Metabolic Interventions In Myocardial Preservation

Franklin Rosenfeldt, MD,FRACS

Cardiac Surgical Research Laboratory, Baker Medical Research Institute and Alfred Hospital F’rahran, Victoria, Australia

I t is possible to manipulate myocardial metabolism to improve recovery after ischaemic insults. We studied 2 metabolic manipulations: addition of amino acids during cardioplegia, and treatment with erotic acid before cardioplegla. Aspartute improved recovery after cardioplegic arrest, and glutamate exerted a minimal protective effect. We studied the recently infarcted heart to determine whether erotic acid therapy could improve recovery after cardioplegia. The infarct placed the non-infarcted myocardlum under stress and increased its sensitivity to hypothermic cardioplegia. This sensitivity was reduced by treatment with erotic acid. Metabolic supplementation may be useful during cardiac surgery, after myocardial infarction, and in the early period after cardiac surgery. (AustralAs J Cardiac Thorac Surg 1993:2(2):80 - 82)

In recent years, biochemical substances have been used to manipulate myocardial metabolism. At the Baker Medical Research Institute, we have studied 2 metabolic manipulations: addition of amino acids during cardio- plegia; and treatment with erotic acid before cardioplegia.

h'&TABOLICSUPPLEMENTATIONWITHAMINOACIDS

Introduction Before arrest and after reperfusion, glucose is

metabolised anaerobically to pyruvate, which is oxidised via the tricarboxylic acid (TCA) cycle. During cardio- plegic arrest, glucose is metabolised anaerobically to pyruvate and then to lactate. During cardioplegic arrest there is depletion of some of the intermediates in the TCA cycle, such as oxaloacetate and alpha-ketoglutarate, with the result that on reperfusion, the TCA cycle cannot run efficiently. By transamination, aspartate can be con- verted to oxaloacetate, and glutamate to alpha-ketoglutarate. Thus by adding aspartate or glu- tamate to the cardioplegia, it might be possible to prevent or reverse the depletion of TCA cycle intermediates and enhance myocardial recovery. Rosenkranz and Buckberg recommended a combination of glutamate and aspartate for warm induction and reperfusion but have not shown whether amino acids are effective during hypothermia or whether the combination of both amino acids is superior to aspartate alone I.

The aim of the present study 2 was to assess the indi- vidual and additive protective effects of aspartate and glutamate during hypothermic and normothermic cardio- plegic arrest. Methods

Rat hearts underwent one hour of cardioplegic arrest on the isolated working rat heart apparatus. Cardioplegic arrest comprised 28 minutes at 37°C or 5 hours at 2°C. The cardioplegia contained 20 mM of KCl, alone or with 20 mM aspartate, 20 mM glutamate, or both (6 to 10 hearts per group).

Return of heart function after the arrest period was defined as the percentage recovery of pre-arrest work.

Results At 37’C aspartate-treated hearts (n=13) recovered 50

f 2.8% (SEM) of control cardiac work, which was signif- icantly greater than recovery in the glutamate-treated hearts (34.6 f 2.0%) and in the untreated heart (31.7 + 2.8%, ~~0.01, Fig. 1). Under hypothermia, aspartate- treated hearts recovered 59.4 f 1.8% of work (n=lO) which was significantly greater than the recovery in glu- tamate-treated hearts (45.2 + 2.3%) or untreated hearts (40.4 + 4.0%). Comparisons were also made between hearts treated with aspartate, and aspartate plus glu- tamate. There was no significant difference between the recovery in aspartate-treated hearts or aspartate plus glu- tamate-treated hearts at normothermia or hypothermia (Fig. 2).

We concluded that (i) aspartate improved recovery after cardioplegic arrest under normothermic and hypothermic conditions; (ii) glutamate exerted a minimal protective effect on was not additive to the protective effect of aspartate.

Discussion There are 2 likely mechanisms for the protective

effect of aspartate. The first is that during ischaemia there is a loss of intermediates from the TCA cycle and that aspartate may replenish these intermediates via oxaloacetate so that the cycle is “primed” for immediate, efficient, energy production during reperfusion. The second is that aspartate may act via the malate-aspartate shuttle to shift, from the cytoplasm to the mitochondria, the excess of NADH which builds up during ischaemia and which is normally removed by TCA-cycle activity. Excessive cytoplasmic NADH is one of the factors that stops anaerobic glycolysis during ischaemia. Thus aspartate may allow glycolysis to continue longer during ischaemia with accompanying increased production of energy.

80

AustralAs J Cardiac Thorac Surg 1993:2(2):80 - 82 Rosenfeldt Myocardial preservation

of - Function

70

60

50

40

30

20

10

0 I C G A 2X/5 hours 37W26 mins

Fig 1: Recovery of function (% pre-arrest cardiac work) after arrest using simple potassium (C) cardioplegia, glutamate cardioplegia (G), or aspartate cardioplegia (A). *** = p < 0.01 for A vs G or C.

We have applied aspartate cardioplegia in 2 classes of patients at the Alfred Hospital. Firstly, all transplant donor hearts receive aspartate in the induction crystalloid cardioplegic solution, in re-infusions of cold, aspartate- enriched blood cardioplegia, given on return of the donor heart to the hospital, and as an aspartate-enriched warm reperfusate. Success with aspartate cardioplegia in 103 heart transplants and 29 heart-lung transplants has allowed us to extend the ischaemic times of transplant donor hearts up to 7 hours and 30 minutes. This has enabled us to obtain donor hearts from Darwin, Perth and Cairns. We also use aspartate in blood and crystalloid cardioplegic solutions for routine cardiac surgery.

SENSITIV~OFTHERECENTLYINFARCTEDHEARTTO CARDIOPLEGICARREST: BENEFICIALEFFECTOFOROTICACID

Introduction Cardiac surgery performed in the early period after

acute myocardial infarction is associated with a higher risk than surgery during chronic myocardial infarction, even though the preoperative impairment of ventricular function may be the same. There is evidence of a vul- nerable period between approximately 6 hours and 7 days after infarction, during which operative mortality and morbidity from cardiac surgery are increased. The cause is unknown. We postulated that the surviving, non- infarcted myocardium is responsible for the reduced tolerance to surgically-induced ischaemia because of the mechanical and metabolic stress placed upon it by the infarct. The apparently normal, non-infarcted myocardium in the presence of a large, acute infarction is under abnormal metabolic and workload stress. Low levels of high energy phosphates, low noradrenaline content, glycogen depletion and other metabolic changes provide evidence of the stress placed on the non-infarcted myocardium by the loss of contracting fibres and cate- cholamine overstimulation.

Orotic acid has been suggested as being beneficial to the acutely stressed heart. We postulated that adminis- tration of erotic acid in the period between infarction and

% % Recovery Recovery

of of Function Function

70 70

60 60

50 50

40 40

30 30

A+G A A+G 37”C/26 mins 2W5 hours

Fig 2: Recovery of function after arrest using aspartate cardioplegia (A) or glutamate cardioplegia (G).

cardioplegic arrest might improve the tolerance of the heart to cardioplegia. We studied the tolerance of the recently infarcted heart to cardioplegic arrest in rats 3.4 and dogs 5 to determine whether erotic acid therapy could improve the response of the recently infarcted heart to hypothermic cardioplegia. Studies in rats

Myocardial infarction was produced by coronary ligation. The rats were divided into 2 groups: treated with erotic acid (10 mg/kg/day) or untreated. A sham-operated non-infarcted group served as controls. After 2 days, the hearts underwent one hour of cardioplegic arrest at 23°C on the isolated working rat heart apparatus (n=12 per group) .

Before arrest, cardiac work capacity in the untreated infarct group was lower than the normal group (~~0.05). In the erotic acid treated group, function was similar to the normal group. After arrest, there was severe depression of cardiac function in the untreated infarct group: only 57% recovery of the pre-arrest cardiac work, compared with 86% in the normal group (p<O.OOl , Fig 3). In the erotic acid treated group, recovery of function was 90%, which was not significantly different to the normal group. Recovery of heart function in the untreated group was 54%.

Thus, a recent infarct reduces the tolerance of the rat heart to hypothermic cardioplegia. Treatment with erotic acid improves the function of the infarcted heart fol- lowing cardioplegic arrest.

Studies in dogs In 21 greyhounds, myocardial infarction was

produced by ligation of the left anterior descending coronary artery. Ten received erotic acid, 100 mg/kg/day for 4 days, and I I were untreated. A sham-operated group of 8 dogs had a thoracotomy only.

Four days later, all 29 dogs underwent 60 minutes of cardioplegic arrest at 28°C on cardiopulmonary bypass. Before arrest, stroke work index was lower and myocardial oxygen consumption at comparable work levels was higher in erotic acid and untreated infarct groups than in the normal group (Fig. 4). After arrest and

81

AustralAs J Cardiac Thorac Surg 1993:2(2):80- 82 Rosenfeldt Myocardial preservation

100-

go-

60-

70-

60- c

.L” 2 50-

2 6

40-

2 at 30- 0’ !i 20- a 8 10-

O-

-L T

l *,J . . . .

I.... n.... B.... e....

. ..*

. . . .

. . . . l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..q

SHAM MI MI+OA

Fig 3: Recovery of pre-arrest function after 1 hr of cardioplegic arrest in normal hearts (sham), infarcted hearts (MI) and infarcted hearts treated with erotic acid (Ml + OA).

reperfusion, there was severe depression of ventricular function in the untreated infarct group, with only 18% recovery of pre-arrest stroke work. In the erotic-acid treated infarct group, recovery of function was 43%; recovery in the normal group was 56%. Both values were greater than the 18% recovery of the untreated infarcted group (p<O.OOl). After reperfusion, the untreated infarct group had lower oxygen consumption, lower myocardial levels of ATP and glycogen, and higher lactate and water content than before arrest. In the erotic acid and normal groups, these variables returned to pre-arrest levels.

Thus, a recent infarct places the non-infarcted myocardium under stress and increases its sensitivity to hypothermic cardioplegia. This sensitivity is markedly reduced by treatment with erotic acid.

After these experiments, we began to use erotic acid in patients undergoing coronary bypass surgery within one week of myocardial infarction. Patients of one of the cardiac surgeons at the Alfred Hospital routinely received preoperative magnesium orotate therapy (800 mg orally, 3 times per day) before any cardiac operation. Patients showed no side effects due to orotate. There were no obvious differences in mortality or morbidity between patients receiving or not receiving orotate. Without a prospective randomised trial, no conclusions about the effects of erotic acid in humans can be drawn. We are planning a prospective randomised trial of erotic acid therapy in patients in 2 situations: patients undergoing cardiac surgery within one week of myocardial infarction, and those with end-stage cardiac disease awaiting cardiac transplantation. Other metabolic treatments

In laboratory animals, we investigated the effect of ribose on cardiac protection. We found that ribose is

2.!

2s

0.5

5-

,-

,-

,-

-

I- O

I , Normal

5 10 rs i0

Left atrial pressure (mmlig)

Fig 4: Left ventricular function curves before and after arrest in normal, untreated infarcted (MI), and erotic acid-treated hearts (MI + OA). The dotted line indicates the work level critical for survival in this preparation; below this level it was not possible to discontinue bypass.

directly metabolised by the myocardium and produces a small protective effect in recently infarcted hearts undergoing cardioplegic arrest 6. Conclusion

Metabolic supplementation has a wide range of applications during cardiac surgery, in energy-depleted hearts after myocardial infarction, and in the early period after cardiac surgery.

References I. Rosenkranz ER, Okamoto F, Buckberg GD, Robertson JM, Vinten-

Johansen J, Bugyi HI. Safety of prolonged aortic clamping with blood cardioplegia. III. Aspartate enrichment of glutamate-blood cardioplegia in energy-depleted hearts after ischemic and reper- fusion injury. J Thorac Cardiovasc Surg 1986;91:428-35.

2. Rosenfeldt FL, Pisarenko 0, Langley L, Conyers RAJ. The indi- vidual and combined protective effects of aspartate and glutamate during hypothermic cardiac storage. J Mel Cell Cardiol 1993, in press (Abstr).

3. Munsch CM, Rosenfeldt FL, O’Halloran K, Langley LH, Conyers RAJ, Williams JF. The effect of erotic acid on the response of the recently infarcted rat heart to hypothermic cardioplegia. Eur J Cardiothorac Surg 1991;5:82-93.

4. Munsch C, Williams JF, Rosenfeldt FL. The impaired tolerance of the recently infarcted rat heart to cardioplegic arrest: the protective effect of erotic acid. J Mel Cell Cardiol 1989;2 1:75 1-4.

5. Newman MAJ, Chen X-Z, Rabinov M, Williams JF, Rosenfeldt FL. Sensitivity of the recently infarcted heart to cardioplegic arrest. J Thorac Cardiovasc Surg 1989;97:593-604.

6. Cowling J, Richards SM. Conyers RAJ, Craik DJ, Rosenfeldt FL. Protection of post-ischaemic cardiac function by ribose does not involve increased purine salvage. J Molt Cell Cardiol, in press (Abstr)

82