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Response time of myocardial oxygen consumption to cardiac work jumps at 28 degrees C varies with exogenous carbon substrate.

In isolated rabbit heart perfused with hemoglobin-free Tyrode's solution at 28 degrees C the response time of myocardial oxidative phosphorylation to steps in heart rate, which is about 8 s, is decreased by about 2.5 s when lactate of pyruvate are given as exogenous carbon substrate. A hypothesis that may explain the decrease in response times is that glycolytic buffering in compartments near the energy consuming ATPases delays the transport of the energetic signal between sites of ATP consumption and the mitochondria. Pyruvate inhibits this glycolytic buffering, which would explain the faster response time. Whatever the mechanism may be, the type of exogenous substrate available to the heart has a major effect on the speed of response of oxidative phosphorylation to quick changes in cardiac workload.