Rates of glycolysis and glycogenolysis during ischemia in glucose-insulin-potassium-treated perfused hearts: A 13C, 31P nuclear magnetic resonance study

Na+ overload-induced mitochondrial damage in the ischemic heart

Ischemia induces a decrease in myocardial contractility that may lead more or less to contractile dysfunction in the heart. When the duration of ischemia is relatively short, myocardial contractility is immediately reversed to control levels upon reperfusion. In contrast, reperfusion induces myocardial cell death when the heart is exposed to a prolonged period of ischemia. This phenomenon is the so-called "reperfusion injury". Numerous investigators have reported the mechanisms underlying myocardial reperfusion injury such as generation of free radicals, disturbance in the intracellular ion homeostasis, and lack of energy for contraction. Despite a variety of investigations concerning the mechanisms for ischemia and ischemia–reperfusion injury, ionic disturbances have been proposed to play an important role in the genesis of the ischemia–reperfusion injury. In this present study, we focused on the contribution of Na+ overload and mitochondrial dysfunction during ischemia to the genesis of this ischemia–reperfusion injury.Key words: mitochondria, myocardial ischemia, Na+ channels, Na+/H+ exchanger, Na+ overload.

Metabolic effects of glucose-insulin infusions: myocardium and whole body

In target organs, insulin switches substrate utilization from free fatty acids to glucose, a change that: (i) is oxygen-efficient; (ii) repletes glycogen stores; (iii) removes potentially toxic fatty acids; and (iv) restores intracellular potassium. During or after an ischaemic challenge, the insulin metabolic mode should protect cellular functions provided that insulin can reach the ischaemic tissue. Insulin, however, also exerts non-metabolic effects, such as membrane hyperpolarization, the stimulation of adrenergic activity, and inhibition of parasympathetic tone, which may counter its beneficial metabolic actions. The net balance between the favourable and unfavourable effects of insulin on ischaemic tissues depends on: (i) the dose-response of the various effects; (ii) the presence of insulin resistance; (iii) the coexistence of hyperglycaemia; and (iv) the stage of ischaemic tissue damage. At present, a role for glucose-insulin-potassium infusions in clinical practice seems to be clearly established in the case of diabetic patients with acute coronary syndromes, and in patients undergoing urgent or elective cardiac surgery. Its role as an adjunctive therapy in the management of myocardial infarction in non-diabetic individuals has been tested in several clinical trials; however, the evidence emerging from them is inconclusive.

Increased calcium loading and inotropy without greater cell death in hypoxic rat cardiomyocytes

To test whether contractile function in "hypoxic" myocytes treated with high glucose (19.5 mM) can be improved by increasing intracellular Ca2+ without accelerating cell contracture or death, we challenged metabolically inhibited, paced myocytes with high extracellular Ca2+ concentration ([Ca2+]o) and measured simultaneously cell shortening and intracellular Ca2+ concentration ([Ca2+]i). NaCN exposure at a physiological [Ca2+]o level (1.2 mM) caused a decline of contractile function to 58 +/- 8% of the pre-NaCN value (P < 0.001) but increased systolic and diastolic [Ca2+]i by 104 +/- 17 and 37 +/- 9% above baseline (P < 0.01), respectively. Consequent doubling of [Ca2+]o to 2.4 mM, in the presence of NaCN, immediately restored contractile function, and twitch amplitude after 18 min was 123 +/- 14% (P < 0.001) of baseline pre-NaCN values, whereas systolic [Ca2+]i increased further to 225 +/- 63% (P < 0.05) and diastolic [Ca2+]i to 73 +/- 16% above baseline (P < 0.01). This marked increase in [Ca2+]i had no deleterious effect on myocyte diastolic function or survival. These results suggest that if adequate metabolic substrate is provided, contractile function in metabolically inhibited, hypoxic myocytes can be restored by increasing [Ca2+]i without causing short-term cell injury.

Glucose and glycogen utilisation in myocardial ischemia--changes in metabolism and consequences for the myocyte

Experimentally, enhanced glycolytic flux has been shown to confer many benefits to the ischemic heart, including maintenance of membrane activity, inhibition of contracture, reduced arrhythmias, and improved functional recovery. While at moderate low coronary flows, the benefits of glycolysis appear extensive, the controversy arises at very low flow rates, in the absence of flow; or when glycolytic substrate may be present in excess, such that high glucose concentrations with or without insulin overload the cell with deleterious metabolites. Under conditions of total global ischemia, glycogen is the only substrate for glycolytic flux. Glycogenolysis may only be protective until the accumulation of metabolites (lactate, H+, NADH, sugar phosphates and Pi ) outweighs the benefit of the ATP produced. The possible deleterious effects associated with increased glycolysis cannot be ignored, and may explain some of the controversial findings reported in the literature. However, an optimal balance between the rate of ATP production and rate of accumulation of metabolites (determined by the glycolytic flux rate and the rate of coronary washout), may ensure optimal recovery. In addition, the effects of glucose utilisation must be distinguished from those of glycogen, differences which may be explained by functional compartmentation within the cell.

Calculation of absolute metabolic flux and the elucidation of the pathways of glutamate labeling in perfused rat heart by 13C NMR spectroscopy and nonlinear least squares analysis

Absolute metabolic fluxes in isolated perfused hearts have been determined by a nonlinear least squares analysis of glutamate labeling kinetics from [1-13C]glucose, [4-13C]beta-hydroxybutyrate, or [2-13C]acetate using 13C NMR spectroscopy. With glucose as substrate, the malate-aspartate shuttle flux was too slow to account for the reducing equivalents generated by glycolysis and to predict the observed oxygen consumption rate. For acetate and beta-hydroxybutyrate, the malate-aspartate shuttle had to be reversed for the network to agree with the observed oxygen consumption and glutamate labeling. Thus, an additional redox shuttle was required to reoxidize the NADH produced by cytoplasmic malate dehydrogenase. Using this model there was good agreement between the experimentally determined oxygen consumption and glutamate labeling and the calculated values of these parameters from the model for all substrates. The contribution of exogenous substrate to the overall tricarboxylic acid (TCA) cycle flux, 89.6 +/- 6.5% (mean +/- S.D.) as measured in the tissue extracts compared well with 91.4 +/- 4.2% calculated by the model. The ratio of TCA cycle flux to oxygen consumption for acetate, was 2.2 +/- 0.1, indicating that NADH production is principally accounted for by TCA cycle flux. For glucose or beta-hydroxybutyrate, this ratio was 2.9 +/- 0.2, consistent with the existence of other NADH producing reactions (e.g. glycolysis, beta-hydroxybutyrate oxidation).

Reduction of ischemia-induced electrophysiologic abnormalities by glucose-insulin infusion

OBJECTIVES

This study was designed to determine the effects of glucose-insulin infusion on ischemia-induced changes in extracellular potassium ([K+]o) accumulation and the associated electrophysiologic abnormalities in the canine heart.

BACKGROUND

Although glucose-insulin-potassium infusion has been shown to limit myocardial injury in acute ischemia, its effect on ischemia-induced electrophysiologic alterations has not been investigated.

METHODS

Recordings of [K+]o and local electrograms from the normal, border and ischemic zones were obtained during serial (10-min) left anterior descending coronary artery occlusions in the control state and after infusion of glucose-insulin (eight dogs), glucose alone (six dogs) or insulin alone (eight dogs).

RESULTS

Glucose-insulin infusion caused significant reduction in the rise of [K+]o during the entire period of ischemia in both ischemic and border zones associated with significant improvement in the degree of intramyocardial conduction delay. At 10 min of ischemia, [K+]o was reduced from a mean control level of 15.9 +/- 3.7 to 10.1 +/- 4.3 mmol/liter (p < 0.005) in the ischemic zone and from 6.8 +/- 1.9 to 5.5 +/- 1.1 mmol/liter (p < 0.05) in the border zone. The electrogram duration was shortened from a mean control value of 102 +/- 13 to 78 +/- 12 ms in the ischemic zone and from 79.2 +/- 7.8 to 58.1 +/- 6.6 ms in the border zone (p < 0.005). Glucose alone caused significant reduction in [K+]o during the initial 6 min of ischemia, only in the ischemic zone. Conversely, insulin caused no changes in [K+]o accumulation during ischemia. Neither glucose nor insulin alone had any effect on ischemia-induced intramyocardial conduction delay.

CONCLUSIONS

The present study demonstrated that the combination of glucose and insulin is essential for the salutary effect of reducing [K+]o accumulation during ischemia and improving the associated intramyocardial conduction delay. It could be postulated that glucose in the presence of insulin increases the glycolytic flux, thereby providing adequate adenosine triphosphate for suppressing the cardiac adenosine triphosphate-sensitive potassium ion channels. The latter are, at least partially, responsible for the [K+]o rise in the early phase of ischemia. This study highlights the antiarrhythmic potential of interventions that modulate the metabolic consequences of ischemia.

Effects of amino acids on substrate selection, anaplerosis, and left ventricular function in the ischemic reperfused rat heart

The effect of aspartate and glutamate on myocardial function during reperfusion is controversial. A beneficial effect has been attributed to altered delivery of carbon into the citric acid cycle via substrate oxidation or by stimulation of anaplerosis, but these hypotheses have not been directly tested. 13C isotopomer analysis is well suited to the study of myocardial metabolism, particularly where isotopic and metabolic steady state cannot be established. This technique was used to evaluate the effects of aspartate and glutamate (amino acids, AA) on anaplerosis and substrate selection in the isolated rat heart after 25 min of ischemia followed by 30 or 45 min of reperfusion. Five groups of hearts (n = 8) provided with a mixture of [1,2-13C]acetate, [3-13C]lactate, and unlabeled glucose were studied: control, control plus AA, ischemia followed by 30 min of reperfusion, ischemia plus AA followed by 30 min of reperfusion, and ischemia followed by 45 min of reperfusion. The contribution of lactate to acetyl-CoA was decreased in postischemic myocardium (with a significant increase in acetate), and anaplerosis was stimulated. Metabolism of 13C-labeled aspartate or glutamate could not be detected, however, and there was no effect of AA on functional recovery, substrate selection, or anaplerosis. Thus, in contrast to earlier reports, aspartate and glutamate have no effect on either functional recovery from ischemia or on metabolic pathways feeding the citric acid cycle.

Intracellular pH determination by 13C-NMR spectroscopy

A noninvasive method for the determination of pH by the 13C-nuclear magnetic resonance (NMR) chemical shift of the C-3 carbon of sn-glycerol 3-phosphate is described. Nonlinear least-squares analysis of the chemical shift variation of the C-3 resonance of sn-glycerol 3-phosphate with pH at 37 degrees C in solutions and in perchloric acid extracts of tissue yielded a pKa of 6.2, making it a very sensitive indicator of pH in the approximate range of 5-7. Intracellular pH determined by the present 13C-NMR method correlated well with simultaneous measurements of pH by 31P-NMR spectroscopy over a wide range during normal perfusion and ischemic conditions in intact rat hearts. These findings indicate that this approach is particularly suited for quantification of intracellular pH over the physiological range in intact tissues and that observed in ischemic myocardium.

Complementarity of magnetic resonance spectroscopy, positron emission tomography and single photon emission tomography for the in vivo investigation of human cardiac metabolism and neurotransmission

The three techniques allowing the noninvasive study of cardiac metabolism, namely magnetic resonance spectroscopy (MRS), positron emission tomography (PET) and single photon emission computed tomography (SPET), all use external detection with stable or radioactive isotopes. These techniques yield different information. PET is quantitative and very sensitive, and therefore only tracer amounts of molecules need to be injected. It allows neurotransmitters and receptors to be studied and a global view of metabolism (oxygen consumption, glucose and fatty acid utilization) to be obtained. SPET also has good sensitivity, but uses gamma-emitting isotopes of heteroatoms. Their longer half-lives allow follow-up for hours or days. MRS is based on stable elements with high (hydrogen 1, phosphorus 31, fluorine 19...) or low (carbon 13, Deuterium) natural abundance. It has very low sensitivity and only millimolar concentrations of substrates can be detected, but various parts of metabolism can be studied. The in vivo measurement of myocardial concentration of substances has many problems that are common to all three techniques (measurement of the volume, measurement of the quantity of each molecule, resolution, partial volume effect, improvement of the signal-to-noise ratio, movement of the organ). The complementarity of the techniques is illustrated by their applications to the study of cardiac metabolism. For instance, the energy metabolism can be studied by 31P-MRS, which detects the high-energy compounds ATP and phosphocreatine, and 13C-MRS yields information on the tricarboxylic acid cycle activity. PET and SPET allow the utilization of fatty acids, the normal fuels of the heart, to be studied. During ischaemia, PET with 18F-fluorodeoxyglucose (18FDG) can determine the glucose consumption and 1H-MRS shows the increase in lactic acid, reflecting anaerobic glycolysis. Comparison of the use of acetate labelled with 11C for PET or 13C for MRS shows the potentials and limitations of each technique. Myocardial perfusion can be evaluated directly with various PET tracers or indirectly with thallium 201 or various technetium-99m-labelled tracers by SPET. No MRS marker of perfusion is so far clinically available. Mainly SPET and PET are used clinically for the investigation of ischaemic heart disease as well as cardiomyopathies, but some initial results using 31P-MRS are being obtained.

Hypoxia and glucose independently regulate the beta-adrenergic receptor-adenylate cyclase system in cardiac myocytes

We explored the effects of two components of ischemia, hypoxia and glucose deprivation, on the beta-adrenergic receptor (beta AR)-adenylate cyclase system in a model of hypoxic injury in cultured neonatal rat ventricular myocytes. After 2 h of hypoxia in the presence of 5 mM glucose, cell surface beta AR density (3H-CGP-12177) decreased from 54.8 +/- 8.4 to 39 +/- 6.3 (SE) fmol/mg protein (n = 10, P less than 0.025), while cytosolic beta AR density (125I-iodocyanopindolol [ICYP]) increased by 74% (n = 5, P less than 0.05). Upon reexposure to oxygen cell surface beta AR density returned toward control levels. Cells exposed to hypoxia and reoxygenation without glucose exhibited similar alterations in beta AR density. In hypoxic cells incubated with 5 mM glucose, the addition of 1 microM (-)-norepinephrine (NE) increased cAMP generation from 29.3 +/- 10.6 to 54.2 +/- 16.1 pmol/35 mm plate (n = 5, P less than 0.025); upon reoxygenation cAMP levels remained elevated above control (n = 5, P less than 0.05). In contrast, NE-stimulated cAMP content in glucose-deprived hypoxic myocytes fell by 31% (n = 5, P less than 0.05) and did not return to control levels with reoxygenation. beta AR-agonist affinity assessed by (-)-isoproterenol displacement curves was unaltered after 2 h of hypoxia irrespective of glucose content. Addition of forskolin (100 microM) to glucose-supplemented hypoxic cells increased cAMP generation by 60% (n = 5; P less than 0.05), but in the absence of glucose this effect was not seen. In cells incubated in glucose-containing medium, the decline in intracellular ATP levels was attenuated after 2 h of hypoxia (21 vs. 40%, P less than 0.05). Similarly, glucose supplementation prevented LDH release in hypoxic myocytes. We conclude that (a) oxygen and glucose independently regulate beta AR density and agonist-stimulated cAMP accumulation; (b) hypoxia has no effect on beta AR-agonist or antagonist affinity; (c) 5 mM glucose attenuates the rate of decline in cellular ATP levels during both hypoxia and reoxygenation; and (d) glucose prevents hypoxia-induced LDH release, a marker of cell injury.

Alterations in the energy metabolism of the isolated perfused frog heart during calcium depletion and subsequent repletion

The changes in myocardial energy metabolism of isolated perfused Rana ridibunda hearts subjected to prolonged calcium depletion and reperfusion with calcium-containing medium were studied. Calcium-free perfusion resulted in an increase in the concentrations of glucose, glucose-6-phosphate, alpha-ketoglutarate and malate. The myocardial contents of high-energy phosphates were maintained while concentrations of key amino acids were significantly altered. During the reperfusion period the tissue high-energy phosphate content fell abruptly. A marked increase in glycolytic flux and lactate production was observed. The tissue contents of citric acid cycle intermediates and key amino acids decreased. Examination of the activities of marker enzymes during the calcium-free and reperfusion periods showed that only cytoplasmic enzymes are lost during reperfusion, while the activities of other enzymes remained unchanged. The results suggest that the fluxes of both glycolysis and the citric acid cycle are significantly altered during calcium depletion and following repletion in the amphibian heart. The major characteristics of calcium paradox-induced damage in Rana ridibunda heart are the depletion of high-energy stores, the impairment of mitochondrial oxidative metabolism, and a significant increase in anaerobic metabolism.

Improvement of hypoperfusion with norepinephrine injury by ex vivo insulin in isolated diabetic rat hearts

Effects of insulin on contractile and energy metabolic dysfunctions during hypoperfusion (2 ml/min/g heart wt., 60 min) with 10(-6) M norepinephrine were studied in paced hearts isolated from streptozotocin-diabetic rats. Insulin (2 mU/min/g heart wt.) was infused 20 min before and during hypoperfusion (pre-treated group) or 30 min after the onset of hypoperfusion (post-treated group). Hearts in the non-treated group were hypoperfused without insulin and other hearts in the control group were not hypoperfused. In the non-treated group, resting contractile force (CF) and resting left ventricular pressure (LVP) were significantly elevated to maximum levels within 30 min after hypoperfusion and these elevations were restored in the pre-treated group but not in the post-treated group. Developed CF was depressed in the non-treated group and improved significantly in the pretreated group but not in the post-treated group. Developed LVP was depressed in the non-treated group, and depression was slightly larger in the pre-treated group. In the non-treated group, ATP and creatine phosphate contents in the left ventricle significantly decreased. Decreases in ATP and creatine phosphate contents in the inner layer were partially restored in the pre-treated group but not in the post-treated group. Lactate significantly increased in the non-treated group and increased even further in the insulin treated groups. These results indicate that contractile dysfunction during hypoperfusion with norepinephrine is improved by pre-treated insulin, as is partial recovery of energy metabolism.

Comparative 13C and 31P NMR assessment of altered metabolism during graded reductions in coronary flow in intact rat hearts

13C NMR spectroscopy may offer a unique ability to characterize the metabolic response to graded reduction in coronary flow since it allows repeated, nondestructive identification of products of intermediary metabolism in the same heart. The sensitivity of 13C parameters of glucose metabolism was compared with changes in levels of phosphocreatine, ATP, and pH as determined by 31P NMR in the intact, beating rat heart model during graded reductions in coronary flow. Experiments were performed during 60 min of perfusion with [1-13C]glucose (5 mM) at normal flow (15 ml/min) and at the reduced flow rates of 5 and 2 ml/min. During flow at 5 ml/min, isovolumic developed pressure fell to 51 +/- 4% of control. Although phosphocreatine, ATP, and pH were not changed, [3-13C]lactate was increased (1.46 +/- 0.12 mumol/g of wet weight vs. 0.63 +/- 0.08 during normal flow). In addition, the time to 50% maximum enrichment of [2-13C]glutamate was prolonged (17 +/- 1 min vs. 9 +/- 1 min during normal flow), indicating that glucose-supported flux through the tricarboxylic acid (TCA) cycle was decreased. The relative anaplerotic contribution to citrate synthase-supported TCA flux was increased from 6% to 35%. These 13C metabolic changes could not be reproduced by reduced [1-13C]glucose delivery in the absence of ischemia, although similar reduced TCA flux indices were reproduced in additional hearts when workload was reduced by low calcium (0.7 mM) perfusion. Therefore, the information provided by 13C NMR spectroscopy can be a more sensitive indicator of flow-induced alterations in cardiac metabolism than that provided by the much more commonly used 31P NMR technique.