Myocardial metabolism: III. Myocardial intermediates and substrate oxidation during in vitro perfusion of palmitate and glucose

Abstract

Isolated hearts from fasted rats were perfused with Krebs bicarbonate buffer containing glucose and palmitic acid, either individually or in combination. Glucose-U- 14C was perfused for 75 minutes, and in some cases, unlabeled palmitate was added after 45 minutes' perfusion. Similarly, palmitic-1- 14C acid, complexed to albumin, was perfused for 75 minutes with or without addition of unlabeled glucose. Substrate uptake and oxidation were determined at 15-minute intervals. Incorporation of isotope into various lipid fractions of heart, and the levels of certain glycolytic and Krebs cycle intermediates, were also determined. Addition of unlabeled palmitate to perfusates containing glucose-U- 14C caused a complete cessation of 14CO 2 production and a decrease in glucose uptake by the myocardium. However, this block was not reflected by changes in the levels of myocardial glucose 6-phosphate or fructose 6-phosphate. Accumulation of these intermediates was apparent only after 75 minutes' perfusion with palmitate alone. Addition of unlabeled glucose to perfusates containing palmitate-1- 14C caused a rapid increase in 14CO 2 evolution within 5 minutes after addition of the glucose. However, there was a decrease in the levels of glucose and fructose phosphates, indicating a release in the block of glycolysis at the level of the phosphofructokinase reaction. Also, the addition of glucose resulted in increased incorporation of label into tissue triglyceride. In this case, all of the labeled palmitate taken up by the heart was accounted for either as CO 2 or tissue lipids. It is proposed that the increased 14CO 2 evolution caused by addition of glucose to the perfusion medium containing labeled palmitate represents rapid depletion of the acyl coenzyme A intermediates rather than activation and oxidation of additional palmitic acid. The release of inhibition of glycolysis due to the presence of high concentrations of circulating glucose resulted in a decrease in glucose and fructose phosphates. This increased glycolysis would then provide α-glycerol phosphate for phospholipid and glyceride synthesis, and would also provide intermediates such as pyruvate, which are necessary for optimum turnover of the Krebs cycle.