Energetics of isovolumic contractions of the isolated rabbit heart.

1. In the isolated beating rabbit heart the heat production (measured by Dewar-flask calorimetry) and oxygen consumption (measured polarographically) increased in a similar way to force development (assessed as the time integral of left ventricular developed pressure) when the diastolic size of the heart was increased. 2. The energy expenditure of the heart consists of an element which is independent of the force developed and another which varies with the force developed. 3. The calorific equivalent of oxygen in the beating heart when it was provided with pyruvate as a substrate was found to be 20-48 mJ/mul: O2 at 25 degrees C. 4. The anaerobic metabolism was well below 5% of the total energy liberation and was constant at all levels of mechanical activity.