During low-to-high work transition in adult mammalian heart in vivo the concentrations of free ADP, ATP, PCr (phosphocreatine), Pi and NADH are essentially constant, in striking contrast with skeletal muscle. The direct activation by calcium ions of ATP usage and feedback activation of ATP production by ADP (and Pi) alone cannot explain this perfect homoeostasis. A comparison of the response to adrenaline (increase in rate-pressure product and [PCr]) of the intact beating perfused rat heart with the elasticities of the PCr producer and consumer to PCr concentration demonstrated that both the ATP/PCr-producing block and ATP/PCr-consuming block are directly activated to a similar extent during physiological heart activation. Our finding constitutes a direct evidence for the parallel-activation mechanism of the regulation of oxidative phosphorylation in heart postulated previously in a theoretical way.

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