Acute hibernation decreases myocardial pyruvate carboxylation and citrate release

Abstract

In the well-perfused heart, pyruvate carboxylation accounts for 3-6% of the citric acid cycle (CAC) flux, and CAC carbon is lost via citrate release. We investigated the effects of an acute reduction in coronary flow on these processes and on the tissue content of CAC intermediates. Measurements were made in an open-chest anesthetized swine model. Left anterior descending coronary artery blood flow was controlled by a extracorporeal perfusion circuit, and flow was decreased by 40% for 80 min to induce myocardial hibernation (n = 8). An intracoronary infusion of [U-(13)C(3)]lactate and [U-(13)C(3)]pyruvate was given to measure the entry of pyruvate into the CAC through pyruvate carboxylation from the (13)C-labeled isotopomers of CAC intermediates. Compared with normal coronary flow, myocardial hibernation resulted in parallel decreases of 65% and 79% in pyruvate carboxylation and net citrate release by the myocardium, respectively, and maintenance of the CAC intermediate content. Elevation of the arterial pyruvate concentration by 1 mM had no effect. Thus a 40% decrease in coronary blood flow resulted in a concomitant decrease in pyruvate carboxylation and citrate release as well as maintenance of the CAC intermediates.

Reverse flux through cardiac NADP+-isocitrate dehydrogenase under normoxia and ischemia

“…Compared with normoxia, the tissue levels of citrate were decreased (two-to threefold), whereas those of succinate were increased (six-to ninefold) under LFI/NE. The total pool size of CAC intermediates was similar or greater under LFI/NE than under normoxia, which is in agreement with the data of other researchers (28,30). Note that under all conditions, tissue citrate levels remained severalfold (24-90) higher than those of isocitrate and ␣-KG.…”

“…Citrate release, which is modulated by substrates and/or O 2 supply (24, 28, 29, 43), represents at most 1% of CAC flux. Mitochondrial citrate efflux appears normally to be compensated largely by flux through anaplerotic reactions such as pyruvate carboxylation, which represents between 2 and 8% of CAC flux (5,28,29).Citrate synthesis could also occur through a reductive process, which involves the participation of the CAC enzymes aconitase and NADP ϩ -linked isocitrate dehydrogenase (NADP ϩ -ICDH). Aconitase catalyzes the reversible interconversion between citrate and isocitrate.…”

“…1). Citrate release, which is modulated by substrates and/or O 2 supply (24,28,29,43), represents at most 1% of CAC flux. Mitochondrial citrate efflux appears normally to be compensated largely by flux through anaplerotic reactions such as pyruvate carboxylation, which represents between 2 and 8% of CAC flux (5,28,29).…”

Cardiac metabolism measured noninvasively by hyperpolarized 13C MRI

“…1). Its fate in the heart muscle has been described extensively both under normal conditions (31,32) and during different cardiac diseases (33,34). Pyruvate enters the muscle cells and is metabolized to lactate, alanine, and CO 2 , of which the latter will promptly be in equilibrium with HCO 3 Ϫ .…”

Mitochondria from the left heart ventricles of both normotensive and spontaneously hypertensive rats oxidize externally added NADH mostly via a novel malate/oxaloacetate shuttle as reconstructed in vitro

“…In this case, even though the malate concentration in the heart is between 100 and 500 nmols/g wet (35), the division between cytosol and mitochondria remains difficult to determine with certainty due to reaching an equilibrium in the time necessary to isolate mitochondria.…”