Calcium sparks in mouse ventricular myocytes at physiological temperature

In cardiac muscle, Ca²⁺ is released from the sarcoplasmic reticulum (SR) in units called Ca²⁺ sparks. Ca²⁺ spark characteristics have been studied almost entirely at room temperature. This study compares characteristics of spontaneous sparks detected with fluo 3 in resting mouse ventricular myocytes at 22 and 37°C. The incidence and frequency of Ca²⁺ sparks decreased dramatically at 37°C compared with 22°C. Also, spark amplitudes and times to peak were significantly reduced at 37°C. In contrast, spatial width and decay times were unchanged. During field stimulation, peak spatially averaged transients were similar at 22 and 37°C, and experiments with fura 2 demonstrated that diastolic and systolic Ca²⁺ concentrations were unchanged. However, SR Ca ²⁺ content decreased significantly at 37°C. Restoration of SR Ca²⁺ by superfusion with 5 mM Ca²⁺ increased spark frequency but did not reverse the effects of temperature on spark parameters. Thus effects of temperature on spark frequency may reflect changes in SR stores, whereas changes in spark amplitude and rise time may reflect known effects of temperature on ryanodine receptor function.