Regulation of respiration in myocardium in the transient and steady state.

1H/31P NMR has followed the metabolic response to increased work in the glucose- and pyruvate-perfused rat myocardium during a heart cycle and at the steady state. With electrical pacing and dobutamine, the heart O2 consumption increases by 56%. The phosphocreatine (PCr) level initially declines, but recovers within 15 min to its control level; the oxymyoglobin (MbO2) saturation decreases by 15%. Because the MbO2 signal reflects the intracellular [Formula: see text], the capillary-to-cell O2 gradient has increased to match the increased O2 need. However, no transient metabolic fluctuation is observed in either PCr or MbO2 throughout the entire cardiac cycle in both glucose and pyruvate-/glucose-perfused hearts. No systolic-diastolic variation is detectable under either high workload or hypoxic conditions. The results reveal that neither O2 nor ADP is regulating respiration under increased energy demand in the steady or transient state.