Temperature Effects on the Myoplasmic Free Ca2+ Transients in FDB Muscle Fibers of the Mouse

The fast, low affinity, Ca2+ dye Mag-Fluo-4 loaded in its AM moiety, in mouse FDB fast-twitch fibers has allowed to study, with high temporal resolution, the decay phase of single or tetanic (350 ms-100 Hz), Ca2+ transients (Calderon et al. JMRCM 2009;30). At 23°C, a double exponential function with time constants (always expressed in ms), τ1=1.4±0.1, τ2=15.3±1.3 (n=20), and τ1=23.4±2.5, τ2=594±102.4 (n=13), respectively, describe the decay phase of single and tetanic transients. These time constants have been associated with Ca2+ binding to parvalbumin, and SERCA, respectively. We tested for differential effect of tetanic Ca2+ loads and temperature on the putative Ca2+ removal mechanisms. Increasing temperature, from 23 to 33°C, reversibly diminishes the single transient amplitude (ΔF/F), by 31%, and increases the base line by 32% (n=25). At 33°C, in 15 out of 20 fibers, the decay of single transients followed a single exponential, τ=4.7±0.7, while in the other 5 fibers, two exponentials provided a better fit, τ1=0.6± 0.1 and τ2=5.1±0.4; two exponentials τ1=10.1±1.8 and τ2=394.9±51.2 (n=13), provided the best fit for tetanic decay. The results show that the two time constants, that describe Ca2+ single and tetanic responses decay, are differentially affected by temperature and by tetani imposed Ca2+ loads. The fast decay components of single and tetanic responses, represented by τ1, and associated with parvalbumin Ca2+ binding, are affected similarly by Ca2+ load (17x increase) or by temperature (2.3x increase for 10°C increase). The slower components of the single and tetanic responses, represented by τ2, and associated with SERCA operation, are differentially affected by a 10°C temperature raise (3.1x vs 1.5x) and by the tetanic Ca2+ load (39x vs 77x) respectively.