Non-diffusive slow heat dissipation induces high local temperature in living cells

Recently, intracellular thermometry has revealed temperature variations within cells. Although the biological significance of intracellular temperature change is recognized, the physical principles of intracellular temperature change remain a mystery. Here, we investigate intracellular heat transfer through intracellular temperature mapping using a fluorescent polymeric thermometer and high-speed fluorescence lifetime imaging microscopy. Through infrared laser irradiation-assisted heating, we track changes in temperature distribution to examine the mechanism of intracellular heat dissipation in comparison with heat conduction. Continuous heating induces the significantly slower relaxation of the average temperature of single cells compared with that of liposomes containing homogeneous aqueous solutions of comparable size; to the scale of seconds. We additionally elucidate that these phenomena are impacted by intracellular structures and molecules. Finally, we discover that this slow intracellular temperature relaxation originates from non-diffusive heat dissipation distinct from the conventional heat conduction model. Our results provide insights into the mechanisms of temperature variation in cells that are unresolved based on our current understanding, establishing a framework for understanding intracellular thermodynamics under non-equilibrium conditions.