Temperature dependence of cardiac sarcoplasmic reticulum function in rainbow trout myocytes

To explore how the cardiac sarcoplasmic reticulum (SR) functions over a range of temperatures, we used whole-cell voltage clamp combined with rapid caffeine application to study SR Ca2+ accumulation, release and steady-state content in atrial myocytes from rainbow trout. Myocytes were isolated from rainbow trout acclimated to 14degreesC, and the effect of varying stimulation pulse number, frequency and experimental temperature (7degreesC, 14degreesC and 21degreesC) on SR function was studied. To add physiological relevance, in addition to 200 ms square (SQ) voltage pulses, myocytes were stimulated with temperature-specific action potentials (AP) applied at relevant frequencies for each test temperature. We found that the SR accumulated Ca2+ more rapidly and to a greater concentration (1043+/-189 mumoll(-1) Ca2+, 1138+/-173 mumoll(-1) Ca2+, and 1095+/-142 mumoll(-1) Ca2+ at 7degreesC, 14degreesC and 21degreesC, respectively) when stimulated with physiological AP waveforms at physiological frequencies compared with 200ms SQ pulses at the same frequencies (664+/-180 mumoll(-1) Ca2+, 474+/-75 mumoll(-1) Ca2+ and 367+/-42 mumoll(-1) Ca2+ at 7degreesC, 14degreesC and 21degreesC, respectively). Also, and in contrast to 200ms SQ pulse stimulation, temperature had little effect on steady-state SR Ca2+ accumulation during AP stimulation. Furthermore, we observed SR-Ca2+-dependent inactivation of the L-type Ca2+ channel current (I-Ca) at 7degreesC, 14degreesC and 21degreesC, providing additional evidence of maintained SR function in fish hearts over an acute range of temperatures. We conclude that the waveform of the AP may be critical in ensuring adequate SR Ca2+ cycling during temperature change in rainbow trout in vivo.