body vascular volume measurements based on the differential distribution of glycocalyx-selective tracers appear appropriate for the detection of generalized glycocalyx degradation in experimental animals and humans.