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(Circulation. 1999;99:1646-1649.)
© 1999 American Heart Association, Inc.

Correspondence

Limitations to the Assessment of Reperfusion Injury With Radiolabeled 2-Deoxyglucose

Torsten Doenst, MD

Department of Medicine Division of Cardiology, University of Texas–Houston Medical School, Houston, Tex

James E. Holden, PhD

Department of Medical Physics University of Wisconsin, Madison, Wis

Heinrich Taegtmeyer, MD, DPhil

Department of Medicine Division of Cardiology, University of Texas–Houston Medical School, Houston, Tex

	Introduction

Top Introduction References Introduction  References

 
To the Editor:

Matsumura et al¹ propose a new method to determine reperfusion injury in dog heart. The phosphorylation rate constant, k₃, for two 2-deoxyglucose moieties ([¹⁴C]2-DG and ¹⁸F-2-deoxy-2-fluro-D-glucose [FDG]) was compared with histological assessment of ischemic damage. The results suggest that a large portion of the infarcted myocardium loses viability during the first hours of reperfusion. If true, this would be the first demonstration of a bimodal time course of reperfusion injury in vivo.

Two factors may have affected the interpretation of the results. First, the assumption is made that the total ¹⁴C radioactivity in each sample is proportional to the phosphorylation rate. However, the contribution of unphosphorylated deoxyglucose, although declining, is never negligible, and in some circumstances, it can represent the majority of the tissue radioactivity. Correction for this is the main supposition of the deoxyglucose method.² Neglecting this correction is particularly problematic because the relationship between phosphorylated and unphosphorylated deoxyglucose concentrations is almost certain to be regionally variable.

Second, and more important, is the cancellation of the lumped constant (LC) from the calculations for the 2 tissue regions. We have shown that the LC is subject to considerable variability depending on the experimental conditions.³ It has been postulated that the LC is high when transport is rate limiting and low when phosphorylation is rate limiting.⁴ After 90 minutes of total ischemia, the available glucose in the ischemic area is likely to be very low. Any uptake of glucose at the onset of reperfusion is likely to be limited by transport because intracellular glucose is low and hexokinase is not saturated. Thus, the LC would be high. It is conceivable that during reperfusion, transport begins to exceed phosphorylation because transport capacity is high and glycolysis becomes inhibited by the resumption of fatty acid oxidation.⁵ Now hexokinase will become rate limiting, resulting in a low value for the LC. If one applies this scenario to the results in the triphenyltetrazolium chloride (TTC)–negative regions, the initially high k₃ (5 minutes), which suggests viability, may be the result of an overestimation of glucose uptake (LC high), whereas the low k₃ after 3 hours may be due to an underestimation of glucose uptake (low LC) and may not be related to reperfusion injury at all.

In conclusion, we agree that a method for the detection of reperfusion injury in vivo has long been elusive. In our view, the goal remains elusive.

	References

Top Introduction References Introduction  References

 
1. Matsumura K, Jeremy RW, Schaper J, Becker LC. Progression of myocardial necrosis during reperfusion of ischemic myocardium. Circulation. 1998;97:795–804.

2. Sokoloff L, Reivich M, Kennedy C, Des Rosiers MH, Patlak CS, Pettigrew KD, Sakurada O, Shinohara M. The [¹⁴C] deoxyglucose method for the measurement of local cerebral glucose utilization: theory, procedure, and normal values in the conscious and anesthetized albino rat. J Neurochem. 1977;28:897–916.

3. Hariharan R, Bray MS, Ganim R, Doenst T, Goodwin GW, Taegtmeyer H. Fundamental limitations of [¹⁸F] 2-deoxy-2-fluoro-D-glucose for assessing myocardial glucose uptake. Circulation. 1995;91:2435–2444.

4. Kuwabara H, Evans A, Gjedde A. Michaelis-Menten constraints improved cerebral glucose metabolism and regional lumped constant measurements with [18F]fluorodeoxyglucose. J Cereb Blood Flow Metab. 1990;10:180–189.

5. Lopaschuk GD, Spafford MA, Davies NJ, Wall SR. Glucose and palmitate oxidation in isolated working rat hearts reperfused after a period of transient global ischemia. Circ Res. 1990;66:546–553. 30,1999

Response

Richmond W. Jeremy, MBBS, PhD

Division of Cardiology Department of Medicine, Johns Hopkins Medical Institutions, Baltimore, Md, Department of Medicine, University of Sydney, Sydney, Australia

Kaname Matsumura, MD

Division of Nuclear Medicine Department of Radiology, Johns Hopkins Medical Institutions, Baltimore, Md, Department of Radiology, Mie University, Mie, Japan

Lewis C. Becker, MD

Division of Cardiology Department of Medicine, Johns Hopkins Medical Institutions, Baltimore, Md

	Introduction

Top Introduction References Introduction  References

 
Dr Taegtmeyer and colleagues raise issues about sequential measurements of 2-deoxyglucose uptake in reperfused myocardium.¹ Their first concern is possible contamination by unphosphorylated tracer. The proportion of phosphorylated 2-deoxyglucose reaches steady state after 60 minutes, a time course consistent with our experimental design. If a lot of unphosphorylated tracer contributed to measured tissue activity, then a decline in activity should be observed in sequential measurements, owing to loss of unphosphorylated tracer from myocytes. We observed no significant tracer loss from normal or reperfused myocardium over 4 hours.

Another concern is use of the same lumped constant for calculation of ₃ during early and late reperfusion. Some in vitro data show that addition of insulin or a change in fatty acid substrate availability results in a discrepancy between measured rates of glucose and 2-deoxyglucose uptake,² but the same data show that rates of glucose and 2-deoxyglucose uptake are similar during steady-state conditions and are unaffected by changes in workload. Other data show that the lumped constant does not change in reperfused myocardium³ and is independent of blood flow and workload. Our ₃ values are comparable to previous studies in reperfused canine myocardium.⁴

Could changes in substrate utilization alter hexokinase activity in reperfused myocardium? First, there was unlikely to be a significant change in substrate availability during our experiments. Second, the myocardium prefers fatty acids as a substrate for oxidation, even after only 20 minutes of reperfusion.⁵ It is suggested that we may have overestimated ₃, and thus viability, during early reperfusion, but we also compared ₃ during early reperfusion with electron microscopy findings. The threshold ₃ had a predictive accuracy of 88% for detection of viability.

It might be argued that some change in lumped constant might occur and the ₃ threshold for viability might differ between early and late reperfusion. In samples that are necrotic (no-reflow) and those that are salvaged (TTC positive), there is minimal discrepancy between the samples judged viable or necrotic by the sequential tracers (Figure 6 in Reference 1). In TTC-negative samples, the difference in viability between early and late reperfusion is obvious for a wide range of ₃ values. Even if there were a difference in viability threshold between early and late reperfusion, the essential finding (that a proportion of myocardium loses viability during the reperfusion period) would be unchanged.

	References

Top Introduction References Introduction  References

 
1. Matsumura K, Jeremy RW, Schaper J, Becker LC. Progression of myocardial necrosis during reperfusion of ischemic myocardium. Circulation. 1998;97:795–804.

2. Hariharan R, Bray M, Ganim R, Doenst T, Goodwin GW, Taegtmeyer H. Fundamental limitations of [¹⁸F]2-deoxy-2-fluoro-D-glucose for assessing myocardial glucose uptake. Circulation. 1995;91:2435–2444.

3. Krivokapich J, Huang SC, Selin CE, Phelps ME. Fluorodeoxyglucose rate constants, lumped constant, and glucose metabolic rates in rabbit heart. Am J Physiol. 1987;252:H777–H787.

4. Buxton DB, Schelbert HR. Measurement of regional glucose metabolic rates in reperfused myocardium. Am J Physiol. 1991;261:H2068–H2068.

5. Lopaschuk GD, Spafford MA, Davies NJ, Wall SR. Glucose and palmitate oxidation in isolated working rat hearts reperfused after a period of transient global ischemia. Circ Res. 1990;66:546–553.

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