Myocardial metabolism in ischemic heart disease: basic principles and application to imaging by positron emission tomography

Myocardial Metabolism in Ischemic Heart Disease: Basic Principles and Application to Imaging by Positron Emission Tomography P. Camici, E. Ferrannini,

R

ECENTLY there has been a reawakening of interest in the patterns of glucose metabolism of the heart as a result of the use of radiolabelled deoxyglucose to evaluate myocardial utilization of exogenousglucose. Early studies on myocardial metabolism showed that the oxidation of glucose did not account for the major part of the oxygen uptake of the isolated heart-lung preparation.’ Rather, nonglucosefuels such as free fatty acids (FFA) were the most important substrate of the myocardium in the fasting state. In hypoxia, however, glucoseextraction increased concurrently with the formation of lactate, showing that hypoxia could accelerate the pathways of glycolysis.’ On the basisof these observations, it may be expected that glucose extraction by the ischemic heart should be accelerated, thereby allowing increased uptake of the tracer 18F-2fluoro-2-deoxyglucose, an event that can be imaged noninvasively by meansof positron emissiontomography (PET). This article first concentrates on the patterns of metabolism of the human heart, then on the effects of hypoxia and ischemia on the metabolism of glucose in experimental animals, and thereafter on the effects of angina pectoris on myocardial metabolism. Because ischemia, whether mild or severe, influences the pattern of glucose metabolism and glycolysis, these basic studies have definite implications for noninvasive imaging of ischemic heart disease in patients. Thus the overall purpose of this article is to outline current knowledge of human heart metabolism with special reference to the improved understanding recently achieved by noninvasive imaging. PRINCIPLES OF STUDY OF HUMAN MYOCARDIAL METABOLISM

Knowledge of the fuels of the human heart started with the introduction of coronary sinus (CS) catheterization by Bing in 1947.3 The chemical composition of arterial blood entering the heart was compared with that of CS blood

Progress

in Cardiovascular

Diseases,

Vol XXXII,

No 3 (November/December).

and L.H. Opie

leaving the heart. Glucose, lactate, and FFA were established as the heart’s major sourcesof energy (Figs 1, 2). According to the condition selected, each substrate could be the chief provider of energy. In general, the uptake of various substrates by the heart is dependent on the arterial concentration of the fuel concerned. Increasing the contribution of any one substrate leads to a decreasedcontribution by the others, because the various substrates compete for the availability of oxygen which sets the rate of mitochondrial oxidative metabolism. Extraction

Ratio

The procedure generally used in human studies for the assessmentof myocardial uptake of substratesor hormonesis the combined catheterization of the CS and an artery with measurement of substrate concentrations in simultaneously drawn arterial and CS blood samples. These allow the calculation of the extraction ratio, defined as (A-CS)/A. This index reflects the intrinsic capacity of the myocardium to extract a substrate from the arterial input independently of the arterial level of the substrate and of coronary blood flow (CBF). The expected oxygen extraction is the percentage of the concurrent myocardial oxygen extraction that can be accounted for by assuming full oxidation of each of the various circulating substrates (Table I). Thus the essential measurements required are the simultaneous arteriovenous differences (arterial v CS blood) of

From the CNR Institute of Clinical Physiology and Institutes of Medical Pathology and Clinical Medicine, University ofPisa. Italy (first two authors), and the Ischemic Heart Research Unit, University of Cape Town Medical School, and Medical Research Council, South Africa, respectively. Address reprint requests to Professor L.H. Opie. Heart Research Unit, University of Cape Town Medical School, Observatory 7925, Cape Town, South Africa. F I989 by W.B. Saunders Company. 0033-0620/89/3203-0003$5.00/0

1989:

pp 2 17-238

217

218

CAMICI.

GLYCOGEN

Absolute

F-ij’-@‘-a F 1,6 bis P

I

i I

I

i INHIBITION

LACTATE

Rates of Substrate

FERRANNINI.

AND

OPIE

Uptake

On the other hand, if the required information is the rate of absolute substrate uptake or release, then an estimate of CBF is necessary.The most widely used technique for measuring CBF in man is thermodilution in the CS’: the same catheter is used to sample CS blood and to estimate CBF. Two important points should be made. First, the thermodilution method measures blood flow through the CS only, and not total CBF. Furthermore, the CS drains different regions of heart muscle in different individuals. Therefore, absolute values of blood flow cannot be compared in different individuals but only in the samesubject when CBF is changed (eg, atria1 pacing). Second, the catheter technique provides estimatesof net myocardial exchange or balance. If there is simultaneous uptake and releaseof a substrate, the arteriovenous difference will be the

r O2

j-KJ---; ‘frG Pi Fig 1. Inhibited glycolysis in fasted state. Overall control of pathways of glycolysis which is taken as the conversion of G6P to P. The inhibition of glycolysis at the level of phosphofructokinase is the result of accumulation of excess citrate during oxidation of FFA. Pyruvate dehydrogenase is inhibited by accumulated NADH,, the result of &oxidation of fatty acids. G6P. glucose B-phosphate: P. pyruvete; G, glucose; F-6-P, fructose B-phosphate: PFK. phosphofructokinase; F 1.6 bis P, fructose 1 ,&diphosphate; GAPDH, glyceraldehyde phosphate dehydrogenase; NADH,. reduced form of nicotine adenine dinucleotide. (Copyright L.H. Opie)

glucose, lactate, pyruvate, FFA, and oxygen. Crude estimates of the type of fuel used by the heart can also be obtained by the respiratory quotient, which is calculated by dividing the rate of carbon dioxide production by the rate of oxygen uptake. Thus a respiratory quotient of near to one implies oxidation of glucose and/or lactate (Table l), whereas a lower value implies dominant fatty acid oxidation. Becausethe myocardial respiratory quotient was frequently low, early workers such as Cruickshank4 were alerted to the importance of oxidation of lipid even before blood fatty acid concentrations could be measured.

GLYCOGEN I ’

II G-6-P F-6-P F

0

pfK II *----’ 1.6 bis P

gG$qSE LACTATE

‘-7 I

“p

INHIBITION

P

I

’ CITRATE ----*I

Pi Fig 2. Overall patterns of glycolysis in the fed state. Besides direct accdaration of glucose uptake as a result of high circulating levels of glucose and insulin, there is indirect acceleration of glycolysis as blood FFA levels decrease, and the inhibition of phosphofructokinase by citrate is removed. (Copyright L.H. Opie)

MYOCAROIAL

Table

METABOLISM

1. Equations

IN ISCHEMIA

Used in the Calculation Extraction Ratios

219

of Oxygen

RQ Glucose Pyruvic

C,H 12O6 + 6 0, acid

CHJOCOOH Lactic

+ 2.1/2

0,

-

6 CO, + 6 H,O

1 .oo

-

2 H,O + 3 CO,

0.83

-

3 H,O + 3 CO,

1 .oo

-

16 CO, +

0.70

acid

CH,CHOHCOOH Palmitic acid CH,KZH,~,,COOH For convenience, reality the ionized

+ 3 0, + 23 0,

the unionized forms have forms (eg, lactate, pyruvate.

16 H,O

been shown: palmitate)

in are

oxidized. RQ, respiratory

quotient.

algebraic sum of the two processes. To obtain absolute rates of substrate uptake or release, the catheter technique must be combined with the use of labelled substrates. In such a case, the true uptake rate is obtained from the product of the extraction ratio of the labelled substrate and the arterial substrate level, and the true release (if any) is given by the difference between true uptake and net (=catheter) balance. In symbols, Net balance = (A - CS) x CBF Uptake

= (A* - CS*) x A x CBF/A*

Release = Uptake

- Net balance

(1) (2) (3)

in which A* and CS* are labelled substrate concentrations in arterial and CS blood, respectively. These equations are valid only when steady-state conditions hold for both labelled and unlabelled substrates. Extraction and Uptake Fasting State

of Substrates

in the

A small but consistent net uptake of circulating glucose by the heart is normally demonstrable in the fasting state. The reported arteriovenous (AV) differences range from 0.15 to 0.23 mmol/L, which correspond to a fractional uptake of only 3% and an average oxygen extraction ratio of roughly 27% (Table 2).6-” Measurements of the rate of glucose oxidation by radiolabelled techniques in healthy volunteers have shown that, at the most, only about one third of the glucose uptake is rapidly oxidized, and about 15% is converted to lactate.12 Similar data exist for the nonischemic dog heart.13The discrepancy between the data obtained by the use of oxygen extraction ratios and the actual measuredoxida-

tion rates in the case of glucose is because the oxygen extraction ratio data do not distinguish between the rapid oxidation of a substrate and delayed oxidation after passagethrough a storage form such as glycogen. The portion of the glucose uptake not rapidly converted to carbon dioxide or lactate may be converted to glycogen.12 Assuming an average CBF of 100 mL/lOO g/min, the measured glucose extraction rates in man can be converted to a glucoseuptake rate of about 20 pmol glucose/ 100 g/min. This value is within the range of isolated working rat heart data on acutely diabetic rats in which there is substantial inhibition of glycolysis resembling the fasting state in manI At such rates of glucoseuptake, conversion of over 50% to glycogen. asproposedby Wisneski et al,” would imply the formation of 10 pmol glycogen/lOO g/min (expressing glycogen as glucose equivalents). The fasting glycogen levels in man are about 26 pmol/g” sothat if glycogen were formed for 100 minutes at this rate, the cardiac content of glycogen would increase to 36 pmol/g. Eventually. high levels of glycogen would inhibit its synthesis (see section on glycogen turnover). Then other nonoxidative fates of glucose such as formation of aminoacids, the pentose shunt, and lipogenesiswould needto be considered.Nonetheless, the possibility of glycogen acting as a turnover pool for glucose en route to oxidation cannot easily be discounted. There is a general consensusthat FFA are the major fuels for cardiac muscle in the fasting, postabsorptive state. In various studies using the CS catheterization technique, net uptake of FFA from the arterial circulation has been found consistently. At arterial FFA levels in the 0.5 to 0.9 mmol/L range, the reported AV differences range from 0.14 to 0.20 ~mol/mL,‘“~1i~16~‘7 which correspond to oxygen extraction ratios of up to 40% (Table 2). If a total CBF of 250 mL/min is assumed,then the heart of fasting subjects at rest consumesup to about 50 pmol/min of FFA, or up to 10% of the whole body FFA turnover (8 pmol/min/kg), despite receiving only 5% of cardiac output. In general, the fate of FFA is largely complete oxidation in the Krebs’ cycle with a lessercomponent undergoing reesterification to tissue triglycerides. The fact that the respiratory quotient (RQ) of the heart in the

220

CAMICI,

Table

2.

Effect

of Nutritional

Percentage

Glucose

and

Uptake

Exercise

on Fuel

Accounted

for,

Lactate lOER Percent)

Pyruvate IOER Percent)

GlllCOSe (OER Percent)

Conditmns

State

of Oxygen

Total CHO (OER Percent)

and insulin

“Feeding” Postprandial,

for Oxidative

if Various FFA (OER Percent)

66

4

28

92 100

Postprandial, lipid meal Fasting, few hourst Same during exercise

10 31 16

2 0

10 28 61

20 61 77

30 34 21

Same

21

2

36

59

36

18

1

16

35

(67)$

recovery

Fasting overnight, resting

23

3

8

values,

fasting fasting fed KHO)

*Subjects

studied

2-3

TSubjects

studied

in the early

STotal SExact

fatty acid, conditions

11Reference TCorrected

Ketones (OER Percent)

Human

OPIE

Heart:

Oxidized Aminoacids (OER Percent)

RQ

OER, oxygen extraction Data from Opie.”

ratio:

5 2

0 0

3

0

5

6

0.74

1

10

67

66

1 0

13 8

29 38

70 58

22

1

8 12

31 36

53 77

14

27

1

11

38

62

14

7

0.74

5 68

1 4

11 28

17 96

62 5

14

7

0.74 0.94

h after a light low-fat

oxidation

58

afternoon

rates CHO,

1 .O

50

15 30

9

breakfast.

after

includes triglyceride. not specified; overnight

10. for actual

34 30

Approaches 0.94

56

2411 Mean values, corrected7 Mean values,

Fully

5

505

Mean

TG (OER Percent)

of the

Are

AND

none

CHO meal*

with

Metabolism

Substrates

FERRANNINI,

a light

breakfast.

fast assumed.

of glucose.” carbohydrate;

TG, triglyceride;

fasting state is 0.74 on average6 indicates that the greater part of the extracted FFA is oxidized. Furthermore, radiolabelled data in man show that uptake of circulating FFA in the fasting state is about 25% to 40% higher than the net FFA balance measured chemically.‘6.18 The probable explanation for this discrepancy is the release into the CS of unlabelled FFA derived from intramyocardial lipolysis. The fact that labelled CO, release into the CS amounts to 85% of the labelled FFA uptake suggests that there is rapid oxidation of FFA with only limited reesterification.‘* The major determinant of FFA uptake is believed to be their arterial concentration, as shown by linear correlations between arterial FFA concentration and FFA uptake or palmitate oxidation.” Since palmitate oxidation requires 23 pmol (0.52 mL) of 0, per hmol of palmitate

RQ, respiratory

quotient;

- = absence

of data.

(Table l), the expected oxygen usage for wholeheart FFA oxidation is about 1.l mmol/min (25 mL/min). This value is up to 75% of the total myocardial oxygen consumption in the fasting state, with an average of about 60% in seven studies (Table 2). Thus, the heart uses about 11% of the whole body oxygen consumption, and about two thirds of this is accounted for by direct FFA oxidation in the fasting state. Accordingly, if the values for myocardial oxygen consumption are plotted against myocardial FFA uptake, a good direct relationship exists in postabsorptive subjects. Both lactate and pyruvate are also extracted by the resting heart.6W9*‘2*16*17*20 For the former, reported (AV differences range between 0.1 and 0.18 mmol/L at arterial concentrations of 0.6 to 0.7 mmol/L, which corresponds to an extraction

MYOCARDIAL

METABOLISM

221

IN ISCHEMIA

ratio of about 25%. For pyruvate, the AV difference is somewhat variable, 4 to 20 pmol/L at arterial levels of 50 to 120 pmol/L. It is pertinent to recall that for either metabolite these estimates are net balance values. Since both lactate and pyruvate can be derived from endogenous glucose breakdown (as well as from one another by oxidation/reduction), it follows that the true tissue uptake should be larger than the net balance. In fact, when lactate exchange across the heart is measured with the use of U-13Clactate, the isotope extraction ratio is 41%, greater than the 25% net extraction ratio; this corresponds to a true absolute uptake of 65 gmol/min. Recent studies using labelled ketones” have shown that the fasting heart consistently takes up both acetoacetate and B-OH-butyrate at a cumulative rate of roughly 30 pmol/min. Furthermore, B-OH-butyrate is converted into acetoacetate at a rate (13 pmol/min), that is about fivefold higher than the rate of the reverse conversion.*‘*** Finally, of the naturally occurring aminoacids, only glutamate shows significant net uptake in the resting heart. A direct relationship has been observed in the fasting, resting heart between glutamate uptake and alanine release. This finding suggests that alanine is derived mainly from transamination between glutamate and pyruvate. It is to be expected that reduced generation of pyruvate would lead to decreased alanine output and reduced glutamate uptake. Inhibition of glycolysis by iodoacetate or 2deoxyglucose causes decreased myocardial alanine production. Conversely, hypoxia, by stimulating anaerobic glucose metabolism, enhances pyruvate production and alanine output.23 The Fed State

Feeding induces a set of metabolic changes in the whole body that have important effects on myocardial metabolism. Although the composition of the diet can be drastically altered in experimental models designed to assessspecific nutritional influences, the mixed diet of the average adult generates rather consistent substrate and hormonal signals. Of these, by far the most important is the increase in the circulating levels of insulin. Concomitant with insulin-

induced stimulation of glucose metabolism is a drastic reduction in FFA delivery to tissuesdue to the inhibition of adipose tissue lipolysis by insulin. Therefore, the shift in myocardial substrate utilization occurring with feeding (Table 2) is the result of a concerted action of insulin at the whole body level. Since feeding is also associated with hyperglycemia of variable degree, the stimulatory action of insulin is coupled with increased glucose supply: hyperinsulinemia and hyperglycemia thus work synergistically to promote glucose disposal. The absolute rates of myocardial glucose uptake in man can be estimated at about 60 pmol/lOO g/min (for assumptions, seeTable 2 and section on glucose in the fasting state), which is in the range of the values found in the isolated rat heart.14 Hyperglycemia in consciousdogs, achieved by intravenous (IV) glucose infusion, failed to increase myocardial glucose uptake when the endogenous insulin response was blocked with somatostatin,24suggesting that insulin was required for high rates of myocardial glucose uptake. In patients with ischemic heart disease, infusion of hypertonic glucose does lead to increased extraction of glucose by the myocardium, albeit with a very variable patternz5 It should be noted that glucose is a more efficient fuel than FFA in terms of energy production, because 5.01 kcal/L of 0, are generated from glucose (v 4.66 from fat) on complete oxidation (Table 1). Feeding, therefore, causes a rise in efficiency of O2 usage at the same time as it

Table

3.

Most

Common

Tracers With

Radionuclide

Half-ltfe (min)

'50

2.1

Used

in Cardiac

Studies

PET Compound

Water Carbon

dioxide

Molecular oxygen Carbon monoxide

US.3

Blood flow Blood flow Metabolism Blood

volume

‘3N

10.1

Ammonia Aminoacids

Blood flow Metabolism

“C

20.4

Glucose Fatty acids

Metabolism Metabolism

Microspheres Carbon monoxide Deoxyglucose

Blood flow Blood volume Metabolism

Chloride

Blood

‘*F %b

110 1.3

flow

222

CAMICI.

increases 0, consumption (for the absorption, processing and storage of nutrients). GLUCOSE-FFA CYCLE AND CONTROL OF GLYCOLYSIS

Patterns of substrate uptake by the human myocardium therefore show marked oscillation (Figs 1,2; Table 2) between (1) the fasting state, with low rates of uptake of carbohydrate in contrast to the high rates of uptake of lipids such as FFA and sometimes triglyceride, and a low respiratory quotient of 0.74; and (2) the fed state, with high rates of uptake of glucose and lactate, accounting for virtually all of the concurrent oxygen uptake and with a respiratory quotient of nearly 1.O.This change from predominant carbohydrate to predominant lipid usage is explained by the glucose-FFA cycle of Randle et a1.26The basic evidence for the glucose-FFA acid cycle, now to be described, was obtained by studying patterns of glycolysis in isolated rat heart tissue. Inhibition

of Glycolysis

by FFA

Glycolysis is defined here as that metabolic pathway (Emden-Meyerhof) converting glucose 6-phosphate to pyruvate. The pathways of glycolysis can be divided into two parts, the first concerned with the metabolism of 6-carbon hexosesand the secondwith 3-carbon trioses. Glycolysis first converts glucose 6-phosphate (derived from glucoseor glycogen) into a 6-carbon hexose compoundcontaining two phosphategroups, fructose 1,6-bisphosphate (= fructose 1,6-diphosphate). Secondly, glycolysis converts each 6carbon hexose into two 3-carbon trioses, eventually forming pyruvate. which either undergoes conversion to lactate in anaerobic circumstances (anaerobic glycolysis) or conversion to acetyl CoA to enter the citrate cycle in aerobic circumstances (aerobic glycolysis). In the first of the above steps, two molecules of adenosine triphosphate (ATP) are used up per glucose molecule converted to two trioses phosphate molecules. In the secondof the steps, four molecules of ATP are made anaerobically for each glucose6-phosphateultimately converted to pyruvate. An important early observation was that the oxidation of glucoseby the isolated rat heart was

FERRANNINI.

AND

OPIE

markedly inhibited by concurrent provision of FFA.27 The mechanismwas shown to be complex and included inhibition of glucoseuptake, inhibition of glycolysis at the level of phosphofructokinase by the high levels of citrate found in hearts using FFA, and the inhibition of pyruvate dehydrogenase. Inhibition at the level of pyruvate dehydrogenase is mediated by formation of NADH2,28 which stimulates the kinaseconverting the active to the inactive form of pyruvate dehydrogenase.29 On the other hand, during provision of glucose and insulin, adipose tissue lipolysis is inhibited so that the circulating levels of FFA are low, while those of glucose and insulin are high. High rates of glucoseuptake are ensured both by positive factors (high glucose and insulin concentrations yield maximal rates of glucose uptake in isolated hearts) and negative factors (low FFA concentrations remove the inhibition of glucose uptake and glycolysis normally exerted by FFA during the fasting state). These basic observations explain why in the fed state the respiratory quotient of the myocardium is close to 1, whereas in the fasting state it approaches0.7. Cellular Uptake

Mechanisms for Control and Phosphorylation

of Glucose

The physiologic range of blood glucoselevels is relatively high (5 to 10 mmol/L; 90 to 180 mg/ 100 mL), whereasthe content of free glucose within the heart cell is almost too low to be measured.” Thus the transsarcolemmal gradient of glucoseis very high so that the uptake process is normally strongly inhibited. For glucose to enter the heart cell requires (1) an adequate rate of delivery of glucose to the heart cell by coronary blood flow; (2) the transport of glucosefrom the blood acrossthe capillary membrane into the interstitial space;and (3) the transport of glucose across the sarcolemma into the cytosol of the heart cell. Glucose transport into the cell is regulated by a stereospecific glucose carrier system. Glucose transport is limited in the fasting state because insulin is low in concentration and becausealternate fuels such as FFA inhibit the transport mechanism. Conversely, in the fed state, both a high external glucoseconcentration and the pres-

MYOCARDIAL

METABOLISM

223

IN ISCHEMIA

ence of insulin accelerate glucose transport. Other factors known to enhance transsarcolemmal glucose transport are increased heart work and tissue hypoxia. After intracellular penetration, glucose is phosphorylated to glucose 6-phosphate. This step is controlled by the enzyme hexokinase, which has a very low Km for glucose so that only a very low concentration of glucose is required for a fast rate of enzyme reaction. Thus intracellular glucose is rapidly phosphorylated and free glucose does not accumulate within the cell even when the rate of transport of glucose is accelerated by hypoxia. The action of hexokinase is not reversible, because it is an energy-consuming reaction requiring ATP. In addition, glucose-6-phosphate phosphatase activity in the myocardium is thought to be negligible. Thus hexokinase traps the glucose taken up into the cell as glucose 6phosphate. As the latter compound forms, it stimulates its own removal either by the pathways of glycolysis or by glycogen synthesis. If these pathways are blocked, as for example during the oxidative metabolism of FFA, then glucose 6-phosphate accumulates and restrains further uptake of glucose by allosteric inhibition of hexokinase. Although many enzymes participate in the reactions of the glycolytic pathway, only a few are thought to regulate glycolytic flux by acting as control points.30 Of great importance is the role of phosphofructokinase (Figs 1,2); its activity can be increased during the fed state simply by substrate provision (high rates of formation of glucose 6-phosphate giving rise to high concentrations of fructose 6-phosphate), whereas in the fasting state phosphofructokinase is inhibited by citrate which is produced especially by the oxidation of fuels yielding high cytosolic contents of citrate such as FFA and ketone bodies. A third site at which glycolysis is regulated by fatty acid metabolism is the activity of the enzyme pyruvate dehydrogenase, which is deactivated during FFA metabolism (Fig 3). Thereby products of glycolysis are diverted from aerobic metabolism in the citrate cycle to lactate formation. The practical implication is that high rates of FFA metabolism, as in the fasting state, are accompanied by lower rates of myocardial lac-

GLYCOLYSIS

1

CITRATE CYCLE

Fig 3. The major fates of pyruvata. In the normal aerobic heart. lactate is taken up and converted to pyruvata, the major part of which enters the citrate cycle. In the anaerobic heart, lactate is produced from pyruvata derived from glycolysis. Transamination is a pathway of minor significance, although increased in anaerobiosis. crKG, aketogluterata. (Copyright L.H. Opial

tate extraction because glucose is diverted to lactate as a result of the inhibition of pyruvate dehydrogenase. An absolute value of myocardial lactate extraction other than overt production cannot be used as an indicator of abnormal myocardial oxidative metabolism. Therefore, to interpret lactate formation or decreased lactate extraction requires a knowledge of the concurrent rates of FFA uptake by the myocardium (Fig 4).31 Glycogen

Turnover

In the fed state, when there are high rates of uptake of glucose and when insulin is present, the activity of glycogen synthetase (= glycogen transferase) is stimulated by increased levels of intracellular glucose 6-phosphate and especially by insulin. Insulin is able to increase the ratio of glycogen synthetase in the more active I or a form up to 75YL3* The activity of glycogen synthetase is controlled by a phosphorylationdephosphorylation cycle. When phosphorylated,

224

CAMICI. 2.00

5

glycogen are an important FFA cycle.

1

1.50

FERRANNINI.

AND

OPIE

part of the glucose-

E z L

Evidence

1.00 0.50 1

00

04 0

I 100

50 TIME

I 150

(mins)

Fig 4. Exampie of changes in arterial (ART) FFA and myocardial lactate extraction during time course of the experimental protocol. Plot of arterial FFA Y time (top). Plot of arterial and CS lactate levels vtime (bottom). (Reprinted with permission.“)

the enzyme is in its less active form (synthetase I to D conversion), whereas dephosphorylation to the more active form is achieved by another auxilliary enzyme, synthetase phosphatase; as the inorganic phosphate is split off, the more active I form is regained. Thus it might be expected that cardiac glycogen should be high in the fed state. Yet its content is actually lower than after prolonged fasting (or in severe ketotic diabetes) when there are low rates of glucose uptake and little activity of insulin. The probable explanation is that as the level of cardiac glycogen rises during glycogen synthesis, a feedback mechanism turns off the synthetase.33T34 Without such a mechanism, excess accumulation of glycogen could occur during conditions such as severe diabetes or prolonged fasting, when glycogen synthesis takes place even in the absence of insulin by virtue of the high levels of cardiac glucose 6-phosphate, consequent on inhibition of glycolysis by the fatty acids or ketones. During prolonged fasting, on the other hand, the high cardiac glycogen content may consist largely of low turnover glycogen, the indirect result of high blood FFA and low blood insulin concentration. Thus changes in the level and turnover of cardiac

for Glucose-FFA

Cycle In Vivo

These control mechanismsfor glycolysis, described in isolated enzyme systems,are the basis for the glucose-FFA cycle, whereby in the resting human heart there is an inverse relationship between FFA and glucose uptake.10-‘2*‘6S17 Furthermore, when whole-body FFA turnover is inhibited by the administration of nicotinic acid, myocardial FFA uptake is drastically decreased and glucose uptake is increased.16Under these circumstances a proportionate shift in MVO, partition between carbohydrate and lipid oxidation is observed, and the regional RQ increases. On the other hand, all physiologic conditions that are associated with increased lipolysis and FFA turnover inhibit myocardial glucose exchange. For example, prolonged fasting or stressful circumstancesincrease the releaseof insulin antagonistic hormones(glucagon, catecholamines,cortisol, and growth hormone), all of which stimulate lipolysis, thereby enhancing tissue FFA utilization. Although it is generally true that the heart will use FFA in proportion to its arterial delivery, endogenoustriglyceride storescan help to supply oxidative fuel when circulating FFA levels are suppressedby carbohydrate feeding. There can be dissociation between systemic supply and tissue usageof FFA if enough insulin is available to block adipose tissue lipolysis, but the inhibitory action of the hormone is overcome by local lipolytic stimuli, eg. catecholamines. In that case, the myocardial exchange of plasma FFA may be drastically reduced but a net release of glycerol will prove the presence of ongoing intracellular lipolysis. l6 It should also be noted that oxidation is the predominant but not the only fate of FFA in the myocardium. Therefore, there are factors, eg, oxygen availability, that regulate the partition of FFA flux between oxidation and reesterification, as it has been shown in studies using palmitate labelled with positronemitting “C and PET35: the washout curve of “C-palmitate from the myocardium consistsof a rapid component associatedwith releaseof “CO,, and a much slower phase, possibly reflecting incorporation of the tracer into triglycerides.

MYOCARDIAL

METABOLISM

IN ISCHEMIA

Although a glucose-FFA mechanism is surmised whenever a reciprocal relationship between FFA and glucose uptake is observed, it is the extent of FFA oxidation that counterregulates glucose oxidation. It is also worth noting that the heart takes up considerable amounts of lactate, 60% of which is destined to be oxidized. It is therefore not surprising that inverse relationships have also been found to exist between FFA and lactate extraction in the heart (Fig 4).‘633’336Although quantitatively less important, pyruvate has a similar behavior. The operation of Randle’s cycle26 for the reciprocal control of glucose and fat oxidation has received experimental validation. In the conscious dog, the infusion of a triglyceride emulsion during steady-state euglycemic hyperinsulinemia significantly impairs myocardial glucose uptake in comparison with the situation where only hyperinsulinemia is applied. Of interest in the latter studies was the finding that myocardial lactate extraction was not altered by lipid administration, suggesting that the FFA-induced inhibition of glucose metabolism was exerted on phosphofructokinase or enzymes of the upper glycolytic pathway rather than on pyruvate dehydrogenase. Furthermore, it has recently been shown with the use of the euglycemic insulin clamp technique that the temporal sequence of substrate supply is also important in determining the extent of FFA inhibition of glucose metabolism.37 Thus, in general, Randle’s cycle appears to be operative in the fasting as well as in the fed state in human heart, although its impact is dependent upon multiple factors, eg, rate of FFA oxidation, level of insulin stimulation, time sequence, etc. Catecholamines

and the Glucose-FFA

Cycle

When humans are subjected to infusions of norepinephrine, there is evidence for decreased rather than increased glycolysis in the heart, together with a greatly increased uptake and oxidation of FFA.38 Output, instead of the normal uptake, of pyruvate by the heart suggests that glycolysis is restricted by enhanced FFA metabolism. In dogs, FFA uptake and oxidation also increase following increasedcirculating concentrations during an IV infusion of norepinephrine. while a decreasedrespiratory quotient sug-

gests decreasedglucose utilization.39 These data are consonant with a primary effect of catecholamines on adipose tissue lipolysis, setting in motion one component of the glucose-FFA cycle. METABOLIC CHANGES IN THE ISCHEMIC MYOCARDIUM Arteriovenous

Studies

in lschemia

Applying the sametechniques as used for AV substrate differences in nonischemic man (Table 2) should yield useful information on the overall patterns of myocardial metabolism during ischemia. Besides all the technical problems already outlined (seesection on principles of study of human myocardial metabolism), it is important to stress the black-box principle, whereby AV differences merely say what goesin and out of the black box, but cannot say what happens inside it. For an adequate attempt to relate what goesout of the black box to what goesin requires a prolonged steady state. Thus all AV differences dependon a steady state for secureinterpretation of the findings. Such steady-state data are relatively easy to obtain in patients at rest. Exerciseinduced ischemia, therefore, cannot be accurately studied by AV differences because of changing FFA levels and the releaseof catecholaminesduring exercise. Hence, angina is usually induced by pacing, an unnatural stimulus, which may give a variety of angina not exactly corresponding in a strict metabolic sense to effortinduced angina. Lactate Balance in Ischemia Since lactate is the end product of anaerobic glucose breakdown, its release in CS blood has been considered as a metabolic fingerprint of myocardial ischemia (Fig 5). In normal subjects, atria1 pacing does not affect the arterial concentration or the transmyocardial extraction of lactate which remain unchanged relative to basal values.7 Transmyocardial lactate difference in patients with coronary artery diseaseand stable exertional angina has been evaluated during atria1 pacing.9-“~40Lactate release in the CS during pacing stress,although specific, wasfound not to be a sensitive marker of myocardial ischemia since it can be demonstrated in only 50% of patients with pacing-induced left ventricular dysfunction.4’ Up to 0.5 mmol/L of lactate

226

CAMICI.

.YCOGEN -------------

1.6 bts P II TRIOSE

Wash OUt

c

1 ---\ +

OPIE

In

FJ

most studies, transmyocardial

ATP + CP + PI 0

time of lactate discharge have not been reported. Third, myocardial lactate metabolism is heterogeneous, both along and across the heart wall, presumably reflecting the irregular distribution of the effects of coronary artery atherosclerosis. Studies with radiolabelled lactate show that in patients with ischemic heart disease there can be a significant release of lactate from the myocardium at a time when the chemical measurements indicate an overall pattern of myocardial lactate uptake.44 Because of these variables, a decrease of myocardial lactate uptake cannot be considered as diagnostic of myocardial ischemia,42 although myocardial lactate output is good evidence favoring ischemia.44 Yet even lactate output is not totally diagnostic of ischemia because there are incidences in man and in animal preparations in which lactate discharge can occur in the absence of any myocardial hypoxia or ischemia’6*4s and in the presence of normal coronary angiograms.46

I I

I INHIBITION REMOVED

*LACTATE

+FFA

AND

because FFA uptake inhibits that of lactate.” f

I!-6-P ll*--&

FERRANNINI,

-

02

Fig 6. Mechaniama whereby mild iachemia enhances glycolyaia. Note especially a direct effect of hypoxia on glucose tranaport, and acceleration of phoaphofructokinaae (PFK) activity by decreasing ATP. CP, and en increase of inorganic phosphate as well aa a decrease of citrate. Note inhibition of pyruvate dehydrogenaae (PDH) by NADH,. (Copyright L.H. Opie)

can be released from the ischemic myocardium as compared with an average extraction of 0.1 to 0.2 mmol/L during normoxic conditions.7*42 Unfortunately, no relationship between lactate production and the severity of ischemia can be demonstrated.41 Among the patients who are lactate producers, some show extraction at rest and production during pacing, while in others lactate production already occurs at rest and increases significantly with pacing.7 The probable explanations for the discrepancies between the degree of ischemia and the magnitude of lactate discharge into CS blood are threefold. First, the CS catheter can only sample venous blood reaching the tip of the catheter, which may be situated far from the ischemic zone. In animal experiments, siting of the catheter tip can be critical in attempts to sample true local venous blood which contains the metabolic abnormalities typical of myocardial ischemia, including lactate discharge.43 Second, the simultaneous extraction of FFA needs to be assessed

FFA values at the

Pyrztvate and Alanine Pyruvate that is formed from anaerobic glycolysis during ischemic conditions can also undergo transamination (Fig 3) with formation of alanine at the expense of glutamate (which serves as a NH, donor). In patients with coronary artery disease and chronic angina, there was a greater release of alanine in the CS as compared with normal subjects both at rest and during atria1 pacing.42,47 At the same time, a greater glutamate uptake in comparison to normals could also be demonstrated in these patients both at rest and during pacing. These alterations of aminoacid metabolism in patients with chronic ischemic heart disease may suggest a mechanism for enhanced glycolysis due to adaptive biochemical changes associated with previous episodes of hypoperfusion.42*47948 Citrate Transmyocardial release of citrate is thought to be caused by high rates of FFA metabolism, which provide an excess of acetyl CoA and hence of citrate to the mitochondria.49 This citrate may be of cytosolic origin since the inner mitochon-

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drial layer is virtually impermeable to this compound. Cytosolic citrate can be formed from glutamate by the activity of the NADP-linked isocitrate dehydrogenase via 2-oxoglutarate and isocitrate.48 In angina1 patients at rest, there is transmyocardial citrate release. The greater release of citrate in the postpacing recovery phase” could be a phenomenon similar to enhanced FFA metabolism in the postischemic reperfusion period in animal hearts. So First principles would suggest that myocardial citrate release should be inhibited during the ischemic pacing phase because of decreased activity of the citrate cycle. However, some citrate release appears to continue.” Such release may either reflect the impossibility of CS sampling as a sensitive detector of local ischemic blood formed far from the tip of the CS catheter, or there may be large interindividual variability (because the standard deviation of citrate measurements during the pacing period is relatively high and overlaps with the zero line) (Bagger et al, Fig 310). Glucose and FFA Both in normal subjects and patients with stable exertional angina, atria1 pacing does not alter significantly the arterial concentration of glucose.’ In these patients, however. a greater transmyocardial glucose extraction relative to normals seems to take place particularly during the pacing phase. 7~10In addition, a positive correlation between glucose extraction and lactate production can be demonstrated during pacinginduced ischemia.791’ In no instance, however, could a stoichiometric relationship between glucose extraction and lactate production be demonstrated, suggesting that regulation of glycolysis must also be seen in terms of entry points other than glucose (such as glycogen) or exit points other than lactate (such as aerobic metabolism or alanine formation). With regard to FFA, pacing per se does not affect their arterial concentration either in normal subjects or patients with ischemic heart disease.109’1 During pacing-’ induced ischemia, myocardial FFA uptake should be significantly reduced’*” in agreement with the experimental evidence of a reduced oxidation of these compounds during anaerobiosis.” However, extrac-

tion of FFA by the myocardium during pacing may be unchanged. *’ Thus CS catheterization during pacing-induced ischemia offers the possibility of evaluating some metabolic changes that are associated with a reduced tissue oxygen content. However, several problems connected with CS catheterization per se and with the methods for the measurements of concentrations of metabolites limit the information and inferences that can be derived from such studies. Cellular

Control

Mechanisms

in lschemia

The basic control mechanismsoperative during myocardial ischemia have been defined in animal experimental models. The two basic changes are increased glycogen breakdown and increased glucose uptake; both feed their products into the pathways of glycolysis which are accelerated by anaerobiosis (Pasteur effect). In the dog heart with coronary artery ligation, tissue glycogen is the major source of lactate released into coronary venous blood within the first 60 minutes after ligation, but thereafter glucosebecomesthe major source.13 Glycogen in Ischemia During hypoxia or ischemia there is glycogen breakdown. An early event in ischemiais accumulation of cyclic adenosinemonophosphate(AMP) in the ischemic zone, which activates phosphorylase by converting the inactive b to the active a form. Changes between the two forms of phosphorylase involve another phosphorylation-dephosphorylation cycle, also dependent on a kinase and a phosphatase.Thus there is the wellknown cascade of events that governs glycogen breakdown: catecholamine stimulus - adenylate cyclase - cyclic AMP activation of protein kinase - activation of phosphorylaseb kinase change of phosphorylase b to a - breakdown of glycogen. Glycogen phosphorylaseis regulated in a reciprocal manner, opposite to that of the synthetase. Its active form (phosphorylase a) is phosphorylated while its lessactive form (phosphorylase b) is dephosphorylated. Calmodulin, the intracellular Ca*+-binding receptor protein, is one of the subunits of phosphorylase b kinase; the latter enzyme must therefore have calcium present for its activity.

228

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Activity of phosphorylase by itself is not sufficient for full glycogen breakdown. Phosphorylase removes 6-carbon units, one at a time, to form glucose l-phosphate from the outer limit of the carbohydrate chains that comprise glycogen. At a critical branch point, ie, the 1,6 link, a further enzyme, the debranching enzyme, must act (amylo 1,6-glucosidase); then phosphorylase can resume activity to remove 6-carbon units. Achs and Garfinke15* have suggested that phosphorylase is the enzyme controlling the initial burst of glycogenolysis during hypoxia or ischemia, and thereafter the activity of the debranching enzyme becomes rate-limiting. Glucose Uptake in Anaerobiosis Anaerobiosis accelerates glucose transport in two ways (Fig 5). First, there is indirect acceleration of glycolysis as lack of oxygen inhibits the metabolism of FFA, normally exerting an inhibitory effect on glucose uptake and various steps in the glycolytic pathway. Second, even in hearts perfused with glucose as the only exogenous substrate, anaerobiosis increases glycolysis and glucose uptake. During hypoxia, with sustained coronary flow, the transport of glucose across the sarcolemma into the cytosol of the heart cell is accelerated. In contrast, during severe ischemia, glucose uptake can be much reduced because the coronary blood flow is low enough to limit the rate of delivery of glucose to the ischemic cells.53 The mechanism whereby anaerobiosis accelerates glucose uptake is still not determined. However, the early proposals of Randle and Smith54 were that an adequate supply of energy in the form of high energy phosphate compounds acted in an ill-understood way to restrict glucose entry. Because glucose uptake is accelerated by anaerobiosis, glucose 6-phosphate will accumulate to exert feedback inhibition on hexokinase, unless glucose 6-phosphate is removed by glycogen synthesis or by increased glycolysis. As glycogen is broken down during anaerobiosis, glycolysis must be accelerated. Acceleration

of Glycolysis

by Mild Ischemia

The activity of phosphofructokinase during mild ischemia” is specifically accelerated by changes in the energy status of the cells. During

FERRANNINI.

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conditions of adequate oxygenation and substrate supply, the cellular content of ATP is high and the enzyme is ATP-inhibited. During mild ischemia, ATP levels decrease, those of ADP and AMP increase, as does that of Pi (the latter also being derived from breakdown of creatine phosphate). Hence the inhibition of phosphofructokinase by ATP is relieved both by the decrease in ATP and by deinhibition by the breakdown products of ATP (Fig 5). NewsholmeS proposes that the ATP/AMP ratio acts as an amplification system, so that the combination of the decrease in ATP and the increase in AMP is more stimulatory than either change considered singly, thereby amplifying the effect of hypoxia. Another amplification system relates to the breakdown of energy stores in the form of creatine phosphate (CP). Breakdown of CP occurs rapidly after the onset of hypoxia and gives rise to increased inorganic phosphate. Both the decrease of CP and the increase of the inorganic phosphate relieve the inhibition of phosphofructokinase by CP. Inhibition Products

of Anaerobic Glycolysis

by End

Products of anaerobic glycolysis, namely protons, NADH, and lactate, can feed back to inhibit glycolysis at various levels. An important factor regulating phosphofructokinase activity is the cytosolic pH; an increase stimulates activity and a decrease inhibits it. The inhibitory effect of acidosis is one factor explaining the difference between the effects of severe ischemia, which inhibits glycolysis (Fig 6), and mild ischemia which accelerates glycolysis. The crucial difference is whether or not the coronary flow is adequate (as in mild ischemia) to wash out most of the accumulated protons, or inadequate, as in severe ischemia, with accumulation of protons and inhibition of phosphofructokinase. These differences are reflected in the increased glucose uptake of the mildly ischemic rat heart v the decreased uptake of the severely ischemic heart.56-58 Proton production by glycolysis. During normal aerobic metabolism, NADH, produced by glycolysis is oxidized within the mitochondria after passage through a shuttle. In contrast,

MYOCARIXAL

METABWSM

229

IN ISCHEMIA

GLYCOGEN

tion of ADP and ATP with Mg’+, the following equation is derived: glucose + 2 MgADP’-

F I6

bis

+ 2Pi2-

2 lactate’-

+ 2 MgATP’-

In anaerobic conditions, all ATP produced will be broken down so that protons are produced:

P

I I I

TRIOSE

2 MgATP2-

-

2 MgADP’-

+ 2 Pi2- + 2 H+

Fig 6. Mechanisms whereby severe ischemia inhibits glycolysis. Note especially decreased glucose delivery by the very low coronary flow, inhibition of phosphofructokinase by a low intracellular pli, inhibition of glyceraldehyde 3-phoaphodehydrogenase (GAPDH) by lactate and NADH,. (Copyright L.H. Opie)

when glycolysis is anaerobic, the NADH, is reused to form lactate. In neither case are protons produced, except that the change from NAD to NADH, H, + NAD is more correctly

NADH,

(1)

given as

H, + NAD+

-

NADH

(3)

+ H+

(2)

However, for practical purposes, the protons thus produced are handled together with NADH, hence equation 1 is frequently used in practice. Anaerobic glycolysis is usually held to be proton-producing and the potential cause of acidosis. Gevers59 has examined this question in detail. When all the charges are written into the individual glycolytic reactions and allowance is made for the probable degree of complex forma-

(4)

These equations are only approximations and depend on a number of assumptions including the free Mg2+ in the cytosol and intracellular pH (the latter influencing the phosphate change). The details of this scheme have been criticized by Wilkie.60 but the basic concepts seem sound. Normally anaerobiosis will not only cause pyruvate to form lactate, but will break down the newly formed glycolytic ATP and ADP (equation 4), thereby liberating inorganic phosphate and protons. Thus ATP generated from glycolysis (and not lactate) is the immediate source of the protons produced during anaerobic glycolysis. Lactateproduction. A fundamental observation made by Evans et al in 1934 was that arterial hypoxia caused lactate output instead of lactate uptake by the heart.2 The critical metabolic signal that governs whether pyruvate is oxidized or reduced to lactate is the accumulation of NADH, in the cytosol. NADH, accumulates in ischemia or hypoxia to stimulate lactate dehydrogenase and to inhibit pyruvate dehydrogenase. Because pyruvate dehydrogenase can be inhibited by conditions other than ischemia or hypoxia, such as severe diabetes mellitus, the formation of lactate cannot always be equated with a lack of intracellular oxygen.

Conversion of Pyruvate to Alanine Although lactate formation from pyruvate is the commonly recognized end product of anaerobic glycolysis, another possibly anaerobic fate of pyruvate is transamination to alanine (Fig 3).23 In the isolated heart, alanine accumulates during perfusion with pyruvate. Net production of alanine during anaerobic glycolysis, however, is only < 10% that of lactate.

230

CAMICI,

Inhibition of Glyceraldehyde-3-Phosphate Dehydrogenase As lactate accumulates, this enzyme is inhibited. In isolated systems, about 15 to 20 mmol/L lactate is inhibitory.57 However, inhibition of glycolysis during &hernia cannot be complete because glycogen breakdown continues to occur over many hours in the dog heart with coronary artery ligation, despite very high tissue lactate levels (Table 2).61 Severity of Ischemia and Glycolytic

Flux

During mild ischemia, the cellular content of ATP and citrate is low, inorganic phosphate is high, and the result is that activity of phosphofructokinase is accelerated (Fig 5). During severe ischemia, inadequate washout of glycolytic products is added to the effects of hypoxia. Now accumulation of protons, NADH,, and lactate inhibits phosphofructokinase, despite the acceleration by loss of ATP and citrate (Fig 6). As a result of ischemic inhibition, the tissue content of the substrates of phosphofructokinase (glucose 6-phosphate and fructose 6-phosphate) increases, while that of the product (fructose 1,6-bisphosphate) decreases. In contrast, during severe hypoxia with maintained coronary flow or during mild ischemia, decreased inhibition at the level of phosphofructokinase stimulates glycolytic flow so that the concentration of glucose 6-phosphate and fructose 6-phosphate decreases, and that of fructose 1,6-bisphosphate increases. The combination of decreased levels of these substrates (G-6-P and F-6-P), together with an increased +FDG*q 4

WASHOUT

G

+’

[ c+

+ FFA*

OPIE

Substrate Competition in Ischemia During mild ischemia, there is still residual oxygen uptake by the ischemic zone as a result of collateral blood flow still providing a supply of oxygen. There is competition for this residual oxygen uptake. As outlined, in the nonischemic myocardium, FFA compete more successfully than does glucose for the myocardial oxygen uptake. In contrast, in the ischemic myocardium, there is an increased extraction and oxidation of LFDG*-6-1

+ FDG*-

tG

G-6-P

LOW WASHOUT

-

G-6-P

-

-

A Fig 7. Use phosphorylated A) Mild &hernia.

AND

glycolytic flux, can only be explained by increasedactivity of the enzyme, ie, altered regulation at the level of phosphofructokinase. Thus two opposing factors are at work: reduction of coronary flow indirectly reduces delivery of oxygen which will stimulate glycolysis, while the reduced washout allows accumulation of products of glycolysis which in turn inhibits glycolysis. Furthermore, decreased delivery of glucoseto the ischemic cells by the very low rate of coronary flow also decreasesglucose uptake. Thus there is a crucial difference between the effects of mild and severe ischemia on glucose uptake, phosphorylation, and glycolysis. Whereas mild ischemia accelerates glucoseuptake, a process that can therefore be imaged by PET with increasedintracellular content of labelled deoxyglucose 6-phosphate, in severe ischemia there is decreasedglucoseuptake and intracellular deoxyglucose 6-phosphate (Fig 7). Therefore, PET imaging is potentially an important procedure to detect the severity of the metabolic consequences of ischemia.

+

LACTATE

FERRANNINI,

B of F-l&deoxyglucose form of deoxyglucose 8) Severe ischemia.

(FDG.1 (FDG*-6-P) (Copyright

as

a metabolic marker cannot be metabolized L.H. Opie)

for the glucose further, in contrast

uptake process. Note to glucose B-phosphate

that the (G-6-P).

MYOCARDIAL

METABOLISM

IN ISCHEMIA

231

glucose, relative to that of FFA.13 The mechanism is probably explained by the acceleration of glucose uptake and glycolysis found in mild ischemia (Fig 5). Application of Animal lschemia to Man

Experimental

Data on

Several factors limit the inferences that can be made about myocardial metabolism in patients with ischemic heart disease. Metabolic changes that are consistently demonstrated in the isolated rat heart or in the open-chest dog during conditions of reduced oxygen supply may or may not be seen in patients with various forms of ischemic heart disease. For example, lactate release into the CS during atria1 pacing may be demonstrated only in a subgroup of patients with angiographically proven coronary artery disease and stable exertional angina yet not in others who have equally severe coronary artery disease and similar symptoms and signs of ischemia. Such a discrepancy can be accounted for in part by the difficulties created by the specific problems in the interpretation of changes in lactate metabolism during pacing in man (see section, lactate balance in ischemia). The following technical problems must also be considered: a great number of parameters can be kept under control in the experimental animal in which any sort of invasive measurement can be performed. The experiments can therefore be designed to isolate individual metabolic changes independently of other factors. In human studies, on the other hand, acquisition of this kind of information is cumbersome, limited, much less controlled, and is affected by the intrinsic variability of the phenomenon under study. Furthermore, in animals the zone of myocardial ischemia is usually clearly defined, whereas in man its location (in relation to the tip of the CS catheter) is usually not known. To what extent experimental models of myocardial ischemia are relevant to human disease depends on factors that require more detailed discussion. Ischemia, due to either a primary reduction of coronary flow (primary ischemia) or to an imbalance between oxygen supply and demand (secondary ischemia), is the commonest cause of reduced myocardial oxygen delivery in man.62 In addition, myocardial ischemia in man is usually a

regional phenomenon and has a different transmural severity, being more severe in the inner compared with the outer myocardial layers of the left ventricle.62 In the clinical setting, myocardial ischemia can be either transient and reversible (angina pectoris) or prolonged and irreversible, ie, associated with permanent tissue damage (myocardial infarction). In between these two extremes (angina and infarction) many clinical situations can be recognized in which a variable mixture of viable and necrotic tissue coexist. The metabolic basis of human ischemic heart disease differs from most experimental animal models because of the interplay of essentially four factors: (1) ischemia rather than anoxia is the mechanism of reduced oxygen supply; (2) ischemia in man is a regional phenomenon both along the heart walls and across them; (3) ischemia can be primary or secondary; and (4) the duration and severity of flow reduction have important consequences of their own. However, despite possible difficulties and limitations, a number of recent studies with new techniques have been aimed at evaluating myocardial metabolism in patients with ischemic heart disease. ROLE OF PET: NONINVASIVE IMAGING INTRACELLULAR METABOLISM Metabolic

OF

Principles

Noninvasive metabolic imaging of ischemia basically relies on two simple observations. First, the uptake of glucose by the myocardium is increased by hypoxia and mild ischemia, but decreased by severe ischemia (Figs 5, 6). By the use of 18-F-deoxyglucose, the process of glucose transport into the cell can be monitored noninvasively (Fig 7). Second, during both mild and severe ischemia, the extraction uptake and oxidation of FFA are reduced. Hence the uptake of an appropriately labelled fatty acid is seen to be decreased in the ischemic myocardium. Techniques

Nuclear techniques have undergone a rapid growth over the past decades, reflecting the need for more precise information about regional tracer distribution in tissue. While important qualitative deductions, such as an obviously reduced

232

CAMICI,

blood flow to a section of an organ, can be made from visual inspection of a tomographic image, it is the reliability of the numerical representation of the radionuclide distribution that is of critical importance. The tomographic image should not only be free of spatial distortions, but also the measured uptake per unit activity should be independent of anatomic location. This ideal relies on the independence of spatial resolution and correction for photon attenuation caused by the position of the heart in the body. The characteristics of PET imaging greatly enhance the possibility of attaining these optimum conditions.63 A positron (a positively charged electron) emitted from a nucleus will annihilate with an electron and produce simultaneously two 511 KeV y-rays travelling in approximately opposite directions. All elements except the three lightest (hydrogen, helium, and lithium) have associated positron-emitting isotopes that are artificially produced by an accelerator (cyclotron). Table 3 shows the principal radionuclides that have been used in cardiac studies to label either physiologic substances or analogues. The regional distribution of positron-labelled radiopharmaceuticals in different organs of the body can be measured by means of specially designed PET. The majority of PET machines in operation today are those that image one or more transaxial slices through the body and consist of circular or polygonal detector arrays.64 PET Patterns Pectoris

in Patients

With

Stable

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tion of coronary flow. Under these circumstances, patients are not distinguishable from a group of normal volunteers studied under the same conditions. Also, the washout of labelled palmitate, as an index of FFA oxidation in the myocardium, is uniform in various myocardial regions in patients with stable angina at rest. To study the effects of exercise on myocardial metabolism, patients with effort angina were subject to maximal bicycle ergometric exercise in the supine position within a PET camera. Due to the very low physical half-life (78 seconds)of the flow tracer Rb, repeated measurements of regional perfusion were performed in each patient under different conditions (ie, before the stress test, at its peak, and in the recovery phase). In all patients the stresstest induced typical chest pain and ECG signs of ischemia that were accompanied by regional abnormalities of perfusion (Figs 8. 9). An increase of myocardial glucose utiliza-

Angina

In patients with angiographically proven coronary artery diseaseand stable angina on exercise (secondary ischemia), regional myocardial perfusion and glucose utilization were assessedwith PET using the positron emitting cation rubidium82 (Rb)65 and the glucoseanalogue 18F-2-fluoro2-deoxyglucose (FDG),66*67respectively. The regional myocardial utilization of FFA can also be evaluated in these patients during pacinginduced ischemia by meansof “C-palmitate.35 In patients with stable angina studied at rest after overnight fasting and in the absence of medical treatment, the regional myocardial utilization of glucose is very low (because of low insulin and workload) and matches the distribu-

Fig 8. PET images of the chest of a patient with stable angina. In each image the left ventricle free well is in the &to-10 o’clock position, the anterior wall and septum are in the lo-to-3 o’clock position, end the remaining open area is the plane of the mitral valve. Myocardiel uptake of Rubidium82 (Rb) at rest (top left) is homogeneous, while during exercise (top right) cation uptake is severely reduced in the anterior wall. When FDG wes injected at rest (bottom left) after overnight fasting, myocardial tracer uptake was very low, the heart profile being barely detectable. In this patient FDG was also injected in the recovery from the stress test when all the signs of ischemia had disappeared. Under these conditions (bottom right) the region of previous ischemia was clearly identifiable, tracer uptake in the anterior wall being 1.75 times higher then in nonischemic myocardium.

MYOCARDIAL

METABOLISM

IN ISCHEMIA

233

Fig 9. PET images of Rb and FOG uptake in the left ventricle of a patient with stable angina. The perfusion scan recorded at peak exercise (top left) shows e severely reduced Rb uptake in the anterior left ventricle wall. When FDG wes injected during the exercise (top right1 tracer concentration in the ischemic region was 0.76 times lower than in the nonischemic tissue (free well) even though FDG in the ischemic zone was in excess of perfusion. The perfusion scan recorded six minutes after the end of exercise (bottom left) shows s homogeneous flow distribution throughout. The seen recorded following an injection of FOG in the recovery phase, when Rb had normalized (bottom right), shows a higher il.90 times) trecer concentration in the previously ischemic region as compared to the nonischemic tissue (free wall). For figure orientation see legend to Fig 9.

tion was observed during the stress test. This increase, however, was not regionally homogeneous: glucose utilization in the nonischemic areas (ie, the ones showing an increase of perfusion during exercise comparable with that in normals) increased more than in the ischemic regions (ie, the ones developing flow defects during exercise).@ Under similar circumstances (pacing-induced ischemia), a reduction of palmitate oxidation could be demonstrated in the ischemic regions.35 Postexercise

‘Chronic

Metabolic

Ischemia’

When glucose utilization is measured in the recovery period after exercise when all the indices of ischemia including myocardial perfusion have normalized, a greater sugar uptake can be

demonstrated in the postischemic myocardium (Fig 10). Taking enhanced glucose uptake as a sign of metabolic ischemia, this postexercise change could be termed a persistent metabolic abnormality that apparently occurs in the absence of symptoms or signs of frank ischemia. The increased glucose uptake, not sustained by ischemia since coronary flow was comparable to control values, could reflect either an increased glycolytic flux and/or an increased rate of glycogen synthesis due to depletion of the polysaccharide induced by ischemia. The latter hypothesis is supported by experiments performed in the isolated perfused working rat heart where glycogen breakdown and synthesis were measured before and after a period of total global ischemia.69 In addition, preliminary results, obtained with “Cglucose and PET@” in patients with stable angina who showed increased uptake of FDG in the postischemic myocardium, seem further to support the above hypothesis (Fig 10). However, it must be considered that myocardial glycogen would soon be repleted in the postischemic synthetic phase, so that alternate explanations must also be considered. Data on unstable angina are of interest in this regard. Unstable

Angina

Regional myocardial perfusion and glucose utilization with Rb and FDG, respectively, and

Fig 10. “-C-glucose and “-F-deoxyglucose myocardial uptake after exercise. PET images of “C-glucose end FOG in the left ventricle of a patient with stable angina. During exercise the perfusion seen showed a reproducible defect of Rb uptake in the free wall of the left ventricle (not shown). Both glucose and deoxyglucose. injected on two different occasions, 9.6 minutes after the end of exercise, show s comparable regional distribution. Both trecers are more concentreted in the previously ischemic myocardium (free wall). For figure orientation see legend to Fig 9.

CAMICI,

234

PET have also been assessed in patients with severe coronary artery disease and unstable angina characterized by repeated episodes of spontaneous ST-segment depression (primary ischemia) without evidence of acute myocardial infarction.7’-73 The patients were studied at rest, after overnight fasting, off therapy, and in the absence of symptoms or ECG signs of ischemia at the time of the PET study. Myocardial glucose utilization in these patients was different from that observed in normal volunteers and in patients with stable angina at rest. In fact, in patients with unstable angina, myocardial glucose utilization was regionally or globally increased even at rest as compared with that in normal volunteers and in patients with stable angina on effort (Figs 11, 12). This change occurred most often in the absence of clearly detectable abnormalities of myocardial perfusion. No simple explanation for this phenomenon can be found; it may be hypothesized that this pattern of glucose utilization in patients with unstable angina could represent a chronic adaptation of myocardial metabolism to a situation of persistent hypoperfusion, with a chronic stimulation of glycolysis as a result of continued mild tissue hypoxia. However, detailed studies of the simultaneous patterns of extraction of FFA and lactate, the prevailing levels of insulin, as well as an assessment of tissue hypoxia by means of radiolabelled FFA studies are all needed before

Fig 11. PET images of the left ventricle of a patient with unstable angina, characterized by repeated spontaneous episodes of ST-segment depression on the ECG and angiographically proven severe coronary artery disease. The images were obtained at rest when the patient was painless and in the absence of ECG signs of ischemia. The perfusion scan (Rb) shows only a minimal defect at the level of the enterolateral wall of the left ventricle. By contrast, the FDG scan shows a large area of significantly greeter uptake at the level of the inferolateral wall of the left ventricle. For figure orientation sac legend to Fig 8.

0.16

-

0.14

-

0.12

-

g 0.10 2” r; 0.08 E

-

to.06

-

peo.005 rp ‘. 0.005 I

p
Septum

Antenor

FERRANNINI,

AND

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p
-

0.04 0.02

-

000

1 Free wall

Chest wall muscle

Fig 12. The myocardial .metabolic rate of glucose was measured at rest with “F-2-fluoro-2-deoxyglucose and ET in a group of normal volunteers and in patients with stable or unstable angina pectoris. Glucose use at rest in patients with stable angina was not statistically different from that in normals, whereas it was significantly higher in patients with unstable angina. It should be noted that the increased glucose use was confined to the myocardium, since glucose use in skeletal muscle (chest wall muscle) was similar in all three groups. LC, lumped constant. (Reprinted with permission.‘*)

the concept of chronic metabolic ischemia can be fully accepted.” Prediction

of Reversibility

The magnitude of myocardial glucose uptake, as measured by PET scanning, can also be used to predict whether or not regional function of the ischemic myocardium will return to normal after coronary artery bypass surgery.74 In patients with abnormal wall motion in which there was preservation of glucose uptake, 85% of hypocontractile segmentsreversed their ischemic abnormality after coronary artery surgery. On the other hand, those segmentswith decreasedglucose uptake had an irreversible defect of the contractile pattern in 92% of cases.These data show that depressed glucose uptake indicates ischemia sufficiently severe to have irreversible consequenceson contractility. Similar principles appear to hold in the assessment of recovery of segmental function in the early phases (within 72 hours) of the onset of acute myocardial infarction.75 Segments with a depressionof glucoseuptake (measured by F- 18deoxyglucose) were less likely to recover than

MYOCAROIAL

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IN ISCHEMIA

those with preserved uptake of glucose; both patterns of glucose change were associated with reduction of coronary blood flow as assessed by decreased uptake of N- 13 ammonia. An Extension of the Concept Metabolic Ischemia’

of ‘Chronic

In chronic metabolic ischemia, an entity proposed in this article, the uptake of glucose is increased, which is regarded as evidence favoring the view that glycolysis is enhanced and that there is chronic ischemia from the metabolic but not from the coronary flow point of view. However, there need not be an increased glucose uptake in absolute terms to provide evidence for chronic metabolic ischemia; rather, the uptake, as imaged by F-l %deoxyglucose, could be normal yet increased in relation to a depressed coronary flow, as imaged by N-l 3 ammonia.76 The observations already quoted in relation to coronary artery surgery and in patients with recent myocardial infarction support this definition of chronic metabolic ischemia. SUMMARY

The human heart in the fasting state extracts FFA, glucose, lactate, pyruvate, and ketone bodies from the systemic circulation. Of these substrates, FFA utilization accounts for the greater part of oxygen consumption and energy production. The oxidative use of lipid (FFA) and carbohydrate (glucose and lactate) fuels is reciprocally regulated through the operation of Randle’s cycle. Feeding, by increasing both insulin and glucose concentration, shifts myocardial metabolism towards preferential carbohydrate usage, both for oxidative energy generation and for glycogen synthesis. During conditions of reduced oxygen supply, the oxidation of all substrates is decreasedwhile anaerobic metabolism is activated. In patients

with coronary artery diseaseand stable angina pectoris, lactate releasein the CS can be demonstrated during pacing stress. However, this occurs in only 50% of patients, and no relationship can be demonstrated between lactate production and the severity of ischemia. In patients with chronic angina, a significant releaseof alanine in the CS and an increased myocardial uptake of glutamate could be demonstrated at rest and following pacing. These two phenomena result from increased transamination of excess pyruvate to alanine with glutamate serving as NH, donor. In addition, release of citrate (a known inhibitor of glycolysis) in the CS can be demonstrated following pacing in patients with stable angina. The introduction of PET has made it possible to study regional myocardial perfusion and metabolism in humans noninvasively. Two basically different patterns of myocardial glucose utilization have been observed in patients with coronary artery disease studied at rest using ‘*F-flurodeoxyglucose. In patients with stable angina on exercise but studied at rest, regional myocar- dial glucose utilization was homogeneously low and comparable with that of a group of normals. In contrast, in patients with unstable angina, myocardial glucose utilization at rest was increased even in the absenceof symptoms and ECG signs of acute ischemia. In patients with stable angina, a prolonged increase in glucose uptake could be demonstrated in the postischemic myocardium in the absenceof perfusion abnormalities, and a state of chronic metabolic ischemia is proposed. PET imaging has also allowed prospective differentiation between viable and nonviable segmental function in patients with recent myocardial infarction and in those undergoing coronary artery surgery; in both casesviable segmentshave relatively maintained glucose uptakes, whereas nonviable segments have depressedglucoseuptakes.

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