Search

Aims: Technological limitations have restricted in vivo assessment of intracellular pH (pHi) in the myocardium. The aim of this study was to evaluate the potential of hyperpolarized [1-13C]pyruvate, coupled with 13C magnetic resonance spectroscopy (MRS), to measure pHi in the healthy and diseased heart. Methods and results: Hyperpolarized [1-13C]pyruvate was infused into isolated rat hearts before and immediately after ischaemia, and the formation of 13CO2 and H13CO3- was monitored using 13C MRS. The HCO3-/CO2 ratio was used in the Henderson-Hasselbalch equation to estimate pHi. We tested the validity of this approach by comparing 13C-based pHi measurements with 31P MRS measurements of pHi. There was good agreement between the pHi measured using 13C and 31P MRS in control hearts, being 7.12 ± 0.10 and 7.07 ± 0.02, respectively. In reperfused hearts, 13C and 31P measurements of pHi also agreed, although 13C equilibration limited observation of myocardial recovery from acidosis. In hearts pre-treated with the carbonic anhydrase (CA) inhibitor, 6-ethoxyzolamide, the 13C measurement underestimated the 31P-measured pHi by 0.80 pH units. Mathematical modelling predicted that the validity of measuringpHi from the H13CO3-/13CO2 ratio depended on CA activity, and may give an incorrect measure of pHi under conditions in which CA was inhibited, such as in acidosis. Hyperpolarized [1-13C]pyruvate was also infused into healthy living rats, where in vivo pHi from the H13CO3-/13CO2 ratio was measured to be 7.20 ± 0.03. Conclusion: Metabolically generated 13CO2 and H13CO3- can be used as a marker of cardiac pHi in vivo, provided that CA activity is at normal levels. [ABSTRACT FROM PUBLISHER]

The Krebs cycle plays a fundamental role in cardiac energy production and is often implicated in the energetic imbalance characteristic of heart disease. In this study, we measured Krebs cycle flux in real time in perfused rat hearts using hyperpolarized magnetic resonance spectroscopy (MRS). [2-13C]Pyruvate was hyperpolarized and infused into isolated perfused hearts in both healthy and postischemic metabolic states. We followed the enzymatic conversion of pyruvate to lactate, acetylcarnitine, citrate, and glutamate with 1 s temporal resolution. The appearance of 13C-labeled glutamate was delayed compared with that of other metabolites, indicating that Krebs cycle flux can be measured directly. The production of 13C-labeled citrate and glutamate was decreased postischemia, as opposed to lactate, which was significantly elevated. These results showed that the control and fluxes of the Krebs cycle in heart disease can be studied using hyperpolarized [2-13C] pyruvate. [ABSTRACT FROM AUTHOR]

Abstract: Objective: Many patients with heart failure have whole-body insulin resistance and reduced cardiac fluorodeoxyglucose uptake, but whether these metabolic changes have detrimental effects on the heart is unknown. Here, we tested whether there is a link between insulin resistance and ischemic damage in the chronically infarcted Wistar rat heart, postulating that the heart would have decreased insulin sensitivity, with lower GLUT4 glucose transporter protein levels due to high circulating free fatty acid (FFA) concentrations. A decreased capacity for glucose uptake would lower glycolytic adenosine triphosphate (ATP) production and thereby increase ischemic injury in the infarcted heart. Methods and results: In vivo left ventricular ejection fractions, measured using echocardiography, were 40% lower in rats 10weeks after coronary artery ligation than in sham-operated control rats. Insulin-stimulated d[2-3H]glucose uptake was 42% lower in isolated, perfused, infarcted hearts. Myocardial GLUT4 glucose transporter protein levels were 28% lower in the infarcted hearts and correlated negatively with ejection fractions and with fasting plasma FFA concentrations. Compared with controls, chronically infarcted hearts had 46% lower total glucose uptake and three-fold faster ATP hydrolysis rates, measured using phosphorus-31 nuclear magnetic resonance spectroscopy, during 32-min ischemia at 0.4ml/min/gww. During reperfusion, recovery of left ventricular developed pressure in infarcted hearts was 42% lower than in control hearts. Conclusions: Glucose uptake, in response to insulin or ischemia, was lower in the chronically infarcted rat heart and associated with increased circulating FFA concentrations and decreased GLUT4 levels. Thus, infarcted hearts had greater ATP depletion, and consequently incurred greater damage, during ischemia. [Copyright &y& Elsevier]

The transcription of key metabolic regulatory enzymes in the heart is altered in the diabetic state, yet little is known of the underlying mechanisms. The aim of this study was to investigate the role of peroxisome proliferator-activated receptor-α (PPAR-α) in modulating cardiac insulin-sensitive glucose transporter (GLUT-4) protein levels in altered metabolic states and to determine the functional consequences by assessing cardiac ischemic tolerance. Wild-type and PPAR-α-null mouse hearts were isolated and perfused 6 wk after streptozotocin administration or after 14 mo on a high-fat diet or after a 24-h fast. Myocardial D-[2-³H]glucose uptake was measured during low-flow ischemia, and differences in GLUT-4 protein levels were quantified using Western blotting. In wild-type mice in all three metabolic states, elevated plasma free fatty acids were associated with lower total cardiac GLUT-4 protein levels and decreased glucose uptake during ischemia, resulting in poor postischemic functional recovery. Although PPAR-α-null mice also had elevated plasma free fatty acids, they had neither decreased cardiac GLUT-4 levels nor decreased glucose uptake during ischemia and, consequently, did not have poor recovery during reperfusion. We conclude that elevated plasma free fatty acids are associated with increased injury during ischemia due to decreased cardiac glucose uptake resulting from lower cardiac GLUT-4 protein levels, the levels of GLUT-4 being regulated, probably indirectly, through PPAR-α activation. [ABSTRACT FROM AUTHOR]

Perfusion of rat hearts with Krebs-Henseleit bicarbonate buffer containing low concentrations of hydrogen peroxide or t-butylhydroperoxide (50 - 500 μM) caused an imbalance in the relative synthesis versus utilization rates of ATP, leading to a net hydrolysis of ATP and phosphocreatine. Hydrogen peroxide also caused an 80% inactivation of glyceraldehyde-3-phosphate dehydrogenase, resulting in an inhibition of glycolysis and a rapid accumulation of sugar phosphates as detected with 31P-NMR spectroscopy. This inhibition was partially reversible with peroxide-free perfusion, resulting in a cessation of high-energy-phosphate hydrolysis and a decrease in the accumulated inorganic phosphate and sugar phosphate. t-Butylhydroperoxide toxicity was irreversible. Providing an alternative, non-glycolytic substrate (butyrate) did not protect against the toxicity of hydrogen peroxide, but altered the relative importance of sugar phosphate formation versus ATP hydrolysis. Experiments with heart homogenates in vitro suggest that the inhibition of glyceraldehyde-3-phosphate dehydrogenase is a consequence of a direct reaction of the enzyme with hydrogen peroxide or one of its metabolites. Hearts subjected to total global ischemia (10-20 min), followed by reperfusion with oxygenated buffer, showed no detectable inactivation of glyceraldehyde-3-phosphate dehydrogenase, indicating that ischemia and reperfusion do not result in the production of high global concentrations of hydrogen peroxide. [ABSTRACT FROM AUTHOR]

Examines whether the effects of fatty acids on the diabetic heart during ischemia involve altered glycolytic adenosine triphosphate (ATP) and proton production. Suggestion that fatty acids, but not ketone bodies, protect the diabetic heart by decreasing ATP depletion, with no detrimental effects on the normal heart during low-flow ischemia.