Metabolism of the human heart. II. Studies on fat, ketone and amino acid metabolism

Mitochondrial energy metabolism in heart failure: a question of balance

The mitochondrion serves a critical role as a platform for energy transduction, signaling, and cell death pathways relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events and downstream effector pathways involved in mitochondrial energy metabolic derangements known to occur during the development of heart failure.

Mitochondrial energy metabolism in heart failure: a question of balance

The mitochondrion serves a critical role as a platform for energy transduction, signaling, and cell death pathways relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events and downstream effector pathways involved in mitochondrial energy metabolic derangements known to occur during the development of heart failure.

Inhibition of very long chain acyl-CoA dehydrogenase during cardiac ischemia

The heart utilizes primarily fatty acids for energy production. During ischemia, however, diminished oxygen supply necessitates a switch from beta-oxidation of fatty acids to glucose utilization and glycolysis. Molecular mechanisms responsible for these alterations in metabolism are not fully understood. Mitochondrial acyl-CoA dehydrogenase catalyzes the first committed step in the beta-oxidation of fatty acids. In the current study, an in vivo rat model of myocardial ischemia was utilized to determine whether specific acyl-CoA dehydrogenases exhibit ischemia-induced alterations in activity, identify mechanisms responsible for changes in enzyme function, and assess the effects on mitochondrial respiration. Very long chain acyl-CoA dehydrogenase (VLCAD) activity declined 34% during 30 min of ischemia. Loss in activity appeared specific to VLCAD as medium chain acyl-CoA dehydrogenase activity remained constant. Loss in VLCAD activity during ischemia was not due to loss in protein content. In addition, activity was restored in the presence of the detergent Triton X-100, suggesting that changes in the interaction between the protein and inner mitochondrial membrane are responsible for ischemia-induced loss in activity. Palmitoyl-carnitine supported ADP-dependent state 3 respiration declined as a result of ischemia. When octanoyl-carnitine was utilized state 3 respiration remained unchanged. State 4 respiration increased during ischemia, an increase that appears specific to fatty acid utilization. Thus, VLCAD represents a likely site for the modulation of substrate utilization during myocardial ischemia. However, the dramatic increase in mitochondrial state 4 respiration would be predicted to accentuate the imbalance between energy production and utilization.

Influence of calcium-induced workload transitions and fatty acid supply on myocardial substrate selection

Because of differences in energy yield and oxygen demand, the selection of oxidative fuels is important in the hypoxic or ischemic heart muscle. The aim of the present study was to clarify the contradictions observed in the effects of workload and fatty acid supply on myocardial fuel preference in isolated perfused rat hearts. Nuclear magnetic resonance spectroscopy combined with the administration of substrates labeled with the stable isotope carbon 13 and isotopomer analysis of glutamate labeling offers an opportunity to simultaneously measure metabolic fluxes in pathways feeding into the tricarboxylic acid (TCA) cycle. The work output was modulated by changes in extracellular calcium. In the presence of 5 mmol/L glucose, 0.5 mmol/L octanoate in the perfusate dominated the oxidative metabolism, and workload had little effect on the ratio of glucose to fatty acid utilization. This was the case even when the octanoate concentration was lowered to 50 micromol/L. The relative rate of replenishment of the TCA cycle intermediates was higher at a low workload. The redox state of flavoproteins in the intact heart was monitored fluorometrically to obtain an estimate of the mitochondrial reduced/oxidized nicotinamide-adenine dinucleotide ratio (NADH/NAD ratio) for assessment of the dominant level of regulation of cell respiration, and the myoglobin spectrum was simultaneously monitored to evaluate the oxygenation status of the myocardium. Commencement of octanoate infusion (50 micromol/L or 0.5 mmol/L) caused a large but transient reduction of mitochondrial NAD and, conversely, its cessation elicited NADH oxidation and rebound reduction. During glucose oxidation, an increase in workload led to oxidation of the mitochondrial NADH, but this effect was much smaller in the presence of 50 micromol/L octanoate and absent in the presence of 0.5 mmol/L. This indicates that strong control of oxygen consumption during glucose oxidation is exerted in the mitochondrial respiratory chain, whereas equal control during fatty acid oxidation is exerted within the metabolic pathway upstream from the respiratory chain. It is concluded that when a medium-chain fatty acid is available, myocardial workload and energy consumption have little influence on fuel preference and glucose oxidation remains suppressed.

Nuclear receptor signaling and cardiac energetics

The heart has a tremendous capacity for ATP generation, allowing it to function as an efficient pump throughout the life of the organism. The adult myocardium uses either fatty acid or glucose oxidation as its main energy source. Under normal conditions, the adult heart derives most of its energy through oxidation of fatty acids in mitochondria. However, the myocardium has a remarkable ability to switch between carbohydrate and fat fuel sources so that ATP production is maintained at a constant rate in diverse physiological and dietary conditions. This fuel selection flexibility is important for normal cardiac function. Although cardiac energy conversion capacity and metabolic flux is modulated at many levels, an important mechanism of regulation occurs at the level of gene expression. The expression of genes involved in multiple energy transduction pathways is dynamically regulated in response to developmental, physiological, and pathophysiological cues. This review is focused on gene transcription pathways involved in short- and long-term regulation of myocardial energy metabolism. Much of our knowledge about cardiac metabolic regulation comes from studies focused on mitochondrial fatty acid oxidation. The genes involved in this key energy metabolic pathway are transcriptionally regulated by members of the nuclear receptor superfamily, specifically the fatty acid-activated peroxisome proliferator-activated receptors (PPARs) and the nuclear receptor coactivator, PPARgamma coactivator-1alpha (PGC-1alpha). The dynamic regulation of the cardiac PPAR/PGC-1 complex in accordance with physiological and pathophysiological states will be described.

Non-invasive measurements of myocardial carbon metabolism using in vivo 13C NMR spectroscopy

Despite their prime role in maintaining contractile performance, myocardial substrate uptake, substrate preference and metabolism are difficult to assess non-invasively. The objective of the present work was to extend the scope of cardiac 13C nuclear magnetic resonance (NMR) spectroscopy to the in vivo situation ('closed-chest model') and to quantitatively appraise myocardial metabolism in vivo. For this purpose, overnight-fasted Sprague-Dawley rats received intravenous infusions of non-radioactive 13C-labeled glucose, 3-hydroxybutyrate, and acetate as markers for glycolysis, metabolism of ketone bodies and direct incorporation into tricarboxylic acid (TCA) cycle, respectively. In vivo 13C NMR spectra (at 7 T) were acquired from the myocardium with a time resolution of 6 min. At the end of the infusion experiments, tissue extracts were prepared and further analyzed by high-resolution 13C NMR spectroscopy in order to corroborate the findings obtained in vivo. Accordingly, 3-hydroxybutyrate and acetate were rapidly extracted by the myocardium and supplied 42 +/- 6 and 53 +/- 9% of the acetyl-CoA for TCA cycle operation, whereas glucose, although also well extracted, did not contribute to myocardial oxidative metabolism. Myocardial TCA cycle turnover (V(TCA)) in vivo was estimated at 1.34 +/- 0.07 micromol/min/g wet weight, myocardial oxygen consumption (MVO2) at 2.95 +/- 0.16 micromol/min/g wet weight, exchange rate between alpha-ketoglutarate and glutamate (V(x)) at 1.22 +/- 0.08 micromol/min/g wet weight and rate of glutamine synthesis (V(gln)) at 0.14 +/- 0.02 micromol/min/g wet weight. The substantial synthesis of myocardial glutamine is in contrast to experiments with isolated and saline perfused hearts. In conclusion, it is demonstrated that 13C NMR spectroscopy of the heart in intact rats is feasible and provides new quantitative insight into myocardial substrate uptake, preference and metabolism in vivo.

Intraoperative metabolic monitoring of the heart: I. Clinical assessment of coronary sinus metabolites

Numerous clinical studies have corroborated the ability of intraoperative sampling of coronary sinus blood to measure changes in myocardial metabolism induced by ischemia and reperfusion. Among other changes, cardiac arrest induces a period of obligate myocardial lactate production that persists for an indeterminate amount of time after reperfusion. Coronary sinus lactate assays have been established as a standard method to compare various myocardial protection strategies. Current methodology requires detailed sample processing, precluding real-time feedback in the operating room. Newer devices hold promise in allowing the online assessment of myocardial metabolism; however, these methods await precise validation.

Myocardial Free Fatty Acid and Glucose Use After Carvedilol Treatment in Patients With Congestive Heart Failure

Background —Use of β-adrenoreceptor blockade in the treatment of heart failure has been associated with a reduction in myocardial oxygen consumption and an improvement in myocardial energy efficiency. One potential mechanism for this beneficial effect is a shift in myocardial substrate use from increased free fatty acid (FFA) oxidation to increased glucose oxidation.

Methods and Results —We studied the effect of carvedilol therapy on myocardial FFA and glucose use in 9 patients with stable New York Heart Association functional class III ischemic cardiomyopathy (left ventricular ejection fraction ≤35%) using myocardial positron emission tomography studies and resting echocardiograms before and 3 months after carvedilol treatment. Myocardial uptake of the novel long chain fatty acid metabolic tracer 14(R, S)-[ 18 F]fluoro-6-thia-heptadecanoic acid ([ 18 F]-FTHA) was used to determine myocardial FFA use, and [ 18 F]fluoro-2-deoxy-glucose ([ 18 F]-FDG) was used to determine myocardial glucose use. After carvedilol treatment, the mean myocardial uptake rate for [ 18 F]-FTHA decreased (from 20.4±8.6 to 9.7±2.3 mL · 100 g –1 · min –1 ; P <0.005), mean fatty acid use decreased (from 19.3±7.0 to 8.2±1.8 μmoL · 100 g –1 · min –1 ; P <0.005), the mean myocardial uptake rate for [ 18 F]-FDG was unchanged (from 1.4±0.4 to 2.4±0.8 mL · 100 g –1 · min –1 ; P =0.14), and mean glucose use was unchanged (from 11.1±3.1 to 18.7±6.0 μmoL · 100 g –1 · min –1 ; P =0.12). Serum FFA and glucose concentrations were unchanged, and mean left ventricular ejection fraction improved (from 26±2% to 37±4%; P <0.05).

Conclusions —Carvedilol treatment in patients with heart failure results in a 57% decrease in myocardial FFA use without a significant change in glucose use. These metabolic changes could contribute to the observed improvements in energy efficiency seen in patients with heart failure.

Changes in perfusion and fatty acid metabolism of rat heart with autoimmune myocarditis

To elucidate the change in perfusion and aerobic metabolism in myocarditis, tissue counting and dual tracer ex vivo autoradiography with Tl-201 and a free fatty acid analog, I-123- or I-125-labeled (p-iodophenyl)-methyl-pentadecanoic acid (BMIPP), were performed in rats with myocarditis induced by immunization with cardiac myosin. Inflammatory damage was classified histologically. At the acute stage (2-4 weeks after the antigen-injection), total heart uptakes of Tl and BMIPP and the ratio (BMIPP/Tl) were significantly reduced in myocarditis rats (N = 15) compared with the controls (N = 12). Myocardial distribution of Tl and BMIPP was not homogeneous. Relative uptake of Tl and BMIPP (N = 9, 128 regions) was gradually decreased with the extent of inflammation, and the regional BMIPP/Tl was smaller than the control. At the subacute stage (7 weeks after the antigen-injection), total Tl uptake in myocarditis rats (N = 5) recovered to the control level (N = 4), but that of BMIPP was still significantly lower than the control. BMIPP/Tl was still significantly lower in myocarditis. Myocardial distribution of Tl and BMIPP recovered to be more homogeneous. Relative uptake of Tl and BMIPP (N = 6, 78 regions) still gradually but significantly decreased with the extent of inflammation. Regional BMIPP/Tl was still depressed in myocarditis. These results indicate that myocardial perfusion and aerobic metabolism were discrepant and heterogeneously suppressed with severe inflammation during the acute stages, but the difference decreases with time. Examination with Tl-201 and BMIPP may provide information about the severity of myocarditis.

Myocardial substrate uptake and oxidation during and after routine cardiac surgery

OBJECTIVE

This study was designed to clarify whether myocardial substrate uptake and oxidation change after a period of hypothermic cardioplegic arrest during coronary artery bypass grafting procedures.

METHODS

In 30 patients arterial and coronary sinus blood was sampled and coronary sinus flow measurements were performed before and after sternotomy and 10 minutes, 20 minutes, 50 minutes, and 6 hours after release of the aortic crossclamp. Measurement of free fatty acids, lactate, glucose, oxygen content, and carbon dioxide content in arterial and coronary sinus blood allowed calculations of myocardial substrate use, respiratory quotients, and myocardial oxidation rates of carbohydrates and fat.

RESULTS

Uptake of free fatty acids and lactate was significant throughout the study and did not change in association with release of the crossclamp. Free fatty acid and lactate uptake measured 6 +/- 4 micromol/min and 23 +/- 26 micromol/min, respectively, before crossclamping compared with 8 +/- 7 micromol/min and 19 +/- 21 micromol/min, respectively, after release of the clamp. Glucose uptake was significant only during the first hour after crossclamp release and increased from 7 +/- 50 to 28 +/- 34 micromol/L after crossclamp release. Myocardial oxygen consumption did not change significantly (0.5 +/- 0.2 mmol/L compared with 0.35 +/- 0.2 mmol/L) after release of the crossclamp. Myocardial oxygen extraction ratio decreased from 58% +/- 8% to 41% +/- 13% after crossclamp release. Respiratory quotient increased after crossclamp release (0.85 +/- 0. 2 compared with 1.00 +/- 0.2), which implies that carbohydrate oxidation increased at the expense of free fatty acid oxidation.

CONCLUSION

We conclude that hypothermic cardioplegic arrest during coronary artery bypass graft operations is associated with a transiently increased uptake and oxidation of carbohydrates during the immediate reperfusion phase.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

beta-adrenergic stimulation induces transient imbalance between myocardial substrate uptake and metabolism in vivo

At steady state, a balance is expected between net myocardial uptake of the principal exogenous carbon substrates and the rate at which these substrates are metabolized. Such a balance is present when the sum of the oxygen extraction ratios (OERs) for glucose, lactate, and free fatty acids (FFA) is near unity. We have previously observed that systemic administration of the beta-adrenergic agonist isoproterenol (Iso) induces a state of excess myocardial substrate uptake relative to the rate of substrate metabolism, reflected by a sum of OERs significantly >1.0. This occurs in conjunction with an Iso-stimulated increase in circulating insulin levels. The goal of the present study was to determine whether this excess substrate uptake depends on the effects of insulin and time. In open-chest anesthetized pigs, myocardial blood flow, substrate uptake, and oxygen consumption were measured at baseline and during systemic administration of Iso (0.08 microgram. kg-1. min-1 iv) under the following conditions: group 1 (n = 10), normal endogenous insulin release; group 2 (n = 10), inhibition of endogenous insulin release with somatostatin; group 3 (n = 7), at 45 and 90 min Iso; group 4 (n = 7), at 45 and 90 min Iso, with exogenous insulin given during the latter measurement. In group 1, plasma insulin rose fivefold with Iso while the sum of the OERs for glucose, lactate, and FFA increased from 0.92 +/- 0.21 at baseline to 1.57 +/- 0.17 with Iso (P < 0.01). In group 2, somatostatin blunted the increase in insulin with Iso and there was no significant change in the sum of OERs between baseline and Iso. In group 3, the sum of OERs increased from 0.95 +/- 0.11 at baseline to 1.69 +/- 0.20 at 45 min Iso (P < 0.01), similar to the response of group 1. However, the state of excess substrate uptake was transient; by 90 min Iso the sum of OERs declined to 0.69 +/- 0.21 (P < 0.05 vs. 45 min Iso). In group 4, excess substrate uptake could not be sustained at 90 min Iso despite administration of exogenous insulin. Systemic beta-adrenergic stimulation causes a transient condition of myocardial substrate uptake in excess of metabolism. Increased plasma insulin is necessary to produce this condition, but a high insulin level does not prolong the condition.

Myocardial blood flow and oxygen consumption during postprandial lipemia and heparin-induced lipolysis

The role of the physical state of plasma as a determinant of oxygen availability to the myocardium has been investigated during the course of alimentary lipemia. After the development of substantial plasma lactescence, the coronary blood flow (nitrous oxide method) and myocardial oxygen consumption were assessed in 7 normal human subjects and repeated after heparin-induced lipolysis. The lipemic state was further contrasted with a control fasting group, comparable in age and sex.

The mean coronary blood flow for 15 fasting controls was 83 ml. per 100 Gm. of left ventricle per minute with a myocardial oxygen extraction of 11.04 volumes per cent, and a myocardial oxygen consumption of 9.0 ml. per 100 Gm. of left ventricle per minute. By contrast, the mean coronary blood flow during maximal lipemia in the 7 subjects fed cream was 20 per cent below normal, with a value of 67 ml. per 100 Gm. per minute ( p =< 0.01). As the extraction of oxygen was not significantly affected, the calculated myocardial oxygen consumption was proportionately reduced to 7.02 ml. per 100 Gm. per minute ( p =< 0.01). A failure of the anticipated oxygen extraction increment in the face of coronary blood flow reduction suggests an impediment of blood-tissue oxygen transport during lipemia.

After the administration of 60 mg. of heparin to the 7 lipemic subjects, a 65 per cent decline in plasma lactescence was observed by 45 minutes, when the coronary blood flow and myocardial oxygen consumption were elevated to 87 ml. per 100 Gm. per minute ( p =< 0.05) respectively. Thus, the reduced coronary flow and myocardial oxygen consumption were restored in each instance to normal levels during the process of plasma clearing. There were no associated systemic hemodynamic changes to account for such increments. These heparin effects appear dependent on the lipemia-clearing property, for no alteration in coronary dynamics was found in 6 additional patients in whom this activity was not manifest after the same heparin dosage.

The residual lactescence after post-heparin lipolysis was associated with no significant deviation of coronary dynamics from the normal. That a concentration-dependent phenomenon is operative, was confirmed in a separate group of patients in whom low lactescence values developed in the course of alimentary lipemia without affecting myocardial oxygen consumption.

The relevance of the lipemic state, per se, to the pathophysiology of myocardial ischemia appears to depend upon the establishment of an oxygen gradient within the myocardium, presumably through altered pressure-flow relationships produced by lipemic blood within a pathologic vessel.

The removal of triglyceride fatty acids from the circulation of the supradiaphragmatic portion of the rat

When 14 C-labelled chylomicron lipid is injected into the circulation of the supradiaphragmatic portion of the fasting rat a composite disappearance curve which can be represented by two exponential components is obtained. The dominant component, which accounts for the removal of over 90 % of the injected label, has a short half-life similar to that observed for the triglyceride fatty acids of injected chyle in intact animals. The minor component has a much longer half-life and may be accounted for, at least in part, by the presence of 14 C-labelled material in phosphatides and cholesterol esters in the plasma. At 40 min after the injection of the chylomicron lipid, when over 90 % of the dose has left the circulation, most of the labelled material can be recovered in the muscle and skin. Very little radioactivity is present in the brain. Oxidation of the labelled material appears to occur more rapidly in the fasting than in the fed state. Endogenous triglycerides in d < 1.006 lipoproteins are removed from the plasma at a similar rate to injected chylomicron lipid. Triglyceride in d > 1.006 lipoproteins are removed much more slowly. Free fatty acids are removed from the circulation of the supradiaphragmatic portion of the rat with a half-life of 1.5 min. The significance of these findings is discussed in relation to current concepts of fatty acid transport.

Substrate selection in the isolated working rat heart: effects of reperfusion, afterload, and concentration

A study of substrate selection in the isolated heart was made using 13C NMR isotopomer analysis, a method that unequivocally identifies relative substrate utilization. This technique has several advantages over conventional approaches used to study this problem. It detects the labeling of metabolic end-products present in tissue, as opposed to more indirect methods such as measurement of respiratory quotient, arteriovenous differences, or specific activity changes in the added substrate. It also has advantages over methods such as 14CO2 release, which may involve dilution of label with unlabeled pools before CO2 release. Furthermore, it can measure the relative oxidation of up to four substrates in a single experiment, which other labeling techniques cannot conveniently achieve. Substrate selection was considered in light of its effects on myocardial efficiency and recovery from ischemia. A mixture of four substrates (acetoacetate, glucose, lactate, and a mixture of long chain fatty acids), present at physiological concentration (0.17, 5.5, 1.2, and 0.35 mM, respectively), was examined. This is the first use of such a mixture in the study of substrate selection in an isolated organ preparation. At these concentrations, it was found that fatty acids supplied the majority of the acetyl-CoA (49%), and a substantial contribution was also provided by acetoacetate (23%). This suggests that the ketone bodies are a more important substrate than generally considered. Indeed, normalizing the relative utilizations on the basis of acetyl-CoA equivalents, ketone bodies were by far the preferred substrate. The relative lactate oxidation was only 15%, and glucose oxidation could not be detected. No change in utilization was detected after 15 min of ischemia followed by 40 min of reperfusion. The change in substrate selection with afterload was examined, to mimic the stress-related changes in workload found with ischemia. Only minor changes were found. Substrate selection from the same group of substrates, but employing concentrations observed during starvation, was also assessed. This represents the state during which most clinical treatments and evaluations are performed. In this case, acetoacetate was the most used substrate (78%), with small and equal contributions from fatty acids and endogenous substrates; the oxidation of lactate was suppressed.

Effect of metoprolol on myocardial function and energetics in patients with nonischemic dilated cardiomyopathy: a randomized, double-blind, placebo-controlled study

OBJECTIVES

This study examined the effects of metoprolol on left ventricular performance, efficiency, neurohormonal activation and myocardial respiratory quotient in patients with dilated cardiomyopathy.

BACKGROUND

The mechanism by which beta-adrenergic blockade improves ejection fraction in patients with dilated cardiomyopathy remains an enigma. Thus, we undertook an extensive hemodynamic evaluation of this mechanism. In addition, because animal models have shown that catecholamine exposure may increase relative fatty acid utilization, we hypothesized that antagonism of sympathetic stimulation may result in increased carbohydrate utilization.

METHODS

This was a randomized, double-blind, prospective trial in which 24 men with nonischemic dilated cardiomyopathy underwent cardiac catheterization before and after 3 months of therapy with metoprolol (n = 15) or placebo (n = 9) in addition to standard therapy. Pressure-volume relations were examined using a micromanometer catheter and digital ventriculography.

RESULTS

At baseline, the placebo-treated patients had somewhat more advanced left ventricular dysfunction. Ejection fraction and left ventricular performance improved only in the metoprolol-treated patients. Stroke and minute work increased without an increase in myocardial oxygen consumption, suggesting increased myocardial efficiency. Further increases in ejection fraction were seen between 3 and 6 months in the metoprolol group. The placebo group had a significant increase in ejection fraction only after crossover to metoprolol. A significant relation between the change in coronary sinus norepinephrine and myocardial respiratory quotient was seen, suggesting a possible effect of adrenergic deactivation on substrate utilization.

CONCLUSIONS

These data demonstrate that in patients with cardiomyopathy, metoprolol treatment improves myocardial performance and energetics, and favorably alters substrate utilization. Beta-adrenergic blocking agents, such as metoprolol, are hemodynamically and energetically beneficial in the treatment of myocardial failure.

Interrelationship between lactate and cardiac fatty acid metabolism

This overview is presented, in the main, to summarize the following aspects of lactate and cardiac fatty acid metabolism: 1. The utilization of exogenous carbohydrates and fatty acids by the heart. 2. The competition between lactate and fatty acids in cardiac energy metabolism. 3. The effect of lactate on endogenous triacylglycerol homeostasis. 4. Lactate-induced impairment of functional recovery of the post-ischemic heart. 5. The effect of lactate on lipid metabolism in the ischemic and post-ischemic heart. 6. The consequences of hyperlactaemia for cardiac imaging.

Muscle relaxants change myocardial metabolism in patients with ischemic heart disease during high-dose fentanyl anesthesia

Although not unanimously accepted, high-dose fentanyl anesthesia has been associated with hemodynamic stability and little derangement of myocardial oxygen balance. This apparent inconsistency inspired us to investigate the effects on cardiac function and myocardial metabolism of stepwise increasing doses of fentanyl, accumulating to 15, 30, and 50 micrograms.kg-1, with the least possible interference from other drugs. Subjects were unpremedicated patients with ischemic cardiac disease scheduled for coronary artery bypass grafting or major vascular surgery. In an initial study employing succinylcholine for muscle relaxation, we found that heart rate (HR), coronary sinus blood flow (CSF) and coronary vascular resistance (CVR) remained unchanged, while systemic arterial pressure (SBP), rate-pressure product (RPP), coronary perfusion pressure (CPP) and left ventricular work (LVW) decreased. Myocardial uptake of oxygen (MVO2) and free fatty acids (FFA) both decreased in a dose-dependent manner. Arterial lactate concentration and myocardial lactate uptake both increased. These findings opposed the postinduction myocardial ischemia noted by some other investigators. In most of these studies pancuronium bromide had been used for muscle relaxation. Since the latter agent has been claimed to increase cardiac work, a second group of correspondingly diseased patients was studied in which succinylcholine was replaced by pancuronium bromide. In this group HR, RPP, CSF and MVO2 all increased at the lowest dose of fentanyl and HR additionally also at 30 micrograms.kg-1. The cardiac index was higher in the pancuronium group at the lowest and middle dose steps of fentanyl. Lactate uptake decreased with higher doses of fentanyl and relative myocardial lactate extraction declined.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary sinus cannulation via the femoral vein

Cannulation of the coronary sinus has been usually accomplished by advancing a catheter through the brachial vein, subclavian vein, or internal jugular vein by venous cutdown or a sheath method. We here describe a technique for cannulation into the coronary sinus through the femoral vein by using a modified catheter. This catheter was easily inserted into the coronary sinus in all of 40 consecutive patients in whom it was attempted.

Individual free fatty acids and lactate uptake in the human heart during severe sepsis

Coronary haemodynamics and myocardial metabolism of nonesterified fatty acids (NEFA) and lactate were studied in 11 patients with severe sepsis, and compared to 10 control subjects. Coronary sinus blood flow was evaluated by thermodilution. Arterial and coronary sinus blood samples were collected for the measurement of lactate and total and individual NEFA concentrations both in septic and control patients. There was an increase in lactate and total NEFA arterial concentrations with a marked increase in palmitic and linolenic acids. The uptake of the main NEFA (C14:0 to C18:2) was significantly decreased. In the control group, individual NEFA uptake was proportional to their arterial concentrations. This relationship was not observed in patients with sepsis: there was no preferential extraction of any particular NEFA. Furthermore, in patients with sepsis, myocardial oxygen consumption did not correlate with NEFA, but only with lactate uptake. Alterations in NEFA uptake were found to be constant during severe sepsis and are consistent with major disturbances in myocardial metabolism.

Prostacyclin-inhibition of lysine accumulation by the rat left ventricle

1. Left ventricular slices of male Sprague-Dawley rats were incubated with a fixed concentration of 0.5 microCi/ml 3H-lysine and several concentrations of unlabelled lysine ranging from 0.2 to 5.0 mM in control and prostacyclin-treated experiments. The time of incubation ranged from 0.5 to 90 min. 2. Left ventricular slices were cut to have an optimal thickness of 0.47 +/- 0.09 mm. 3. Lysine was taken up against a concentration gradient. Saturation was reached at 0.5 mM and steady state accumulation of lysine was attained within 60 min. 4. Prostacyclin in concentrations ranging from 1.2 x 10(-8) to 4.8 x 10(-8) M inhibited lysine transport in left ventricular slices significantly (P less than 0.01).

Myocardial metabolism of free fatty acids. Studies with 14C-labeled substrates in humans

Free fatty acids are considered to be the major energy source for the myocardium. To investigate the metabolic fate of this substrate in humans, 24 subjects underwent coronary sinus and arterial catheterization. 13 subjects were healthy volunteers and 11 subjects had symptoms of ischemic heart disease. [1-14C]oleate or [1-14C]palmitate bound to albumin was infused at a constant rate of 25 microCi/h. Oxidation was determined by measuring the 14CO2 production. The data demonstrated that a high percentage (84 +/- 17%) of the palmitate and oleate extracted by the myocardium underwent rapid oxidation. A highly significant correlation was present between the arterial level and the amount oxidized (r = 0.82, P less than 0.001 for palmitate; r = 0.77, P less than 0.001 for oleate). The isotope extraction ratio was greater than the chemical extraction ratio. This difference of 6 +/- 2 nmol/ml of blood in the young normal subjects was significantly less than the 12 +/- 4 nmol/ml observed in the ischemic heart disease patients (P less than 0.001).

Failure of a branched chain amino acid-enriched diet to reverse ethanol inhibition of cardiac protein synthesis in the rat

The fractional rate of protein synthesis was determined in the hearts of rats in vivo fed on diets containing 27% of energy as ethanol or on this diet supplemented with 5% of equimolar amounts of branched chain amino acids (BCAA). Administration of ethanol significantly decreased the fractional synthetic rate of mixed cardiac proteins and this depression was not ameliorated by concomitant feeding of BCAA. These data are discussed in relation to the stimulation of cardiac protein synthesis by BCAA observed in vitro.

Studies of fatty acid metabolism with positron emission tomography in patients with cardiomyopathy

Positron emission tomography (PET) permits in vivo as well as noninvasive study of fatty acid metabolism. Parameters of 11C-palmitate kinetics relate to the oxidation of fatty acids, and palmitic acid uptake is impaired in patients with coronary disease and cardiomyopathy. Normal myocardium shows homogeneous fatty acid metabolism and can resort to alternate substrates. Diseased myocardium exhibits regional heterogeneity in fatty acid uptake and utilization. In patients with cardiomyopathy, distinct patterns of fatty acid metabolism can be observed following changes of substrate availability by application of an oral glucose load. This intervention also enhances the heterogeneity of 11C-palmitic acid (CPA) uptake and clearance. Thus, PET studies with CPA permit the noninvasive demonstration of effects on substrate availability and may help to characterize patients with ventricular dysfunction on the biochemical level.

Effects of substrate availability on myocardial C-11 palmitate kinetics by positron emission tomography in normal subjects and patients with ventricular dysfunction

The possibility of demonstrating noninvasively with C-11 palmitate and positron emission tomography (PET) changes in myocardial substrate metabolism in normal and diseased human myocardium in response to altered substrate availability in blood and disease-related abnormalities was examined in five normal volunteers and 16 patients with ventricular dysfunction. C-11 palmitate injection and serial PET imaging were performed after an overnight fast (control period) and again 2 hours later after oral glucose (50 gm). Myocardial C-11 time-activity curves from serial PET images revealed a biexponential clearance pattern. An early rapid phase, defined by relative size and clearance half-time, reflects C-11 palmitate oxidation and the late slow phase tracer deposition in the endogenous lipid pool. During the control period, the tracer fraction entering the early rapid phase averaged 47 +/- 13% (SD) in normal subjects and 45 +/- 12% in patients. Corresponding clearance half-times were 19 +/- 7 and 20 +/- 5 minutes, respectively. Heart rate and blood pressure remained unchanged after glucose, but plasma glucose levels rose by 72.5% in normal subjects and by 98.9% in patients, while free fatty acid levels fell by 72% and 42% (p less than 0.001), respectively. In normal subjects, the tracer fraction in the early rapid phase fell by 43% (p less than 0.005) and the clearance half-time increased by 46% (p less than 0.01). In patients, the response of C-11 palmitate tissue kinetics to glucose was variable. In nine patients, it was similar to that in normal subjects while in the other seven patients a "paradoxic" response occurred. The tracer fraction entering the rapid clearance phase increased after glucose by 30% (p less than 0.05) associated with a 36% (p less than 0.05) decline in clearance half-times. The paradoxic response was unrelated to disease etiology or plasma substrate levels but occurred mostly in left ventricles with more severely depressed function. Thus, PET and C-11 palmitate allow the noninvasive demonstration of the known response of substrate metabolism of the human heart to altered substrate availability. Glucose administration in fasted humans serves as a provocative test of substrate regulation which can be abnormal in myocardial disease and can be demonstrated noninvasively.

Omega-halofatty acids: a probe for mitochondrial membrane integrity. In vitro investigations in normal and ischaemic myocardium

Long-chain omega-halofatty acids, especially omega-123I-iodoheptadecanoic acid (IHA), are widely used clinically as radiopharmaceuticals for functional heart imaging. The metabolic interpretation of the various elimination rates, however, remains in dispute. It has been previously shown (Kloster and Stöcklin 1982) that in isolated perfused guinea-pig hearts halide diffusion from the mitochondrion to the blood is the rate-determining step of IHA pharmacokinetics in normal myocardium. We have now extended these in vitro experiments to normal and globally ischaemic isolated perfused rabbit hearts. Again, in normal hearts a single phase iodide elimination half-time (14.3 +/- 2.1 min) was observed. In hearts made globally ischaemic for 90 min, the iodide elimination was biphasic with a first fast phase (T 1/2 = 3.8 +/- 0.49 min) and a late slow phase (T 1/2 = 60.5 +/- 14.0 min). The first fast phase is attributed to iodide ion released by residual beta-oxidation (more rapid than in normal hearts due to damaged membranes in ischaemia), while the late slow phase is explained by beta-oxidation of IHA slowly released by hydrolysis of intracellular lipid stores. These data were compared with published data from investigations in patients which seem to support our interpretation.

Effects of propranolol on myocardial oxygen consumption, substrate extraction, and haemodynamics in hypertrophic obstructive cardiomyopathy

Myocardial substrate extraction, coronary sinus flow, cardiac output, and left ventricular pressure were measured at increasing pacing rates before and after propranolol (0.2 mg/kg) in 13 patients with hypertrophic obstructive cardiomyopathy (HOCM) during diagnostic cardiac catheterisation. At the lowest pacing rate myocardial oxygen consumption varied considerably between patients and very high values were found in several individuals (range 10.1 to 57.5 ml/min). These large differences between patients were not explicable by differences in cardiac work; consequently, cardiac efficiency, estimated from the oxygen cost of external work, varied between patients and was lower than normal in all but two. The pattern of substrate extraction at the lowest pacing rate was similar to results reported for the normal heart, and measured oxygen consumption could be accounted for by complete oxidation of the substrates extracted; thus there was no evidence of a gross abnormality of oxidative metabolism, suggesting that low efficiency lay in the utilisation rather than in the production of energy. Each of the four patients with the highest myocardial oxygen consumption and lowest values of efficiency sustained progressive reductions in lactate and pyruvate extraction as heart rate increased, and at the highest pacing rate had low (< 3%) or negative lactate extraction ratios. In three of these four, coronary sinus flow did not increase progressively with each increment in heart rate. One patient with low oxygen consumption and normal efficiency also failed to increase coronary flow with the final increment in heart rate, and produced lactate at the highest pacing rate. Thus the five patients in whom pacing provoked biochemical evidence of ischaemia all had excessive myocardial oxygen demand and/or limited capacity to increase coronary flow. Propranolol did not change lactate extraction significantly at any pacing rate in either the ischaemic or non-ischaemic groups. In only one patient was ischaemia at the highest pacing rate abolished after propranolol, and this was associated with a 30 per cent reduction in oxygen consumption. These results do not demonstrate a direct effect of propranolol upon myocardial metabolism in patients with HOCM, but emphasise the potential value of beta-blockade in protecting these patients from excessive increases in heart rate.