Abstract
To elucidate the temperature dependence and underlying thermodynamic determinants of the elementary Ca 2+ release from the sarcoplasmic reticulum, we characterized Ca 2+ 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 Ca 2+ sparks decreased monotonically, but the Δ[Ca 2+] i were relatively unchanged due to increased resting [Ca 2+] i. The time-to-peak of Ca 2+ sparks, which represents the RyR Ca 2+ 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 Ca 2+ concentration. These results provided a basis for quantitative interpretation of intracellular Ca 2+ signaling under various thermal conditions. The relative temperature insensitivity above the transitional 25°C led to the notion that Ca 2+ sparks measured at a “warm room” temperature are basically acceptable in elucidating mammalian heart function.
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