Rabbit papillary muscle has been exposed to calcium concentrations ranging from 2.5 mM to zero. Its mechanical and electrical activity has been monitored and its heat production measured using a myothermic technique. Calcium depletion decreased the magnitude of the tension-independent heat per contraction from a mean of 0.45 mcal/g muscle to 0.31 mcal/g muscle at room temperature (18° to 22°C). Calcium-chelating agents did not abolish action potential conduction under the experimental conditions used but they further reduced the magnitude of the tension-independent heat. Raising the temperature from room level to 32°C decreased the tension-independent heat from a mean of 0.52 to a mean of 0.24 mcal/g muscle. Calcium depletion at 32°C further decreased this heat and it was calculated that the energy now liberated in activating the muscle was about 2% of the total energy normally liberated in the working heart. The results are interpreted in terms of current biochemical and myothermic data.

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