Effects of Ca2+ on the temperature transition of hemoglobin properties in human red blood cells

Intact red blood cells (RBCs) undergo a sudden change from blocking to passing through 1.3 μm-micropipettes at a transition temperature (T_c) of 36.4°C and -2.3 kPa aspiration pressure. This was attributed to a change of the physical state of hemoglobin. Since Ca²⁺-ions induce conformational changes of proteins, we studied how elevated intracellular Ca²⁺ would effect the RBC passage through micropipettes and T_c. Transitions occurred in A23187-treated RBCs at 1 mM extraneous Ca²⁺ as well. Surprisingly, T_c remained at 36.4°C. However, higher aspiration forces (pipette diameter: 1.7 μm) were needed for passages. To look at hemoglobin only, low shear viscosity measurements of hemoglobin solutions at 50 g/dl were carried out as a function of temperature and different Ca concentrations (1.8, 9 and 18 mM). When raising the temperature, a sudden viscosity drop occurred between 35°C-38°C. However, the absolute difference of viscosities at 35°C compared to 38°C was lower at 9 mM and had almost disappeared at 18 mM. Furthermore, higher Ca²⁺-levels led to lower viscosities at all temperatures investigated. This may be due to the binding of calcium to hemoglobin molecules, which in turn may have weakened intermolecular bonds leading to a fluidization of hemoglobin solutions at high calcium levels