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Temperature Dependence and Thermodynamic Properties of Ca²⁺ Sparks in Rat Cardiomyocytes

National Key Laboratory of Biomembrane and Membrane Biotechnology, College of Life Sciences, Peking University, Beijing 100871, China

Correspondence: Address reprint requests to Shi-Qiang Wang, E-mail: wsq{at}pku.edu.cn; or Xue-Mei Hao, E-mail: haoxm{at}pku.edu.cn.

To elucidate the temperature dependence and underlying thermodynamic determinants of the elementary Ca²⁺ release from the sarcoplasmic reticulum, we characterized Ca²⁺ sparks originating from ryanodine receptors (RyRs) in rat cardiomyocytes over a wide range of temperature. From 35°C to 10°C, the normalized fluo-3 fluorescence of Ca²⁺ sparks decreased monotonically, but the [Ca²⁺]_i were relatively unchanged due to increased resting [Ca²⁺]_i. The time-to-peak of Ca²⁺ sparks, which represents the RyR Ca²⁺ release duration, was prolonged by 37% from 35°C to 10°C. An Arrhenius plot of the data identified a jump of apparent activation energy from 5.2 to 14.6 kJ/mol at 24.8°C, which presumably reflects a transition of sarcoplasmic reticulum lipids. Thermodynamic analysis of the decay kinetics showed that active transport plays little role in early recovery but a significant role in late recovery of local Ca²⁺ concentration. These results provided a basis for quantitative interpretation of intracellular Ca²⁺ signaling under various thermal conditions. The relative temperature insensitivity above the transitional 25°C led to the notion that Ca²⁺ sparks measured at a "warm room" temperature are basically acceptable in elucidating mammalian heart function.

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