Regulation of Fatty Acid Utilization in Isolated Perfused Rat Hearts

Abstract Regulation of fatty acid utilization was studied in the isolated, perfused rat heart. The effects of increasing the concentration of perfusate fatty acid and the level of ventricular pressure development on the rates of fatty acid uptake and oxidation and on the tissue levels of CoA and carnitine derivatives were determined. In hearts that were developing low levels of ventricular pressure, the rate of palmitate uptake was proportional to its concentration in the perfusate as the concentration was raised from 0 to 0.6 mm bound to 3% albumin. The faster rate of uptake was associated with only a slight increase in the tissue content of long chain acyl-CoA, acylcarnitine, acetyl-CoA, and acetylcarnitine. Palmitate utilization appeared to be limited by the rate of fatty acid uptake or activation. As the concentration of palmitate was increased from 0.6 to 1.2 mm, the rate of uptake did not increase further; oxygen consumption and 14CO2 production from U-14Cpalmitate increased only slightly, and large amounts of acyl-CoA and acylcarnitine derivatives accumulated in the tissue. These observations indicated that the rate of fatty acid uptake at high levels of exogenous palmitate was limited by the rate of acetyl-CoA oxidation through the citric acid cycle. Fatty acid activation and transfer of the acyl unit from acyl-CoA to acylcarnitine may have been limited by high acetyl-CoA to CoA and acetylcarnitine to carnitine ratios. It was estimated that the concentration of CoA and carnitine decreased to below the level needed for optimal rates of fatty acid activation and oxidation. The mass-action ratio for the carnitine palmityltransferase was constant and independent of the exogenous fatty acid concentration. The mass-action ratio for the carnitine acetyltransferase was shifted toward acetyl-CoA formation as the level of palmitate was raised. A possible role of the carnitine acetyltransferase system in coupling the rate of flux through the citric acid cycle with fatty acid activation and acyl transfer is discussed. Increased ventricular pressure development resulted in (a) a faster rate of oxidative phosphorylation as indicated by increased oxygen consumption (b) an acceleration of the citric acid cycle as indicated by a large increase in CO2 production, and (c) an increase in uptake and β oxidation of palmitate. Acceleration of the citric acid cycle was associated with a decrease in the tissue content of fatty acids, acyl-CoA, acetyl-CoA, and acetylcarnitine and an increase in the levels of acylcarnitine, free CoA, and free carnitine. When high levels of exogenous palmitate were present, the increase in CoA and carnitine could have accelerated the rates of fatty acid activation and acyl transfer from acyl-CoA to acylcarnitine and thus increased the rate of palmitate uptake. With concentrations of palmitate below 0.6 mm, the changes in CoA and carnitine were probably too small to account for the increased fatty acid uptake. The tissue content of long chain acylcarnitine increased with acceleration of oxidative metabolism even though the levels of both acyl-CoA and acetyl-CoA decreased. The mass-action ratio for the carnitine palmityltransferase system shifted toward acylcarnitine formation. In contrast to palmitate, oxidation of octanoate was fast enough to maintain high levels of acetyl-CoA when fatty acid oxidation was accelerated by increased cardiac work. These results suggested that the rate of translocation of acyl units across the inner mitochondrial membrane limited the rate of long chain fatty acylcarnitine oxidation at high levels of ventricular pressure development.