Propionate stimulates pyruvate oxidation in the presence of acetate

Abstract

Flux through pyruvate dehydrogenase (PDH) in the heart may be reduced by various forms of injury to the myocardium, or by oxidation of alternative substrates in normal heart tissue. It is important to distinguish these two mechanisms because imaging of flux through PDH based on the appearance of hyperpolarized (HP) [13 C]bicarbonate derived from HP [1-13 C]pyruvate has been proposed as a method for identifying viable myocardium. The efficacy of propionate for increasing PDH flux in the setting of PDH inhibition by an alternative substrate was studied using isotopomer analysis paired with exams using HP [1-13 C]pyruvate. Hearts from C57/bl6 mice were supplied with acetate (2 mM) and glucose (8.25 mM).13 C NMR spectra were acquired in a cryogenically cooled probe at 14.1 Tesla. After addition of hyperpolarized [1-13 C]pyruvate, 13 C NMR signals from lactate, alanine, malate, and aspartate were easily detected, in addition to small signals from bicarbonate and CO2. The addition of propionate (2 mM) increased appearance of HP [13 C]bicarbonate Ͼ30-fold without change in O2 consumption. Isotopomer analysis of extracts from the freeze-clamped hearts indicated that acetate was the preferred substrate for energy production, glucose contribution to energy production was minimal, and anaplerosis was stimulated in the presence of propionate. Under conditions where production of acetyl-CoA is dominated by the availability of an alternative substrate, acetate, propionate markedly stimulated PDH flux as detected by the appearance of hyperpolarized (25,27). Consequently, in addition to disease processes, physiological fluctuations in the concentrations of fatty acids also influence the appearance of HP [13 C]bicarbonate. An intervention that stimulates flux through PDH in the presence of fatty acids could remove the effects of substrate competition in detecting oxidation of HP [1-13 C]pyruvate. One such approach is co-administration of glucose, insulin, and potassium (16). This mixture has multiple effects on plasma glucose, plasma free fatty acids, and myocardial membrane potential, so any effects on bicarbonate appearance probably reflect a complex interaction between substrate concentrations and direct effects on PDH (4).Propionate, a physiological short chain fatty acid, also influences PDH flux. The interaction of propionate with pyruvate metabolism in the heart has been extensively studied using 14 C tracers. Fatty acids of even carbon chain length markedly inhibit appearance of 14 CO 2 from [1-14 C]pyruvate (14, 32). When compared with acetate, propionate increased the rate of 14 CO 2 production from [1-14 C]pyruvate under steady state conditions (14), suggesting that propionate may be an alternative to glucose, insulin, and potassium for stimulating PDH flux. However, the relevance of these studies for HP 13 C NMR observations is unclear, for two reasons. First, the radiotracer studies examined the influence of individual fatty acids such as acetate or propionate on pyruvate metabolism. The effect of propionate on pyr...

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