In Vivo Myocardial Bioenergetics During Acute Volume And/or Pressure Loading in a Canine Model: A 31P NMR Study

The effects of acute volume and/or pressure loading on myocardial metabolic and mechanical function were studied in 13 dogs. Volume loads were applied by shunting the abdominal aorta to the vena cava using polyethylene tubing (5 mm inner diameter). A plastic regulator allowed shunts to be opened or closed. Dogs were heparinized (100 units/kg) to prevent shunts from clotting. To study the effects of pressure loading, a norepinephrine infusion (1 microgram/kg/min) was administered. Mechanical function of the heart was evaluated using heart rate X systolic blood pressure (HR X SBP), cardiac output (CO), pressure X volume work (systolic blood pressure X stroke volume); (P X V), and oxygen consumption (MVO2) to estimate external myocardial work. Metabolic function was evaluated by 31P NMR. Phosphocreatine/adenosine triphosphate (PCr/ATP) ratios were used to estimate the bioenergetic regulation of oxidative phosphorylation during increased work load. HR X SBP, CO, P X V, and MVO2 were correlated with PCr/ATP. Although there was some variability, generally volume loading was associated with an increase in HR X SBP, CO, P X V, and MVO2 accompanied by no change, or small increases or small decreases in PCr/ATP throughout the loading period. These data indicate that the heart bioenergetics are quite stable during volume and/or pressure loading and that 31P spectroscopy methods can document this stability and tight metabolic regulation during in vivo loading conditions.