Effect of low temperature on the cytosolic free Ca2+ in rat ventricular myocytes

The effect of low temperature on the cytosolic free Ca2+ [( Ca2+]i) has been investigated in isolated ventricular myocytes from adult rats using the fluorescent probe Indo-1. The distribution of Indo-1 between the mitochondrial and cytoplasmic compartments was first determined in the isolated myocytes using the digitonin and Triton X-100 treatments. By subtracting the mitochondrial [Ca2+]i from the total [Ca2+]i measured with Indo-1, the average cytosolic [Ca2+]i was found to increase significantly (P less than 0.05) from 139 nM to 255 and 297 nM when the temperature was decreased from 37 degrees C to 15 degrees and 5 degrees C, respectively. A marked increase in cytosolic [Ca2+]i to a new steady state level was observed when the membrane of myocytes was depolarized by 60 mM KCI; the average magnitude of increase being 110, 243 and 186 nM, at 37 degrees, 15 degrees and 5 degrees C respectively. Our results support the hypothesis that the cardiac arrhythmia typically observed in the hypothermic rat is due to an increased cytosolic [Ca2+]i with decreasing body temperature.