Temperature transition of human hemoglobin at body temperature: Effects of calcium

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