Myocardial oxygen and carbohydrate consumption in fetal lambs in utero and in adult sheep.

We measured myocardial oxygen, glucose, lactate, and pyruvate consumption in chronically instrumented fetal and adult sheep. Although ascending aortic blood concentration of oxygen was significantly lower in fetuses, myocardial consumption of oxygen was similar in the two groups. This was accomplished by a significantly greater myocardial blood flow in the fetuses. Although ascending aortic blood glucose concentration was significantly lower in fetuses, myocardial consumption of glucose was significantly greater in fetuses. Complete oxidative combustion of all glucose consumed by the fetal heart would supply only one-third of myocardial energy demands, as measured by oxygen consumption. Ascending aortic blood concentration of lactate was similar in fetuses and adults, but myocardial consumption of lactate was significantly greater in fetuses. Complete oxidative combustion of all lactate consumed by fetal hearts would supply almost 60% of myocardial energy demands. Small, but significant, amounts of pyruvate are consumed by both fetuses and adults. Our data indicate that fetal lamb myocardium requires substrates other than glucose alone. The large amount of lactate consumed indicates that there is oxidative metabolism in addition to glycolysis and that lactate is of equal, or perhaps greater, importance as a myocardial energy substrate.