Correlation of the internal microviscosity of human erythrocytes to the cell volume and the viscosity of hemoglobin solutions.

The microviscosity of the cytoplasm of human erythrocytes as well as of membrane-free hemoglobin solutions was investigated measuring the rotation of the small spin-label molecule, Tempone. The dependence of the intracellular microviscosity on the extracellular pH and osmotic pressure which was varied by NaCl or sucrose was sufficiently explained on the basis of alterations of the red blood cell volume. The intracellular microviscosity depended exclusively on the hemoglobin concentration. It did not differ from that of comparable membrane-free hemoglobin solutions. It was not necessary to take into account long-range interactions between hemoglobin molecules. The conclusion therefore was that the intracellular viscosity is not modified by cytoplasmic structures or the cell membrane. Above a hemoglobin concentration of 6 mM the viscosity of hemoglobin solutions increased much faster than the microviscosity. From measurements obtained with different spin-labels it followed that also the charge of these molecules is of importance.