Regulation of Cardiac Cytosolic Free Ca2+ At Low Temperature in the Richardson's Ground Squirrel

Low temperature has profound effects on heart function. Cardiac arrhythmias, particularly ventricular fibrillation, appear in most non-hibernating species if body temperature is decreased to 30°-15°C. In isolated ventricular muscles from rats and rabbits, after-contractions and an increase in resting tension can be observed when temperature is cooled from 37°C to about 15°C; both effects indicate that a disturbance in regulation of intracellular Ca²⁺ is responsible for the cardiac arrhythmia and/or dysfunction in the cold. Direct measurement of cytosolic free Ca²⁺ ([Ca²⁺]i) with a fluorescent dye, Indo-1, has shown that [Ca²⁺]i in isolated rat ventricular myocytes increased significantly when temperature was decreased from 37°C to 15 and 5°C. Since [Ca²⁺]i plays a central role in cardiac excitation-contraction coupling, the chapter aims to document the changes of cardiac [Ca²⁺]i in a hibernator, the Richardson's ground squirrel, at low temperatures.