Influence of dextrans on platelet distribution in arterioles and venules

Dextrans bind to the surface of platelets, red blood cells and endothelium. We investigated whether a low doses (30 mg/kg IV) of 40-kDa (Dx40), neutral, 500-kDa (Dx500) or sulphated, 500-kDa (Dx500S) dextrans influence platelet distribution in rabbit mesenteric arterioles and venules (diameter 17-33 microns). Intravital fluorescence videomicroscopy was used to visualize platelets labelled in vivo with acridine red. Their concentration distribution determined within a thin optical section about the median vessel plane was expressed relative to the mean concentration in that vessel. In arterioles, Dx500 and Dx500S increased the relative platelet concentration in the centre [radial position (R): 0.0-0.4 R] from 0.60 to 1.07 (P < 0.001) and 1.20 (P < 0.003), and reduced it near the wall (0.8-0.9 R) from 1.59 to 0.93 (P < 0.02) and 0.95 (P < 0.03) respectively. In venules a similar, but non-significant, effect was observed. Dx40 did not change platelet distribution in arterioles, but decreased their concentration in venules in the centre from 1.08 to 0.71 (P < 0.03) and increased it at the wall from 0.89 to 1.27 (P < 0.04). The deformability of red blood cells was unchanged, but their aggregation tendency increased approximately two-fold after Dx500 and Dx500S injection, while Dx40 had no influence. Leucocyte margination in venules did not affect platelet distribution. Dextran injection did not change microvascular flow velocity or plasma viscosity, suggesting that the observed changes in arteriolar platelet distribution were caused by binding of dextran to the surface of platelets and/or red blood cells.