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

1H/(31)P 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 O(2) consumption increases by 56%. The phosphocreatine (PCr) level initially declines, but recovers within 15 min to its control level; the oxymyoglobin (MbO(2)) saturation decreases by 15%. Because the MbO(2) signal reflects the intracellular PO(2), the capillary-to-cell O(2) gradient has increased to match the increased O(2) need. However, no transient metabolic fluctuation is observed in either PCr or MbO(2) 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 O(2) nor ADP is regulating respiration under increased energy demand in the steady or transient state.