(Circulation. 1997;96:1074-1077.)
© 1997 American Heart Association, Inc.
Articles |
From the Boston University School of Medicine (C.S.A.), Boston, Mass, and The University of TexasHouston Medical School (H.T.).
Correspondence to Carl S. Apstein, MD, Boston University School of Medicine, Cardiac Muscle Research Laboratory, Whitaker Cardiovascular Institute (Rm W-611), 80 E Concord St, Boston, MA 02118 () or Heinrich Taegtmeyer, MD, DPhil, University of TexasHouston Medical School, Internal Medicine/Cardiology, 6431 Fannin St (Rm 1.246 MSB), Houston, TX 77030. e-mail capstein{at}bu.edu
Key Words: Editorials myocardial infarction glucose insulin potassium
| Introduction |
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Before GIK can be considered as adjunctive therapy to revascularization in acute MI, confirmatory results are required from a modern, large-scale, prospectively randomized trial with adequate statistical power. Because urgent reperfusion, through thrombolytic therapy or primary angioplasty, is standard care for acute MI, the next target for therapeutic interventions is the myocardium. Restoration of blood flow and oxygen supply to the ischemic myocardium is critical but may not fully exploit the potential for salvage. Therefore, any trial of GIK should assess its value as an adjunctive agent to (1) delay cell death until reperfusion can occur and (2) optimize energy transfer in the postischemic heart.
| Recent Related Studies |
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| Energy Metabolism of Acutely Infarcting and Reperfused Myocardium |
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The magnitude of myocardial energy transfer is best illustrated by the
simple calculation that the human heart, which weighs
300 g,
produces and uses
5 kg ATP over 24 hours. The energy is derived from
the oxidation of fatty acids, glucose, lactate, ketone bodies, and even
amino acids. In addition to these substrates, heart muscle uses, under
defined circumstances, some of its endogenous substrates,
glycogen and triglycerides.
We propose that in the normal, well-oxygenated heart, efficient energy transfer requires simultaneous oxidation of several substrates through a series of moiety-conserved cycles. Not all substrates are created equal, and the interplay of different energy-providing fuels underpins normal contractile function of the heart.9 Lack of oxygen and the accumulation of metabolic products profoundly change this interactive transfer of energy. Anaerobic glycolysis becomes the predominant source for a limited amount of ATP, which may or may not suffice to support the most essential cellular functions.
Just as the causes of myocardial ischemia are diverse, so are the cellular responses to myocardial ischemia and reperfusion. The latter include the cessation of the metabolism of substrates feeding reducing equivalents into the respiratory chain, production of partially reduced oxygen species (especially superoxide hydrogen peroxide radicals), uncoupling and inhibition of electron transport, increased cytosolic Ca2+ levels, loss of adenine nucleotide translocase activity, reversal of mitochondrial ATPase activity, loss of mitochondrial antioxidant defenses, and loss of mitochondrial and cytosolic enzymes. In addition, glycogen and key intermediates of the citric acid cycle are lost.9 Reperfusion may worsen the damage caused by oxygen-derived free radicals. Although fatty acid oxidation rapidly normalizes in reperfused hearts, the oxidation of glucose remains depressed, as does contractile function.10
| Metabolic Substrates as Therapy |
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| Mechanisms of the Protection by GIK of Ischemic Myocardium |
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More recent work has identified additional mechanisms by which GIK is protective of ischemic myocardium. In the setting of experimental low-flow ischemia, a high level of glucose and insulin has been shown to improve ischemic and postischemic myocardial systolic and diastolic function as well as coronary vasodilation, thereby potentially decreasing the "no-reflow" phenomenon.5 The increased amount of available glycolytic substrate increased glycolytic flux and glycolytic ATP synthesis, attenuated the ischemia-induced decrease in ATP and phosphocreatine levels, and prevented the increase in inorganic phosphate.12 13 The prevention of the ischemia-induced increase in inorganic phosphate resulted in preservation of a higher calculated free energy yield from all ATP hydrolysis, not only from glycolytically derived ATP. High glucose and insulin and an increased glycolytic flux may also increase pyruvate generation, which in turn provides substrate for anaplerotic reactions and preserves all moieties of the citric acid cycle.9 Glycolytic ATP protects membranes,14 drives the transport of Ca2+ into the sarcoplasmic reticulum,15 and improves sodium homeostasis of ischemic myocardium.16
The provision of glucose and insulin also preserves and restores myocardial glycogen stores. Glycogen is rapidly mobilized during ischemia. Reduced glycogen concentrations impair force development, Ca2+ release, and contractile function.17 A positive correlation among enhanced glucose uptake, glycogen levels, and contractile function has been shown in patients undergoing revascularization for coronary artery disease.18
| Interaction of GIK With FFAs and Heparin |
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| Role of GIK in Postischemic Myocardium |
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We have also proposed that the depletion of glycogen stores and citric acid cycle intermediates is a major cause for impaired energy transfer in reperfused hearts.9 The replenishment of the glycogen pool and of the citric acid cycle occurs through anaplerosis. In heart muscle, a major source of anaplerosis of the citric acid cycle is the carboxylation of pyruvate. Because glucose is a direct precursor of pyruvate and pyruvate provides both substrates for the citrate synthase reaction (acetyl-CoA and oxaloacetate), it is easy to envision GIK as a metabolic substrate that provides a "jump start" for a collapsed system of energy transfer in the postischemic heart. This does not mean that glycolysis itself is unimportant in ischemia/reperfusion transitions. In isolated, working hearts made ischemic and then reperfused, glycolysis is a highly adaptive emergency mechanism that can prevent deleterious myocyte deenergization during forced ischemia/reperfusion transitions in the presence of excess oxidative substrate.22 Furthermore, in a similar model of ischemia and reperfusion, the inotropic and metabolic effects of insulin and epinephrine were additive and resulted in improved functional recovery in association with enhanced glucose uptake and utilization.23 Thus, the different rationales for the use of metabolic support of the heart during both ischemia and reperfusion complement each other.
| The Long History of GIK |
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The use of GIK in acute MI has been inhibited by concerns that GIK
could starve the cell of energy (glucose
phosphorylation requires ATP) and worsen
ischemic injury by worsening myocardial acidosis as a result of
increased lactate production.24 These concerns
have been alleviated by recent studies showing that collateral flow
exists in the acute infarct region of the large majority of patients at
a level
15% of the perfusion level in the non-MI
region8 ; such a level of coronary flow is adequate
to wash out myocardial lactate and prevent the action by lactate to
inhibit glycolysis.5 Direct measurements of ATP content
and intracellular pH during such a degree of low-flow ischemia
have shown that a high glucose-plus-insulin substrate increases ATP
levels and does not worsen tissue acidosis (Reference 1313 and A.C. Cave,
J. Friedrich, J.S. Ingwall, C.S. Apstein, and F.R. Eberli, Creatine
Kinase Reaction and Other ATP Synthesis Pathways During Low-Flow
Ischemia: Influence of Increased Glycolytic Substrate,
submitted manuscript, 1997), an observation consistent with the
analysis that during ischemia, proton generation from
the hydrolysis of ATP far exceeds that contributed by
lactate.25
The potential use of GIK has also been delayed by a pharmaceutical industry that is indifferent to sponsoring research on therapy without the possibility of patents and profits. We have often wondered how rapidly GIK might have been developed, or at least have been tested in a large modern clinical trial, had there been any commercial interest.
The large number of conflicting and inconclusive prior clinical trials of GIK for acute MI has contributed importantly to the general lack of enthusiasm for its use. Fath-Ordoubadi and Beatt1 performed a great service by reviewing these studies, discarding those in which small amounts of GIK were given or therapy was initiated too late to be useful, and summating the randomized trials in which adequate doses were given and therapy was initiated early; these studies were well planned but were terminated before any reached adequate statistical power to definitively test the GIK hypothesis. Their aggregate results suggest a statistically significant benefit of GIK that no single study could conclude.
We have also heard the argument that GIK therapy is "not worth
it" because thrombolytic therapy has reduced the
acute MI mortality rate to such low levels that further reductions via
adjunctive agents would be difficult to achieve and prove
statistically. Hence, it might not be worth the trouble of monitoring
blood glucose and electrolyte levels every 6 hours. This argument is
weak because frequent blood samples are already required to be obtained
for diagnostic purposes. Most important, one must consider
the large number of MIs that occur each year:
1.5 million in the
United States alone. If the use of GIK can save 49 lives per 1000
cases,1 then 75 000 lives could be saved annually in the
United States through the use of this simple, safe, and synergistic
therapy. Surely, this therapeutic potential warrants a definitive
clinical trial before too many more years go by!
| Footnotes |
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| References |
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2.
GUSTO IIb investigators. A clinical trial comparing
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4.
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5.
Eberli FR, Weinberg EO, Grice WN, Horowitz GL, Apstein
CS. Protective effect of increased glycolytic substrate against
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6. Arsenian M, New PS, Cafasso CM. Safety, tolerability, and efficacy of a glucose-insulin-potassium-magnesium-carnitine solution in acute myocardial infarction. Am J Cardiol. 1996;78:476-479.
7.
Malmberg K, Ryden L, Hamsten A, Herlitz J, Waldenstrom
A, Wedel H. Effects of insulin treatment on cause-specific
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8. Sabia PJ, Powers ER, Ragosta M, Sarembock IJ, Burwell LR, Kaul S. An association between collateral blood flow and myocardial viability in patients with recent myocardial infarction. N Engl J Med. 1992;327:1825-1831.[Abstract]
9. Taegtmeyer H. Energy metabolism of the heart: from basic concepts to clinical applications. Curr Probl Cardiol. 1994;19:57-116.
10.
Liu B, Clanachan HS, Schultz R, Lopaschuk GD.
Cardiac efficiency is improved after ischemia by altering both
the source and fate of protons. Circ Res. 1996;79:940-948.
11. Sodi-Pallares D, Testelli M, Fishleder F. Effects of an intravenous infusion of a potassium-insulin-glucose solution on the electrocardiographic signs of myocardial infarction. Am J Cardiol. 1962;9:166-181.[Medline] [Order article via Infotrieve]
12. Mallet RT, Hartman DA, Bunger R. Glucose requirement for postischemic recovery of perfused working heart. Eur J Biochem. 1990;188:481-493.[Medline] [Order article via Infotrieve]
13. Cave A, Eberli FR, Ngoy S, Rose J, Ingwall JS, Apstein CS. Increased glycolytic substrate protects against ischemic diastolic dysfunction: 31P-NMR studies in the isolated blood perfused rat heart. Circulation. 1993;88(suppl I):I-43. Abstract.
14.
Weiss JN, Lamp ST. Glycolysis preferentially
inhibits ATP-sensitive K+ channels in isolated guinea pig
cardiac myocytes. Science. 1987;238:67-69.
15.
Xu KY, Zweier JL, Becker LC. Functional coupling
between glycolysis and sarcoplasmic reticulum Ca2+
transport. Circ Res. 1995;77:88-97.
16. Cross HR, Radda GK, Clarke K. The role of Na+/K+-ATPase activity during low-flow ischemia in preventing myocardial injury: a 31P, 23Na and 87Rb NMR spectroscopic study. Magn Reson Med. 1995;34:673-685.[Medline] [Order article via Infotrieve]
17.
Chin ER, Allen DG. Effects of reduced muscle
glycogen concentration on force, Ca2+ release and
contractile protein function in intact mouse skeletal muscle.
J Physiol. 1997;498:17-29.
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19. Oliver MF, Opie LH. Effects of glucose and fatty acids on myocardial ischemia and arrhythmias. Lancet.. 1994;343:155-158.[Medline] [Order article via Infotrieve]
20. Kurien V, Yates P, Oliver M. Free fatty acids, heparin, and arrhythmias during experimental myocardial infarction. Lancet.. 1969;2:185-187.[Medline] [Order article via Infotrieve]
21. Lopaschuk GD, Wambolt RB, Barr RL. An imbalance between glycolysis and glucose oxidation is a possible explanation for the detrimental effects of high levels of fatty acids during aerobic reperfusion of ischemic hearts. J Pharmacol Exp Ther. 1993;1264:135-144.
22. Schafer S, Prussel E, Carr LJ. Requirement of glycolytic substrate for metabolic recovery during moderate low-flow ischemia. J Mol Cell Cardiol. 1995;27:2167-2176.[Medline] [Order article via Infotrieve]
23. Richwine RT, Carmical S, Goodwin GW, Taegtmeyer H. Insulin improves post-ischemic recovery and glucose metabolism in the working rat heart perfused with oleate. Circulation. 1996;94(suppl I):I-115. Abstract.
24. Neely JR, Morgan HE. The relationship between carbohydrate and lipid metabolism and the energy balance of heart muscle. Annu Rev Physiol. 1974;36:413-439.
25. Dennis SC, Gevers W, Opie LH. Protons in ischemia: where do they come from, where do they go to? J Mol Cell Cardiol. 1991;23:1077-1086.[Medline] [Order article via Infotrieve]
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