Determination of high energy phosphates and glycogen in cardiac and skeletal muscle biopsies, with special reference to influence of biopsy technique and delayed freezing

Heart, lipids and hormones

Cardiovascular disease is the leading cause of death in general population. Besides well-known risk factors such as hypertension, impaired glucose tolerance and dyslipidemia, growing evidence suggests that hormonal changes in various endocrine diseases also impact the cardiac morphology and function. Recent studies highlight the importance of ectopic intracellular myocardial and pericardial lipid deposition, since even slight changes of these fat depots are associated with alterations in cardiac performance. In this review, we overview the effects of hormones, including insulin, thyroid hormones, growth hormone and cortisol, on heart function, focusing on their impact on myocardial lipid metabolism, cardiac substrate utilization and ectopic lipid deposition, in order to highlight the important role of even subtle hormonal changes for heart function in various endocrine and metabolic diseases.

Metabolic alterations induce oxidative stress in diabetic and failing hearts: different pathways, same outcome

SIGNIFICANCE

Several authors have proposed a link between altered cardiac energy substrate metabolism and reactive oxygen species (ROS) generation. A cogent evidence of this association has been found in diabetic cardiomyopathy (dCM); however, experimental findings in animal models of heart failure (HF) and in human myocardium also seem to support the coexistence of the two alterations in HF.

CRITICAL ISSUES

Two important questions remain open: whether pathological changes in metabolism play an important role in enhancing oxidative stress and whether there is a common pathway linking altered substrate utilization and activation of ROS-generating enzymes, independently of the underlying cardiac pathology. In this regard, the comparison between dCM and HF is intriguing, in that these pathological conditions display very different cardiac metabolic phenotypes.

RECENT ADVANCES

Our literature review on this topic indicates that a vast body of knowledge is now available documenting the relationship between the metabolism of energy substrates and ROS generation in dCM. In some cases, biochemical mechanisms have been identified. On the other hand, only a few and relatively recent studies have explored this phenomenon in HF and their conclusions are not consistent.

FUTURE DIRECTIONS

Better methods of investigation, especially in vivo, will be necessary to test whether the metabolic fate of certain substrates is causally linked to ROS production. If successful, these studies will place a new emphasis on the potential clinical relevance of metabolic modulators, which might indirectly mitigate cardiac oxidative stress in dCM, HF, and, possibly, in other pathological conditions.

Metabolic and genetic regulation of cardiac energy substrate preference

Proper heart function relies on high efficiency of energy conversion. Mitochondrial oxygen-dependent processes transfer most of the chemical energy from metabolic substrates into ATP. Healthy myocardium uses mainly fatty acids as its major energy source, with little contribution of glucose. However, lactate, ketone bodies, amino acids or even acetate can be oxidized under certain circumstances. A complex interplay exists between various substrates responding to energy needs and substrate availability. The relative substrate concentration is the prime factor defining preference and utilization rate. Allosteric enzyme regulation and protein phosphorylation cascades, partially controlled by hormones such as insulin, modulate the concentration effect; together they provide short-term adjustments of cardiac energy metabolism. The expression of metabolic machinery genes is also dynamically regulated in response to developmental and (patho)physiological conditions, leading to long-term adjustments. Specific nuclear receptor transcription factors and co-activators regulate the expression of these genes. These include peroxisome proliferator-activated receptors and their nuclear receptor co-activator, estrogen-related receptor and hypoxia-inducible transcription factor 1. Increasing glucose and reducing fatty acid oxidation by metabolic regulation is already a target for effective drugs used in ischemic heart disease and heart failure. Interaction with genetic factors that control energy metabolism could provide even more powerful pharmacological tools.

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure.

Maternal Hyperglycemia Improves Fetal Cardiac Function During Tachycardia-Induced Heart Failure in Pigs

Background— Fetal tachycardia often leads to cardiac failure, which in experimental settings can be prevented by direct fetal glucose-insulin administration. In this study, we hypothesize that similar effects can be obtained indirectly by inducing maternal hyperglycemia.

Methods and Results— Systolic and diastolic indices (dP/dt max and τ) of left ventricular function were measured by use of high-fidelity catheters during 180 minutes of aggressive atrial pacing (≈300 bpm) in 12 preterm porcine fetuses. In 6 fetuses, maternal hyperglycemia (15 mmol/L) was induced for the last 120 minutes of pacing. The remaining fetuses served as controls. Glucose, insulin, and free fatty acid levels were determined, as was fetal myocardial glycogen content. Maternal glucose infusion led to significant fetal hyperglycemia and hyperinsulinemia but did not change the inherently low fetal levels of free fatty acids. There were no differences between groups with regard to dP/dt max (1025±226 and 1037±207 mm Hg, P =NS) and τ (20.6±2.0 and 21.4±1.6 ms, P =NS) at baseline (100%). During the 180 minutes of pacing, systolic function (dP/dt max ) and diastolic function (τ) deteriorated more in the control group than in the hyperglycemic group ( P <0.001 for both). At 180 minutes, dP/dt max was 62±18% of baseline in controls and 85±11% in hyperglycemic fetuses ( P =0.03), and τ was 117±12% and 98±4%, respectively ( P =0.004).

Conclusions— Induced maternal hyperglycemia improves fetal cardiac function during fetal tachycardia and suggests a possible additional therapeutic option to improve the function of the failing fetal heart before or during antiarrhythmic therapy. The findings may be relevant in fetal heart failure in general.

Force development, energy state and ATP production of cardiac muscle from turtles and trout during normoxia and severe hypoxia

The effects of hypoxia on energy economy of cardiac muscle were compared between the hypoxia-tolerant freshwater turtle at 20 degrees C and the hypoxia-sensitive rainbow trout at 15 degrees C. Isolated ventricular preparations were left either at rest or stimulated at 30 min(-1) to develop isometric twitch force. Under oxygenated conditions, twitch force and oxygen consumption were similar for the two species. Overall metabolism was reduced during severe hypoxia in both resting and stimulated preparations and under these conditions most of the ATP production was derived from anaerobic metabolism. During hypoxia, a metabolic depression of approximately 2/3 occurred for non-contractile processes in both turtle and trout preparations. During hypoxia, lactate production and residual oxygen consumption were similar in turtle and trout. Cellular energy state and phosphorylation potential decreased during severe hypoxia in both species and this reduction was more severe in preparations stimulated to contraction. However, in turtle ventricular preparations the energy state and phosphorylation potential stabilised at higher levels than in trout, and turtle preparations also maintained a higher twitch force throughout the hypoxic period. Moreover, twitch force relative to total ATP hydrolysis was markedly increased during hypoxia in turtle while this ratio was unchanged for trout. The main findings of this study are: (1) cellular energy liberation and the energy demand of non-contractile processes decreased to similar levels in hypoxic turtle and trout myocardium; (2) turtle myocardium maintained a substantially higher cellular energy state and twitch force development than trout myocardium during hypoxia and (3) the ratio of twitch force to ATP hydrolysis increased during hypoxia in turtle but was unchanged in trout. It is possible that this superior economy of the contracting turtle myocardium contributes to the remarkable hypoxia tolerance of freshwater turtles.

Glucose-insulin infusion improves cardiac function during fetal tachycardia

OBJECTIVES

The aim of this work was to study the effects of substrate deficiency and supplementation on cardiac function during fetal tachycardia.

BACKGROUND

Although sustained fetal tachycardia may lead to cardiac failure and intrauterine death, neonatal tachycardia is generally better tolerated. Fetal myocardial energy production relies almost solely on glucose as substrate. We hypothesized that increased substrate availability by glucose-insulin (GI) infusion would improve fetal myocardial responses to tachycardia.

METHODS

We used three porcine models: 1) an isolated fetal heart model; 2) an in vivo fetal model; and 3) an in vivo closed-chest neonatal model. Each animal was randomized to control or GI treatment during tachycardia. In model 1, the controls were perfused with conventional Krebs-Henseleit solution containing a glucose concentration of 5.5 mmol/l; the GI hearts received double glucose concentration and added insulin. In models 2 and 3, the GI animals received insulin in a 20% glucose solution. All hearts were exposed to 90 min of pacing at 250 to 330 beats/min.

RESULTS

The isolated fetal hearts in the GI group showed no decline in dP/dt(max) during pacing, while the controls declined. In the in vivo fetal hearts, dP/dt(max) remained unchanged in the GI group and decreased significantly in the control group. Myocardial glycogen content was higher in the GI group than in controls. Functional indexes remained unchanged among both neonatal groups despite a higher glycogen content in the GI group.

CONCLUSIONS

Glucose-insulin infusion during fetal tachycardia has a beneficial effect on myocardial metabolism and cardiac function. These observations may have direct clinical relevance to the management of fetal arrhythmia.

Effects of temperature and anoxia upon the performance ofin situperfused trout hearts

Rainbow trout (Oncorhynchus mykiss) are likely to experience acute changes in both temperature and oxygen availability and, like many other organisms, exhibit behavioural selection of low temperatures during hypoxia that acts to reduce metabolism and alleviate the demands on the heart. To investigate whether low temperature protects cardiac performance during anoxia, we studied the effects of an acute temperature change, from 10°C to either 5°C, 15°C or 18°C, upon the performance of in situ perfused trout hearts before, during and after exposure to 20 min of anoxia. Routine cardiac workload mimicked in vivo conditions at the given temperatures, and the effects of anoxia were evaluated as maximal cardiac performance before and after 20 min of anoxic perfusion. Functional data were related to maximal activities of glycolytic enzymes and energetic status of the heart at the termination of the experiment.

At high oxygenation, maximum cardiac output and power output increased with temperature (Q10 values of 1.8 and 2.1, respectively) as a result of increased heart rate. Hypoxia tolerance was inversely related to temperature. At 5°C, the hearts maintained routine cardiac output throughout the 20 min period of anoxia, and maximal cardiac performance was fully restored following reoxygenation. By contrast, cardiac function failed sooner during anoxia as temperature was increased and maximal performance after reoxygenation was reduced by 25%, 35% and 55% at 10°C, 15°C and 18°C, respectively. Increased functional impairment following anoxic exposure at elevated temperature occurred even though both cardiac glycolytic enzyme activity and the rate of lactate production were increased proportionally with cardiac work. Nonetheless, there was no indication of myocardial necrosis, as biochemical and energetic parameters were generally unaffected by anoxia.

Failing atrial myocardium: energetic deficits accompany structural remodeling and electrical instability

The failing ventricular myocardium is characterized by reduction of high-energy phosphates and reduced activity of the phosphotransfer enzymes creatine kinase (CK) and adenylate kinase (AK), which are responsible for transfer of high-energy phosphoryls from sites of production to sites of utilization, thereby compromising excitation-contraction coupling. In humans with chronic atrial fibrillation (AF) unassociated with congestive heart failure (CHF), impairment of atrial myofibrillar energetics linked to oxidative modification of myofibrillar CK has been observed. However, the bioenergetic status of the failing atrial myocardium and its potential contribution to atrial electrical instability in CHF have not been determined. Dogs with (n = 6) and without (n = 6) rapid pacing-induced CHF underwent echocardiography (conscious) and electrophysiological (under anesthesia) studies. CHF dogs had more pronounced mitral regurgitation, higher atrial pressure, larger atrial area, and increased atrial fibrosis. An enhanced propensity to sustain AF was observed in CHF, despite significant increases in atrial effective refractory period and wavelength. Profound deficits in atrial bioenergetics were present with reduced activities of the phosphotransfer enzymes CK and AK, depletion of high-energy phosphates (ATP and creatine phosphate), and reduction of cellular energetic potential (ATP-to-ADP and creatine phosphate-to-Cr ratios). AF duration correlated with left atrial area (r = 0.73, P = 0.01) and inversely with atrial ATP concentration (r = -0.75, P = 0.005), CK activity (r = -0.57, P = 0.054), and AK activity (r = -0.64, P = 0.02). Atrial levels of malondialdehyde, a marker of oxidative stress, were significantly increased in CHF. Myocardial bioenergetic deficits are a conserved feature of dysfunctional atrial and ventricular myocardium in CHF and may constitute a component of the substrate for AF in CHF.

Effect of creatine manipulation on fast-twitch skeletal muscle of the mouse

1. The effect of short-term, reversible alteration of muscle total creatine content (Crtot) on force development was sought in fast-twitch extensor digitorum longus (EDL) muscles of female mice. 2. Three in vivo interventions were investigated: 1% creatine-supplementation, treatment with the creatine-uptake inhibitor beta-guanidino propionic acid (beta-GPA; 1%) or beta-GPA treatment followed by creatine supplementation for 5 days. 3. The Crtot of isolated muscles, determined using reverse-phase high-performance liquid chromatography, was 133 +/- 38 mmol/kg dry in 11 EDL control muscles and was not significantly affected by dietary creatine-supplementation (152 +/- 25 mmol/kg dry; n = 8). Significant creatine depletion was observed in the beta-GPA-fed group (65 +/- 6 mmol/kg dry; n = 9) and this was reversed by 5 days of creatine supplementation (133 +/- 21 mmol/kg dry; n = 10). 4. Creatine depletion did not affect maximum tetanic stress. However, when muscle creatine was restored by creatine supplementation, a substantial increase in tetanic stress was observed. Significant correlations were observed between Crtot and maximum tetanic stress (r = 0.56) and relaxation rate (r = 0.43). The enhancement of force was not due to effects of creatine on muscle fibre type because neither mechanical tests of fibre characteristics nor the fibre types of the muscles were affected. 5. We conclude that, in muscles that contain large numbers of fast-twitch fibres, maximum tetanic stress is determined, in part, by muscle creatine stores.

Captopril-induced glutamate release at the start of reperfusion after cold cardioplegic storage of pig hearts

OBJECTIVE

We sought to evaluate the effects of captopril on glucose-related metabolism during hypothermic cardioplegic storage and subsequent reperfusion.

METHODS

We compared hearts from control pigs with hearts from pigs treated with increasing oral doses of captopril for 3 weeks (12.5-150 mg daily), an intravenous bolus (25 mg) before operation, and captopril-containing cardioplegic solution (1 mg/L). The hearts were excised after infusion of cold crystalloid cardioplegic solution and stored in saline solution (4 degrees C-6 degrees C). In one series we studied myocardial blood flow and arteriovenous differences in oxygen, glucose, lactate, glutamate, and alanine during 60 minutes of postcardioplegic blood reperfusion. In this series captopril-treated hearts were reperfused with captopril-containing blood (1 mg/L). In another series we obtained biopsy specimens from the left ventricle throughout 30 hours of hypothermic cardioplegic storage and monitored tissue content of energy-rich phosphates, glycogen, glutamate, and alanine.

RESULTS

Captopril increased glutamate and alanine release 11- to 17-fold at the start of reperfusion (P <.001). Furthermore, captopril increased myocardial oxygen and glucose uptake during reperfusion (P <.001 for both), whereas lactate release and myocardial blood flow were unaffected by captopril. At the start of reperfusion, there was a positive correlation between glutamate release and glucose uptake in captopril-treated hearts (r = 0.66, P =.05). We found no statistically significant differences between captopril and control hearts in tissue content of adenosine triphosphate, glycogen, glutamate, alanine, or lactate during 30 hours of cardioplegic storage.

CONCLUSIONS

The metabolic effects of captopril are strictly related to reperfusion, during which oxidative metabolism of glucose is improved. The captopril-induced increase in glutamate and alanine release at the start of reperfusion after cardioplegic storage may reflect a switch in metabolism of glucose-related amino acids.

In vivo models of myocardial metabolism during ischemia: application to drug discovery and evaluation

This review examines the in vivo techniques that are available for evaluation of the metabolic effects and efficacy of agents intended for the treatment of myocardial ischemia. Energy substrate metabolism is complex, and requires simultaneous measurement of a variety of processes in order to obtain a thorough understanding of the biochemical mechanisms underlying any functional response. Small animals (from the mouse to the rabbit) are generally not very useful in the study of cardiac metabolism in vivo because it is not possible to sample the coronary venous drainage and measure the rate of substrate uptake or metabolite efflux. Anesthetized open-chest swine or dog models allows simultaneous serial measurement of myocardial substrate use, and repeated tissue sampling for the activities and contents of key enzymes and metabolites. The swine model is particularly good because pigs, like humans, lack innate collateral vessels, thus one can induce regional myocardial ischemia in the left anterior descending coronary artery and sample the venous effluent from the anterior interventricular vein. In this review the biochemical and physiological methods that can be used in conjunction with this preparation are described.

Myocardial adenine nucleotides, glycogen, and Na, K-ATPase in patients with idiopathic dilated cardiomyopathy requiring mechanical circulatory support

Acute decompensation leading to progressive pump failure is a main cause of death in patients with congestive heart failure. To find possible metabolic defects associated with the onset of this fatal occurrence, we measured myocardial adenine nucleotides, glycogen, and Na,K-ATPase in patients with end-stage idiopathic dilated cardiomyopathy. The biopsy specimens were obtained from the right ventricle of beating hearts during implantation of a biventricular assistance device in 23 patients (group I) suffering from irreversible cardiogenic shock and during heart transplantation in 20 patients (group II) in compensated heart failure. Left ventricular ejection fraction (LVEF) was determined preoperatively by echocardiography. Left ventricular function in group I was more severely impaired than in group II (LVEF 16.8%+/-4.6% vs 22.1%+/-5.1 %; p <0.01). Myocardial adenosine triphosphate (ATP) in group I was significantly reduced in comparison with group II (119.4+/-10.2 vs 27.7+/-7.4 nmol/mg noncollagen protein; p <0.01). There was no difference in glycogen levels. Na,K-ATPase concentration in group I (n = 8) was lower than that of group II (n = 20) (425+/-80 vs 498+/-75 pmol/g wet weight; p <0.05). Linear regression analyses showed a significant correlation between adenosine triphosphate (ATP) and LVEF (r = 0.41, p <0.01) and between Na,K-ATPase and LVEF (r = 0.55, p <0.01). These results indicate that loss of myocardial ATP and Na,K-ATPase could partially contribute to the development of spontaneous deterioration of the chronically overloaded heart.

Applicability of small endomyocardial biopsies for evaluation of high energy phosphates and glycogen in the heart

To evaluate variability of biochemical determination of energy stores in endomyocardial biopsies, we compared myocardial contents of high energy phosphates and glycogen in endomyocardial and transmural myocardial biopsies from 12 75-kg pigs before, during, and after cardioplegia. Before cardioplegia, comparable amounts of adenine nucleotides and glycogen were found in left and right ventricular endomyocardial and left ventricular transmural biopsies. Phosphocreatine levels were lower in endomyocardial than in transmural biopsies. Significant correlations were observed between endomyocardial and transmural adenine nucleotide and glycogen contents but not phosphocreatine content. During cardioplegia, myocardial ATP and phosphocreatine contents increased and glycogen concentration tended to decrease. During reperfusion, ATP and glycogen levels decreased, whereas phosphocreatine levels increased remarkably. Transmural changes in left ventricular adenine nucleotide and glycogen levels were reflected in endomyocardial biopsies but those in phosphocreatine were not. By increasing the number of endomyocardial biopsies from one to three, within-subject variance was reduced from 33-47% to 14-23% of total variance whereas four or more biopsies only added minor further reduction in variability. In conclusion, endomyocardial biopsies yield representative estimates of the average myocardial content of adenine nucleotides and glycogen but not of phosphocreatine in the normal heart. Endomyocardial biopsies offer a sensitive estimate of the changes in myocardial adenine nucleotides and glycogen induced by cardioplegia and reperfusion. However, metabolite content in endomyocardial biopsies shows a high variability. Three or more endomyocardial biopsies are necessary to reduce variability to acceptable levels.