Temperature Transition of Human Hemoglobin at Body Temperature: Effects of Calcium

AbstractWe studied the effects of calcium ion concentration on the temperature dependence of rheological behavior of human red blood cells (RBCs) and concentrated hemoglobin solutions. Our previous study (G. M. Artmann, C. Kelemen, D. Porst, G. Büldt, and S. Chien, 1998, Biophys. J., 75:3179–3183) showed a critical temperature (Tc) of 36.4±0.3°C at which the RBCs underwent a transition from non-passage to passage through 1.3-μm micropipettes in response to an aspiration pressure of −2.3kPa. An increase in intracellular Ca2+ concentration by using the ionophore A23187 reduced the passability of intact RBCs through small micropipettes above Tc; the micropipette diameter needed for >90% passage increased to 1.7μm. Viscometry of concentrated hemoglobin solutions (45 and 50g/dl) showed a sudden viscosity transition at 36±1°C (Tcη) at all calcium concentrations investigated. Below Tcη, the viscosity value of the concentrated hemoglobin solution at 1.8mM Ca2+ was higher than that at other concentrations (0.2μM, 9mM, and 18mM). Above Tcη, the viscosity was almost Ca2+ independent. At 1.8mM Ca2+ and 36±1°C, the activation energy calculated from the viscometry data showed a strong dependence on the hemoglobin concentration. We propose that the transition of rheological behavior is attributable to a high-to-low viscosity transition mediated by a partial release of the hemoglobin-bound water