Cardiac 5'-nucleotidase activity increases with age and inversely relates to recovery from ischemia.

The metabolic basis for the enhanced tolerance of immature hearts to ischemia remains to be elucidated. Loss of high-energy phosphate nucleotides occurs during ischemia/reperfusion in mature (adult) hearts through the breakdown of adenosine triphosphate, diphosphate, and monophosphate (nondiffusible) to adenosine (freely diffusible). However, previous work has shown that after ischemia nondiffusible nucleotides are better retained by immature (neonatal) hearts than by mature hearts. The enzyme responsible for the conversion of adenosine monophosphate to adenosine is 5'-nucleotidase. We therefore hypothesized lower activity of this enzyme in neonatal than in adult myocardium. The purposes of this study were (1) to document 5'-nucleotidase activities in neonatal and adult rabbit myocardium and (2) to correlate differences of 5'-nucleotidase activity with functional recovery from ischemia. Neonatal (5- to 10-day-old) and adult (4- to 6-month-old) rabbit hearts were isolated and perfused (retrograde Langendorff). A left ventricular balloon measured functional parameters. Hearts were subjected to 20 minutes of global 37 degrees C ischemia and 10 minutes of reperfusion followed by freeze clamping. Tissue homogenates were assayed for 5'-nucleotidase by the linked formation of nicotinamide-adenine dinucleotide at 340 nm (Arkesteijn method). Postischemic recovery of developed pressure was 86% +/- 3% in neonates (n = 5) versus 38% +/- 3% in adults (n = 8) (mean +/- standard deviation) (p less than 0.01). 5'-Nucleotidase activity was 4400 +/- 1208 nmol/min/gm in neonates (n = 5) versus 13,938 +/- 830 nmol/min/gm in adults (n = 8) (mean +/- standard deviation) (p less than 0.01). We conclude that (1) 5'-nucleotidase activity is 68% lower in neonatal than in adult myocardium and (2) functional recovery after ischemia inversely relates to 5'-nucleotidase activity.     

Effects of coronary perfusion during myocardial hypoxia. Comparison of metabolic and hemodynamic events with global ischemia and hypoxemia.

The effects of metabolic accumulation on myocardial metabolism during global heart oxygen deprivation were evaluated in a working in situ swine heart preparation with controlled total coronary blood flow. Myocardial oxygen consumption was depressed to a similar extent by either reducing total coronary flow 60 per cent (ischemia, low coronary perfusion) in 10 swine or by decreasing coronary perfusate PO2 to 30 mm. Hg at normal flows (hypoxemia, high coronary perfusion) in 13 swine. Compared with findings in 13 control hearts, ischemia significantly (p less than 0.05) decreased myocardial oxygen consumption (640 to 390 mumole per hour per gram), glucose uptake (185 to 16 mumole per hour per gram), and free fatty acid consumption (32 to 17 mumole per hour per gram). ttissue levels of glycogen, creatine phosphate, and adenosine triphosphate (tatp) were significantly reduced (p less than 0.005), and tissue lactate, adenosine diphosphate (ADP), and adenosine monophosphate (AMP) were increased (p less than 0.001). During hypoxemia, glucose uptake was increased (240 mumole per hour per gram) and free fatty acid consumption was somewhat less depressed (19 mumole per hour per gram). Creatine phosphate and ATP were higher than with ischemia (p less than 0.01), and lactate, ADP, and AMP accumulations were less (p less than 0.01). Thus, in the period immediately following myocardial oxygen deprivation, inadequate coronary perfusion caused greater metabolic buildup which inhibited myocardial substrate utilization and energy production. High coronary perfusion, even though the perfusate was unoxygenated, was associated with greater preservation of substrate utilization, higher levels of high-energy phosphates, less accumulation of metabolic products, and a longer survival. These data suggest a critical role of coronary perfusion in protecting myocardial metabolism in the immediate period following global heart hypoxia.     

Enhanced myocardial protection during global ischemia with 5'-nucleotidase inhibitors.

Depletion of adenosine triphosphate precursors, such as myocardial adenosine, during global ischemia results in poor postischemic adenosine triphosphate repletion and functional recovery. Neonatal hearts may be more resistant to this deleterious effect of ischemia, because they are characterized by low 5'-nucleotidase activity, which may result in higher sustained endogenous myocardial adenosine triphosphate precursor levels during ischemia. Adult hearts, however, have high levels of 5'-nucleotidase activity leading to depleted precursors during ischemia and poor postischemic functional recovery. Augmenting myocardial adenosine exogenously during ischemia in adult hearts has a beneficial effect on recovery. The present study tested if preservation of nucleotide precursors, better adenosine triphosphate repletion, and enhanced postischemic myocardial recovery in adult hearts could be achieved with a "neonatal" strategy. Therefore 5'-nucleotidase inhibitors were administered to isolated, perfused adult rabbit hearts subjected to 120 minutes of ischemia (at 34 degrees C) to determine if this improved functional recovery. Hearts received St. Thomas' Hospital cardioplegic solution (control hearts) or cardioplegic solution containing 5'-nucleotidase inhibitors: pentoxifylline, thioinosine, [s-(p-nitrophenyl)-4-thioinosine], or thioinosine's dimethyl sulfoxide vehicle alone. After ischemia and reperfusion, recovery of systolic function, diastolic function, and myocardial oxygen consumption was significantly better with 5'-nucleotidase inhibition. No changes in coronary flow were noted. We speculate and are pursuing the theory that the mechanism of 5'-nucleotidase inhibition's favorable action is due to preventing the catabolism, transport, and loss of nucleotide precursors during ischemia, maintaining adenosine triphosphate precursor availability.     

Optimal level of hypothermia for prolonged myocardial protection assessed by 31P nuclear magnetic resonance.

The optimal level of hypothermia during myocardial preservation for cardiac transplantation is not known. Phosphorus 31 nuclear magnetic resonance spectroscopy was used to assess the effect of different preservation temperatures (15 degrees C in group 1, 4 degrees C in group 2) on the myocardial high-energy phosphate profiles during prolonged global ischemia and subsequent reperfusion of isolated rat hearts. Adenosine triphosphate depletion during ischemia was more gradual in group 2, leading to significant differences in myocardial adenosine triphosphate concentrations between the two groups after 3 hours of ischemia. The fall in intracellular pH during ischemia was significantly less pronounced in hearts preserved at 4 degrees C as compared with those at 15 degrees C. The postischemic recovery of both the left ventricular peak systolic pressure and the maximum rate of increase of left ventricular pressure was enhanced in group 2, although the ischemic period was 3 hours longer than in group 1. Hypothermia at 4 degrees C as compared with 15 degrees C appears to prolong myocardial protection with respect to adenosine triphosphate preservation, prevention of the fall in intracellular pH, and the enhancement of postischemic hemodynamic recovery.     

Myocardial energy depletion during profound hypothermic cardioplegia for cardiac operations

Myocardial biopsy specimens were taken from 10 patients undergoing aortic valve replacement using extracorporeal circulation and continuous perfusion blood cardioplegia at extremely low myocardial temperature (10 degrees C). They were analyzed for adenosine triphosphate, creatine phosphate, creatine, and lactate before, after 10 minutes, and after 60 minutes of cardioplegia. Patient inclusion criteria were heart volume less than 700 ml/m2 body surface area and no significant coronary atherosclerosis as judged from preoperative angiograms. The profound hypothermic cardioplegia resulted in a smaller intramyocardial lactate accumulation but a greater decrease in adenosine triphosphate and creatine phosphate than a moderate reduction of myocardial temperature (15 degrees C) as previously reported in a similar patient group. This suggests that at the lower temperature energy-generating processes are thwarted more than energy consumption. In addition, the profound hypothermic cardioplegia led to a reduction of the myocardial pool of total creatine, which may delay restitution of myocardial high-energy phosphate and function after cardioplegia.     

Influence of inhibitors of ATP catabolism on myocardial recovery after ischemia

The loss of the catabolic products of adenosine triphosphate in the form of purine nucleosides and oxypurines during ischemia and subsequent reperfusion may limit adenine nucleotide regeneration. This study compared the effects of infusion of inhibitors of the major reactions involved in the degradation of adenosine triphosphate to inosine on the postischemic recovery of high energy phosphate and myocardial function. Isolated rat hearts were made totally ischemic after a 5-min infusion of p1,p5-diadenosine pentaphosphate, alpha, beta-methylene adenosine diphosphate, nitrobenzyl-6-thioinosine, or erythro-9-(2-hydroxy-3-nonyl) adenine, which are inhibitors of adenylate kinase, 5'-nucleotidase, adenosine translocase, and adenosine deaminase, respectively. Following 30 min of ischemia, only hearts infused with alpha, beta-methylene adenosine diphosphate recovered significantly better ventricular function than did the control (P less than 0.05), but all hearts had increased adenosine triphosphate and creatine phosphate regeneration (P less than 0.05). The formation and washout of greater than 30% of the total adenine pool metabolites were not prevented by any drug. Nevertheless all manipulations of adenine metabolism resulted in recruitment of high energy phosphate during preischemic infusion which may have potential benefits in elective ischemic arrest.     

Embryonic versus adult myocardium: adenine nucleotide degradation during ischemia

Neonatal myocardium demonstrates better recovery from ischemia than does adult tissue. We tested the hypothesis that developmental differences in adenine nucleotide degradation might facilitate recovery by quantitating depletion of high-energy phosphates in nine-day-old embryonic (n = 9) and 15-month-old adult (n = 14) chicken hearts at 15-, 30-, 45-, and 60-minute intervals of normothermic ischemia in vitro. Nucleotides adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate and nucleosides adenosine, inosine, hypoxanthine, and xanthine were determined by high-performance liquid chromatography. Several observations in metabolite degradative response to ischemia were noted. The embryonic myocardium maintained higher adenosine triphosphate and adenosine monophosphate levels over the course of the investigation than did mature myocardium. Moreover, the adult group showed an increase in diffusible nucleoside pool metabolites. Relative immaturity of enzymes responsible for nucleotide degradation may facilitate postischemic recovery by preserving nondiffusible high-energy phosphate precursors to participate in salvage resynthesis of adenosine triphosphate.     

Enhanced myocardial protection with high-energy phosphates in St. Thomas' Hospital cardioplegic solution. Synergism of adenosine triphosphate and creatine phosphate

The potential for improving myocardial protection with the high-energy phosphates adenosine triphosphate and creatine phosphate was evaluated by adding them to the St. Thomas' Hospital cardioplegic solution in the isolated, working rat heart model of cardiopulmonary bypass and ischemic arrest. Dose-response studies with an adenosine triphosphate range of 0.05 to 10.0 mmol/L showed 0.1 mmol/L to be the optimal concentration for recovery of aortic flow and cardiac output after 40 minutes of normothermic (37 degrees C) ischemic arrest (from 24.1% +/- 4.4% and 35.9% +/- 4.1% in the unmodified cardioplegia group to 62.6% +/- 4.7% and 71.0% +/- 3.0%, respectively, p less than 0.001). Adenosine triphosphate at its optimal concentration (0.1 mmol/L) also reduced creatine kinase leakage by 39% (p less than 0.001). Postischemic arrhythmias were also significantly reduced, which obviated the need for electrical defibrillation and reduced the time to return of regular rhythm from 7.9 +/- 2.0 minutes in the control group to 3.5 +/- 0.4 minutes in the adenosine triphosphate group. Under more clinically relevant conditions of hypothermic ischemia (20 degrees C, 270 minutes) with multidose (every 30 minutes) cardioplegia, adenosine triphosphate addition improved postischemic recovery of aortic flow and cardiac output from control values of 26.8% +/- 8.4% and 35.4% +/- 6.3% to 58.0% +/- 4.7% and 64.4% +/- 3.7% (p less than 0.01), respectively, and creatine kinase leakage was significantly reduced. Parallel hypothermic ischemia studies (270 minutes, 20 degrees C) using the previously demonstrated optimal creatinine phosphate concentration (10.0 mmol/L) gave nearly identical improvements in recovery and enzyme leakage. The combination of the optimal concentrations of adenosine triphosphate and creatine phosphate resulted in even greater myocardial protection; aortic flow and cardiac output improved from their control values of 26.8% +/- 8.4% and 35.4% +/- 6.3% to 79.7% +/- 1.1 and 80.7% +/- 1.0% (p less than 0.001), respectively. In conclusion, both extracellular adenosine triphosphate and creatine phosphate alone markedly improve the cardioprotective properties of the St. Thomas' Hospital cardioplegic solution during prolonged hypothermic ischemic arrest, but together they act additively to provide even greater protection.     

Effects of potassium cardioplegia on high-energy phosphate kinetics during circulatory arrest with deep hypothermia in the newborn piglet heart

The protective effects of hypothermia and potassium-solution cardioplegia on high-energy phosphate levels and intracellular pH were evaluated in the newborn piglet heart by means of in vivo phosphorus nuclear magnetic resonance spectroscopy. All animals underwent cardiopulmonary bypass, cooling to 20 degrees C, 120 minutes of circulatory arrest, rewarming with cardiopulmonary bypass, and 1 hour off extracorporeal support with continuous hemodynamic and nuclear magnetic resonance spectroscopic evaluation. Group I (n = 5) was cooled to 20 degrees C; group II (n = 4) was given a single dose of 20 degrees C cardioplegic solution; group III (n = 7) was given a single dose of 4 degrees C cardioplegic solution; and group IV (n = 4) received 4 degrees C cardioplegic solution every 30 minutes. At end ischemia, adenosine triphosphate, expressed as a percent of control value, was lowest in group I 54% +/- 6.5% but only slightly greater in group II 66% +/- 7.0%. Use of 4 degrees C cardioplegic solution in groups III and IV resulted in a significant decrease in myocardial temperature, 9.9 degrees C versus 17 degrees to 20 degrees C, and significantly higher levels of adenosine triphosphate at end ischemia; with group III levels at 72% +/- 6.0% and group IV levels at 73% +/- 6.0%. Recovery of adenosine triphosphate with reperfusion was not related to the level of adenosine triphosphate at end ischemia and was best in groups I and II, with a recovery level of 95% +/- 4.0%. In group IV, no recovery of adenosine triphosphate occurred with reperfusion, resulting in a significantly lower level of adenosine triphosphate, 74% +/- 6.0%, than in groups I and II. Recovery of ventricular function was good for all groups but was best in hearts receiving a single dose of 4 degrees C cardioplegic solution. In this model, multiple doses of cardioplegic solution were not associated with either improved adenosine triphosphate retention during arrest or improved ventricular function after reperfusion, and in fact resulted in a significantly lower level of adenosine triphosphate with reperfusion. The complete recovery of adenosine triphosphate in groups I and II, despite a nearly 50% adenosine triphosphate loss during ischemia, may result from a decrease in the catabolism of the metabolites of adenosine triphosphate consumption in the newborn heart.     

Electrode-derived myocardial pH measurements reflect intracellular myocardial metabolism assessed by phosphorus 31-nuclear magnetic resonance spectroscopy during normothermic ischemia.

To determine the ability of extracellular myocardial tissue pH measured with an intramural electrode to reflect myocardial intracellular metabolic status during normothermic ischemia, we studied 14 open-chest dogs with in vivo phosphorus 31-nuclear magnetic resonance spectroscopy during left anterior descending coronary artery occlusion (experimental group, group I, n = 7) or after a sham operation (control group, nonischemic, group II, n = 7). Phosphorus nuclear magnetic resonance spectra were acquired every 5 minutes at 4.7 tesla (256 averages, TR = 1000 msec, pulse width = 30 microseconds) with a 2 cm two-turn radiofrequency surface coil. Intracellular myocardial adenosine triphosphate peak area was normalized to an external phosphate standard. The change in adenosine triphosphate peak area was expressed as percent of baseline value. During 3 hours of normothermic ischemia the observed extracellular myocardial pH correlated with nuclear magnetic resonance-calculated myocardial pH in the ischemic dogs with an average r value of 0.94 (p less than 0.0001). During this same interval, the fall in extracellular myocardial pH correlated with the loss of adenosine triphosphate peak in each ischemic dog, with an average r value of 0.91 (p less than 0.0001). Thus extracellular myocardial pH, measured with an intramural electrode, correlated with nuclear magnetic resonance-derived myocardial pH and loss of myocyte adenosine triphosphate peak content and reflected the metabolic status of the myocyte during ischemia. These data validate the use of extracellular myocardial pH to assess the adequacy of myocardial preservation during aortic crossclamping for cardiac operations.     

Accelerated transmural gradients of energy compound metabolism resulting from left ventricular hypertrophy

Eighteen dogs underwent transmural left ventricular biopsies for adenosine triphosphate and suturing of the noncoronary cusp, creating valvular aortic stenosis. Three months after aortic stenosis and the subsequent development of left ventricular hypertrophy, animals underwent repeat transmural left ventricular biopsies followed by total myocardial ischemia at 37 degrees C. Left ventricular tissue samples for adenosine triphosphate and lactate levels were determined at 15-minute intervals and compared with 15 control animals. No significant difference between subendocardial and subepicardial adenosine triphosphate levels was found between left ventricular samples taken before left ventricular hypertrophy and 3 months after left ventricular hypertrophy. Significant differences in adenosine triphosphate utilization occurred between subendocardial and subepicardial layers in control and left ventricular hypertrophy myocardium, however. The gradient between the subendocardium and the subepicardium was significantly increased by left ventricular hypertrophy (p less than 0.05). Significant differences also occurred within the same layer when left ventricular hypertrophy and control groups were compared. During total ischemia, lactate concentration was significantly greater within the subendocardium than within the subepicardium in left ventricular hypertrophy. The onset of ischemic contracture was 48.2 +/- 2.1 minutes in left ventricular hypertrophy versus 62.3 +/- 1.8 minutes in control hearts (p less than 0.01). Subendocardial intramyocardial pressure increased significantly earlier than subepicardial in both left ventricular hypertrophy and control hearts. Adenosine triphosphate was used, and lactate accumulated more rapidly in animals with a more pronounced hemodynamic gradient. These data show that after left ventricular hypertrophy, adenosine triphosphate stores in the subendocardium and the subepicardium are unchanged from control values, yet the rates of adenosine triphosphate utilization and lactate accumulation during total ischemia are significantly increased. Furthermore, the subendocardial to subepicardial gradient of adenosine triphosphate utilization during ischemia found in normal hearts is markedly increased by left ventricular hypertrophy.     

The acute effects of AICAR on purine nucleotide metabolism and postischemic cardiac function

The purine precursor AICAR (5-amino-4-imidazolecarboxamide) has been advocated as a substrate for myocardial adenine nucleotide repletion during postischemic reperfusion. The purpose of this study was to investigate the acute effects of this agent on adenine nucleotides, inosine monophosphate, and postischemic ventricular function in an isolated rat heart preparation. The hearts were perfused at constant flow, either continuously for 90 minutes or for a 30 minute period followed by 10 minutes of global normothermic (37 degrees C) ischemia. The ischemic hearts were then reperfused for 15, 30, and 60 minutes. Both groups were treated with AICAR in a concentration of 100 mumol/L throughout the perfusion protocols. In the nonischemic time control group there was no effect on the levels of adenosine nucleotides or developed pressure over 90 minutes of perfusion. In contrast, AICAR treatment increased tissue inosine monophosphate content four-fold and sevenfold at 60 and 90 minutes, respectively (p less than 0.05), but had no effect on tissue adenosine monophosphate levels. During ischemia, there was a 50% decrease in adenosine triphosphate content in the AICAR-treated hearts and a thirteen-fold increase in adenosine monophosphate levels (p less than 0.05). After 60 minutes of reperfusion, adenosine triphosphate and monophosphate levels in the AICAR-treated hearts recovered to only 52% and 59% of preischemic values, respectively. These findings were similar to those observed in the untreated ischemic hearts. In contrast, tissue inosine monophosphate content in the AICAR-treated hearts during reperfusion remained significantly elevated and was fivefold greater than the reperfusion values in the untreated group. Concurrently, AICAR failed to enhance the recovery of postischemic left ventricular developed pressure. These results suggest that inhibition of the conversion of inosine monophosphate to adenosine monophosphate limits the usefulness of the agent in evaluating the temporal relationships between postischemic adenosine triphosphate repletion and recovery of myocardial function in the acute setting.     

Mild hypothermia ameliorates lung ischemia reperfusion injury in an ex vivo rat lung model

BACKGROUND: Ischemia reperfusion (I-R) injury of the lung frequently occurs after cardiopulmonary bypass, pulmonary thromboendarterectomy, lung transplantation, and major pulmonary resection with vascular reconstruction. Mild hypothermia ameliorates ischemia reperfusion injury of the brain and the liver. However, the effect of mild hypothermia on I-R injury of the lung has not been investigated. METHODS: The lungs of Lewis rats underwent 80 min of ischemia followed by 60 min of reperfusion in an ex vivo perfusion model. The ambient temperature was maintained at either normothermia (38 degrees C, n=6) or mild hypothermia (35 degrees C, n=6) during the ischemia and reperfusion. RESULTS: Pulmonary shunt fraction, peak inspiratory pressure, mean pulmonary arterial pressure during reperfusion, and the wet/dry weight ratio of the lung tissue at the end of reperfusion in the mild hypothermia group were significantly (p<0.05) lower than those in the normothermia group. Total adenine nucleotide, adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate after reperfusion in the mild hypothermia group were significantly (p<0.05) higher than those in the normothermia group. CONCLUSION: Mild hypothermia attenuates I-R injury of the lung with maintained levels of intrapulmonary high-energy phosphate compounds after reperfusion, suggesting its beneficial effect on warm lung I-R in clinical settings.     

The time course of myocardial high-energy phosphate degradation during potassium cardioplegic arrest

Myocardial high-energy phosphate and glucose-6-phosphate levels were determined in the in vivo pig heart model during ischemic arrest and reperfusion to determine the effectiveness of potassium cardioplegia in myocardial protection. Thirty-five pigs were divided into six experimental groups consisting of 2-hour normothermic arrest, 2-hour hypothemic arrest, 2-hour normothermic cardioplegic arrest, and 1-, 2-, and 3-hour hypothermic cardioplegic arrest. Myocardial biopsies from the left ventricle were obtained prior to arrest, every 30 minutes during the arrest interval, and at 30 and 60 minutes of reperfusion. The measurement of adenosine triphosphate and creatine phosphate showed that (1) cardioplegic arrest requires hypothermia to preserve high-energy phosphate levels in myocardial tissue; (2) hypothermia, while not completely protective alone, is more effective than potassium cardioplegia alone in providing myocardial preservation during 2-hour ischemic arrest; (3) the combination of potassium cardioplegia and hypothermia is additive in providing an effective means of maintaining myocardial high-energy phosphate stores during 1, 2, and 3 hours of ischemic arrest; (4) myocardial reperfusion does not allow a return to preischemic adenosine triphosphate (ATP) levels after 2 hours of arrest, except following hypothermic cardioplegia; and (5) extension of the duration of ischemic arrest to 3 hours using hypothermic cardioplegia prevents recovery of high-energy phosphate stores to preischemic levels during reperfusion. Optimal preservation can be achieved during 2 hours of ischemic arrest by using hypothermic potassium cardioplegia. The effects of myocardial reperfusion, however, prevent full ATP and creatine phosphate (CP) recovery following 3 hours of arrest. No other technique studied was as effective in providing myocardial preservation.     

Metabolic intervention to affect myocardial recovery following ischemia.

Myocardial recovery during reperfusion following ischemia is critical to patient survival in a broad spectrum of clinical settings. Myocardial functional recovery following ischemia correlates well with recovery of myocardial adenosine triphosphate (ATP). Adenosine triphosphate recovery is uniformly incomplete during reperfusion following moderate ischemic injury and is therefore subject to manipulation by metabolic intervention. By definition ATP recovery is limited either by (1) energy availability and application in the phosphorylation of adenosine monophosphate (AMP) to ATP or (2) availability of AMP for this conversion. Experimental data suggest that substrate energy and the mechanisms required for its application in the creation of high energy phosphate bonds (AMP conversion to ATP) are more than adequate during reperfusion following moderate ischemic injury. Adenosine monophosphate availability, however, is inadequate following ischemia due to loss of diffusable adenine nucleotide purine metabolites. These purine precursors are necessary to fuel adenine nucleotide salvage pathways. Metabolic interventions that enhance AMP recovery rather than those that improve substrate energy availability during reperfusion are therefore recommended. The mechanisms of various metabolic interventions are discussed in this framework along with the rationale for or against their clinical application.     

Effect of aprotinin to improve myocardial viability in myocardial preservation followed by reperfusion.

Isolated canine hearts were preserved at 4 degrees C with multi-dose cardioplegic solution every hour for 6 hours. Reperfusion was observed for 2 hours under cross-circulation without cardiotonic drugs. The aprotinin group (n = 8), which received cardioplegic solution with added aprotinin (150 KIU/mL), was compared with the control group (n = 6). The increase in tissue adenosine triphosphate and total adenine nucleotide content during reperfusion was significant in the aprotinin group; there was no change in the control group, and the levels at the end of reperfusion tended to be higher in the aprotinin group than in the control group. Tissue adenosine diphosphate levels remained unchanged in both groups. Tissue adenosine monophosphate levels declined during reperfusion in both groups and were slightly lower in the control group. Tissue levels of cyclic adenosine monophosphate remained unchanged in the aprotinin group whereas they increased during ischemia and declined significantly during reperfusion in the control group. Tissue levels of cyclic guanosine monophosphate declined during reperfusion in both groups without difference. Creatine phosphate levels recovered in both groups without difference. Serum cyclic guanosine monophosphate concentration tended to be lower in the aprotinin group than in the control group. Serum creatine kinase-MB level increased slightly during reperfusion in both groups without difference. N-acetyl-beta-D-glucosaminidase levels were significantly suppressed during reperfusion in the aprotinin group as compared with the control group. These results suggest that aprotinin is effective in preserving adenine nucleotide and adenosine triphosphate levels and in stabilizing tissue cyclic adenosine monophosphate levels in prolonged hypothermic cardioplegic preservation followed by reperfusion.     

Comparison of the metabolic response of the hypertrophic and the normal heart to hypothermic cardioplegia. The effect of temperature

The aim of this study was to test for metabolic differences in the response of hypertrophic and normal hearts to hypothermic cardioplegia. Hypertrophic dog hearts and normal control hearts were subjected to 6 hours of hypothermic cardioplegia with the St. Thomas' Hospital solution. Levels before arrest of subepicardial and subendocardial adenosine triphosphate, creatine phosphate, and lactate in eight hypertrophic hearts were the same as those levels in 12 normal hearts. In hypertrophic hearts, but not in normal hearts, the induction of arrest was slow and was associated with an 11% increase in adenosine triphosphate levels, a 59% decrease in creatine phosphate levels, and a 12-fold increase in lactate levels. Seven hypertrophic hearts and eight normal hearts were studied during 6 hours of arrest and showed no further differences in metabolic response. Reducing the myocardial temperature from 20 degrees C to 12 degrees C slowed the rate of depletion of adenosine triphosphate and the rate of accumulation of lactate in both groups. We conclude that in the nonfailing, severely hypertrophic heart, levels before arrest of high-energy phosphates and lactate are normal, but that marked biochemical changes may occur if the induction of arrest is prolonged because of underdosing with cardioplegic solution. Cooling from 20 degrees C to 12 degrees C improves myocardial preservation in both hypertrophic and normal hearts.     

Protection of the Myocardium with High-Energy Solutions

An approach to intraoperative protection of the myocardium is described that attempts to increase glucose utilization by infusion of high-energy solutions during aortic cross-clamping. Infusion of hypertonic glucose or glucose plus insulin prior to aortic cross-clamping has enhanced contractility and increased high-energy phosphate moieties in animals with induced ischemia. Recent pilot experiments in our laboratory suggest that infusions of creatine may result in increased production of creatine phosphate, which in turn induces phosphorylation of adenosine diphosphate to adenosine triphosphate, possibly enhancing myocardial contractility. The intraoperative clinical benefits of these infusions remain to be proved, however.     

The effect of temperature and hematocrit level of oxygenated cardioplegic solutions on myocardial preservation

The ideal temperature and hematocrit level of blood cardioplegia has not been clearly established. This study was undertaken (a) to determine the optimal temperature of blood cardioplegia and (b) to study the effect of hematocrit levels in blood cardioplegia. A comparison of myocardial preservation was done among seven groups of animals on the basis of variations in hematocrit levels and temperature of oxygenated cardioplegic solution. The experimental protocol consisted of a 2-hour hypothermic cardioplegic arrest followed by 1 hour of normothermic reperfusion. Group 1 received oxygenated crystalloid cardioplegic solution at 10 degrees C. Groups 2 through 7 received oxygenated blood cardioplegic solution with the following hematocrit values and temperatures: (2) 10%, 10 degrees C; (3) 10%, 20 degrees C; (4) 10%, 30 degrees C; (5) 20%, 10 degrees C; (6) 20%, 20 degrees C; and (7) 20%, 30 degrees C. Parameters studied include coronary blood flow, myocardial oxygen extraction, myocardial oxygen consumption, and myocardial high-energy phosphate levels of adenosine triphosphate and creatine phosphate during control (prearrest), arrest, and reperfusion. Myocardial oxygen consumption at 30 degrees C during arrest was significantly higher than at 10 degrees C and 20 degrees C, which indicates continued aerobic metabolic activity at higher temperature. Myocardial oxygen consumption and the levels of adenosine triphosphate and creatine phosphate during reperfusion were similar in all seven groups. Myocardial oxygen extraction (a measure of metabolic function after ischemia) during initial reperfusion was significantly lower in the 30 degrees C blood group than in the 10 degrees C blood group at either hematocrit level and in the oxygenated crystalloid group, which suggests inferior preservation. The hematocrit level of blood cardioplegia did not affect adenosine triphosphate or myocardial oxygen consumption or extraction. It appears from this study that blood cardioplegia at 10 degrees C and oxygenated crystalloid cardioplegia at 10 degrees C are equally effective. Elevating blood cardioplegia temperature to 30 degrees C, however, reduces the ability of the solution to preserve metabolic function regardless of hematocrit level. Therefore, the level of hypothermia is important in blood cardioplegia, whereas hematocrit level has no detectable impact, and cold oxygenated crystalloid cardioplegia is as effective as hypothermic blood cardioplegia.     

Effects of temperature on myocardial calcium homeostasis and mitochondrial function during ischemia and reperfusion

An isolated rabbit heart preparation was used to characterize the effects of hypothermia on the deterioration in mitochondrial respiratory function and on the calcium overload that occurs during ischemia and reperfusion. Hearts were perfused aerobically with an asanguineous solution for 120 minutes or made totally ischemic for 90 minutes at 37 degrees, 34 degrees, 28 degrees, 22 degrees C, respectively, and reperfused for 30 minutes at 37 degrees C. Mitochondrial function was assessed by measuring calcium content, yield, oxygen consumption, and adenosine triphosphate-producing capacities. In addition, the mechanical function of the hearts was measured together with tissue adenosine triphosphate, creatine phosphate, and calcium content. In a separate series of experiments, the effect of temperature on the initial rate of respiration-supported calcium accumulation of mitochondria from freshly excised, nonperfused rabbit hearts was determined. The hearts made ischemic at 37 degrees C were severely depleted of tissue adenosine triphosphate and creatine phosphate. Their mitochondria accumulated calcium and the oxidative phosphorylating activity was impaired. During reperfusion, tissue and mitochondrial calcium levels were substantially increased, state 3 of mitochondrial respiration was further impaired, and the adenosine triphosphate-generating capacities were severely reduced. Diastolic pressure increased and there was no recovery of developed pressure. Isolated mitochondrial function of hearts made ischemic at 28 degrees and 22 degrees C was protected. There was a less marked increase in tissue and mitochondrial calcium, and the initial rate and total production of adenosine triphosphate were maintained. In these hearts there was an almost complete recovery of mechanical performance at reperfusion, whereas the ischemia-induced depletion of tissue adenosine triphosphate and creatine phosphate was not significantly reduced by hypothermia. The hearts made ischemic at 34 degrees C were only partially protected. These data suggest that a decrease in temperature from 37 degrees to 22 degrees C during ischemia did not significantly prevent depletion of adenosine triphosphate at the end of ischemia but reduced tissue and mitochondrial calcium overload, maintaining mitochondrial function. Thus in our experiments the protective effect of hypothermia might be related to a direct reduction of tissue and mitochondrial calcium accumulation rather than to a slowing in rates of energy utilization. This possibility is supported by the finding that in freshly excised, nonperfused rabbit hearts, hypothermia significantly reduced the initial rate of mitochondrial calcium transport.