Thermal anomaly of erythrocyte osmotic fragility and aquaporin function at body temperature.

The recently proposed Interfacial Water Quantum Transition (IWQ) model presents a novel framework for functional changes in proteins at critical temperatures. Building on this, we hypothesized that red blood cell (RBC) osmotic fragility (OF) and aquaporin (AQP) function might exhibit similar transitions. We assessed the osmotic resistance of human and chicken RBCs across 24-50 °C. Mean corpuscular fragility (MCF50) was obtained by logistic curve fitting. In human RBCs, MCF50 consistently decreased with increasing temperature in whole blood, washed RBCs, and HgCl2-treated (known to reduce AQP water permeability) RBCs. Washed RBCs were more fragile, indicating a protective role of plasma proteins. An abrupt MCF50 drop at Tc = 36.0 ± 0.4 °C in washed RBCs abolished group differences, suggesting a functional transition at the critical temperature, TC. For chicken RBCs at Tc = 41.0 ± 0.5 °C, a phase transition-like decrease in MCF50 was observed, where it dropped from 0.32 at 40 °C to 0.28 at 41 °C. This decrease remained consistent above TC, unlike the peak-like curve shape in human RBCs around TC. These results suggest a species-specific, temperature-triggered anomaly and an osmotic regulation mechanism. The OF transitions found align with known hemoglobin temperature transitions, highlighting the physiological relevance of thermo-sensitive protein-water interactions. The findings of this study could foster the understanding of temperature-regulated water transport in red blood cells and for the development of AQP-specific applications in transfusion medicine or bioengineered membranes.