The hydrodynamic radii of macromolecules and their effect on red blood cell aggregation

Abstract

The effects of nonionic polymers on human red blood cell (RBC) aggregation were investigated. The hydrodynamic radius (Rh) of individual samples of dextran, polyvinylpyrrolidone, and polyoxyethylene over a range of molecular weights (1,500-2,000,000) were calculated from their intrinsic viscosities using the Einstein viscosity relation and directly measured by quasi-elastic light scattering, and the effect of each polymer sample on RBC aggregation was studied by nephelometry and low-shear viscometry. For all three polymers, despite their different structures, samples with Rh <4 nm were found to inhibit aggregation, whereas those with Rh >4 nm enhanced aggregation. Inhibition increased with Rh and was maximal at approximately 3 nm; above 4 nm the pro-aggregant effect increased with Rh. For comparison, the Rh of 12 plasma proteins were calculated from literature values of intrinsic viscosity or diffusion coefficient. Each protein known to promote RBC aggregation had Rh >4 nm, whereas those with Rh <4 nm either inhibited or had no effect on aggregation. These results suggest that the influence of a nonionic polymer or plasma protein on RBC aggregation is simply a consequence of its size in an aqueous environment, and that the specific type of macromolecule is of minor importance.

FIGURE 1

The effect of POE, dextran, and PVP on red blood cell aggregation for cells suspended in plasma (n = 5, mean ± SD) as a function of molecular weight. Data are normalized by control values; a value >1 indicates enhanced aggregation; a value <1 indicates inhibition of aggregation. Open symbols represent a fixed polymer concentration of 5 mg/mL (* = 2.5 mg/mL) and solid symbols represent a fixed suspending phase viscosity of 1.76 mPa.s. The transition from anti- to pro-aggregant occurs at a different molecular mass for each polymer type (POE ≈ 15,000, PVP ≈ 20,000, and dextran ≈ 40,000).

FIGURE 2

The effect of POE (□), dextran (▵), and PVP (○) on red blood cell aggregation (A) and blood viscosity (B) for cells suspended in polymer solution (n = 5 ± SD) as a function of molecular weight. Polymer concentrations are 3% w/v (half-solid symbols), 1.5% w/v (solid symbols), 0.5% w/v (open symbols), and 0.25% w/v (shaded symbols). The ratio of low-shear viscosity (0.15 s⁻¹) to high-shear viscosity (94.5 s⁻¹) provides a viscometric index of RBC aggregation. The transition from a nonaggregating to an aggregating-suspension occurs at a different molecular weight for each polymer type.

FIGURE 3

The effect of POE (□), dextran (▵), and PVP (○) on red blood cell aggregation for cells suspended in plasma (A) and blood viscosity (B) for cells suspended in polymer solution (n = 5 ± SD) as a function of hydrodynamic radius (R_h). For cells suspended in polymer solution, polymer concentrations are 3% w/v (half-solid symbols), 1.5% w/v (solid symbols), 0.5% w/v (open symbols), and 0.25% w/v (shaded symbols). The effects of albumin and fibrinogen (shaded diamond) are shown for comparison. The shaded bar shows the transition from an anti- to a proaggregating system occurring at a hydrodynamic radius of ∼4 nm for all macromolecules studied.