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1 Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia, Canada
* To whom correspondence should be addressed. E-mail: rhsmith{at}dal.ca.
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 were decreased dramatically at 37°C compared to 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 Ca2+ content decreased significantly at 37°C. Restoration of SR Ca2+ by superfusion with 5 mM Ca2+ increased spark frequency but did not reverse 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.
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