A method to prepare isotonic dextran-salt solutions.

To increase, in rheological studies of red cells, the viscosity of suspending media, high-molecular-weight polymers, e.g., dextrans, are added to electrolyte solutions. However, dextrans at typical concentrations (e.g., 15% w/w) contribute an appreciable portion to the osmotic pressure of the suspending medium. To keep the volume of red cells normal, the concentration of electrolyte has to be reduced. In our hands, osmometry of dextran-salt solutions (DSSs) was not reliable, probably due to the increased viscosity. Therefore, a method was devised in which the concentration of electrolyte required to achieve normal red cell volume was determined experimentally using the red cell itself as an osmometer. Packed unwashed red cells were added to DSSs with different concentrations of electrolyte. Swelling or shrinking of the red cells was monitored by a change in the refractive index of the suspending medium. Six different dextrans (MW 40 kDa to 2000 kDa) were tested. The concentration of electrolyte at which the red cell volume did not change upon suspension in these DSSs did not depend on the MW of the dextrans. For a dextran concentration of 15% (w/w), the addition of electrolytes had to be reduced by about 1/4 with respect to an isotonic pure electrolyte solution.