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 Po₂, 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.