Respiratory control in heart muscle during fatty acid oxidation. Energy state or substrate-level regulation by Ca2+?

Fatty acids are the main fuel for the myocardium in vivo. They increase oxygen consumption, but the regulation of their β-oxidation is not well known. Since Ca2+ and matrix volume have been implicated in the regulation of fatty acid oxidation in liver mitochondria, we set out to investigate the effects of Ca2+ on cellular respiration and energetics in the isolated perfused rat heart when oxidizing a short-chain fatty acid. Infusion of hexanoate increased oxygen consumption, while stepwise changes in the perfusate Ca2+ concentration in the range 0.5–2.5 mm caused the mechanical work output and oxygen consumption to change in parallel. Hexanoate addition increased the cellular energy state as determined by 31P NMR and evaluated from the cytosolic [ATP]/[ADP]•[Pi] ratio. During fatty acid infusion the energy state decreased slightly upon Ca2+-induced inotropy, and after discontinuation of the hexanoate infusion the de-energization was more pronounced. The fatty acid caused an extensive partially reversible reduction of flavoproteins and NAD with a slight tendency for oxidation during Ca2+-induced inotropy. The data are in agreement with the notion that oxygen consumption during fatty acid oxidation is mainly determined by the energy expenditure, even in the presence of Ca2+-induced alterations in the inotropic state. The constancy of the redox states of mitochondrial flavins and NADH/NAD during large changes in oxygen consumption is interpreted as indicating stabilization of the mitochondrial redox states by Ca2+-linked regulation.