Fatty acid regulation of glucose metabolism in the intact beating rat heart assessed by carbon-13 NMR spectroscopy: the critical role of pyruvate dehydrogenase.

Although the myocardium is capable of utilizing both glucose and fatty acid substrates, glucose metabolism is inhibited in the presence of fatty acid during normal perfusion conditions. Fatty acid regulation of glucose utilization in intact beating rat hearts was studied with 13C-enriched substrates and 13C and 31P NMR spectroscopy at 8.5 T. During [1-13C]glucose and insulin perfusion, the 13C appeared in alanine, lactate and the glutamate isotopomers, indicating glycolytic flux through pyruvate and glucose-supported tricarboxylic acid (TCA) cycle oxidation, respectively. Following the addition of hexanoic acid, 1 mM, [1-13C]glucose metabolism proceeded through the hexokinase and phosphofructokinase reactions, as evidenced by continued production of [3-13C]alanine and [3-13C]lactate, but was completely inhibited at the pyruvate dehydrogenase (PDH) reaction as evidenced by a lack of appearance of the 13C label in the glutamate isotopomers. This inhibition of PDH was associated with increased PCr/ATP levels and was readily reversed by removal of hexanoic acid. Addition of dichloroacetate, 5 mM, which increases the active form of PDH, to fatty acid and glucose containing perfusate reinstituted carbon flux through the PDH reaction, indicating that the mechanism of fatty acid cessation of PDH flux is by reversible inactivation of the PDH enzyme complex. Thus the point of inhibition and mechanism of action of fatty acid modulation of glucose metabolism can be continuously and non-destructively studied in the intact beating heart with 13C and 31P NMR and is primarily attributable, in this model, to reversible PDH enzyme inactivation.