Myocardial Lipid and Carbohydrate Metabolism in Healthy, Fasting Men at Rest: Studies During Continuous Infusion of ³H‐Palmitate

Myocardial metabolism of lipid and carbohydrate substrates was studied in 17 healthy men at rest by measuring the arterial‐coronary sinus [(a—cs)] concentration differences. A continuous intravenous infusion of albumin‐bound 3 H‐palmitate was given to provide a tracer for the plasma free fatty acids (FFA) and to produce endogenous labelling of plasma triglycerides (TG). A statistically significant positive (a—cs) difference in triglyceride (TG) concentration was detected in 10 of the 17 subjects and averaged 18±4 (SEM) μmol/1 plasma for the 17 subjects. This was 1.0% of the average arterial TG concentration. A significant positive (a—cs) difference in TG radioactivity was found in 12 of the 17 subjects but it was not possible to quantitate myocardial TG extraction from these radioisotope data. Myocardial extraction of FFA based on the radiopalmitate data was on average 39% greater than the extraction of FFA measured chemically. This was interpreted as indicating an efflux of unlabelled fatty acids into the coronary sinus, most probably from a glyceride pool within the myocardium. The finding that this efflux of fatty acids was not accompanied by free glycerol suggested either that the fatty acids were derived from partial hydrolysis of glycerides, or that glycerol was metabolised within the myocardium. Seven of the subjects had significant, positive (a—cs) differences in free glycerol concentration suggesting that the human heart is capable of metabolising glycerol. There were significant, negative linear correlations between arterial FFA concentration and myocardial extraction of glucose, lactate and pyruvate with significant efflux of pyruvate from the heart at higher FFA concentrations. These findings suggested that FFA can decrease glucose extraction by the human heart and that one possible mechanism for this may be the inhibition of pyruvate dehydrogenase. The average (±SEM) oxygen extraction ratios (OERs) for the substrates were: TG 16±4%; FFA 48±3%; glucose 20±3%; lactate 8±2%; pyruvate 1±0.3%. The total OER for these substrates averaged 97% suggesting that in the resting, fasting, state there is little change in the total energy content of endogenous myocardial substrate pools.