Relation between the phosphocreatine to ATP ratio determined by 31P nuclear magnetic resonance spectroscopy and left ventricular function in underperfused guinea-pig heart

High rates of residual fatty acid oxidation during mild ischemia decrease cardiac work and efficiency

It is unknown what effects high levels of fatty acids have on energy metabolism and cardiac efficiency during milder forms of ischemia. To address this issue, isolated working rat hearts perfused with Krebs-Henseleit solution (5 mM glucose, 100 muU/mL insulin, and 0.4 (Normal Fat) or 1.2 mM palmitate (High Fat)) were subjected to 30 min of aerobic perfusion followed by 30 min of mild ischemia (39% reduction in coronary flow). Both groups had similar aerobic function and rates of glycolysis, however the High Fat group had elevated rates of palmitate oxidation (150%), and decreased rates of glucose oxidation (51%). Mild ischemia decreased cardiac work (56% versus 40%) and efficiency (29% versus 11%) further in High Fat hearts. Palmitate oxidation contributed a greater percent of acetyl-CoA production during mild ischemia in the High Fat group (81% versus 54%). During mild ischemia glycolysis remained at aerobic levels in the Normal Fat group, but was accelerated in the High Fat group. Triglyceride, glycogen and adenine nucleotide content did not differ at the end of mild ischemia, however glycogen turnover was double in the High Fat group (248%). Addition of the pyruvate dehydrogenase inhibitor dichloroacetate to the High Fat group resulted in a doubling of the rate of glucose oxidation and improved cardiac efficiency during mild ischemia. We demonstrate that fatty acid oxidation dominates as the main source of residual oxidative metabolism during mild ischemia, which is accompanied by suppressed cardiac function and efficiency in the presence of high fat.

The detection of chronic heart graft rejection by 31P NMR spectroscopy

The usefulness of phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMRS) for detecting heart graft rejection after transplantation has been investigated by several researchers, and it has thus been demonstrated to be a valid technique for detecting acute myocardial rejection. In this study, we investigated the value of 31P NMRS to assess chronic cardiac allograft rejection. Lewis rat hearts were transplanted into the femoral region of F-344 rat recipients which were treated with cyclosporine, 5mg/kg body weight, by a daily intramuscular injection for 30 days beginning on the day of transplantation. The control isografts employed Lewis donors and recipients not given cyclosporine. The ratios of phosphocreatine (PCr) to inorganic phosphate (Pi), beta-adenosine triphosphate (beta-ATP) to Pi, and PCr to beta-ATP were monitored using surface coil 31P NMRS. 31P NMRS was performed 3, 30, and 60 days after transplantation, and the degree of the rejection and arteriosclerosis of the coronary arteries were then assessed histologically. The PCr:Pi and beta-ATP:Pi ratios for the allografts demonstrated a significant decrease on postoperative day (POD) 60 from that on POD 30 (PCr:Pi, P < 0.001; beta-ATP:Pi, P < 0.01). Although a significant difference existed between the isografts and allografts on POD 60 (PCr: Pi, P < 0.01; beta-ATP:P, P < 0.01), no significant difference was found in the PCr:beta-ATP ratio between the allografts and the isografts. On POD 60, the allografts showed significant graft rejection and arteriosclerotic changes in the coronary arteries. These findings therefore demonstrated the effectiveness of 31P NMRS for detecting chronic graft rejection in a rat model.

Functional recovery after human heart transplantation is related to the metabolic condition of the hypothermic donor heart

BACKGROUND

Although strict selection criteria are being used for the acceptance of human donor hearts for transplantation, problems with respect to functional recovery on reperfusion sometimes still occur. Therefore, evaluation of the viability of a human donor heart before implantation during heart transplantation may be of great value.

METHODS AND RESULTS

In the present study, the energy metabolism of 25 excised human donor hearts arrested with St Thomas' Hospital No. 2 cardioplegic solution was evaluated noninvasively by use of 31P magnetic resonance spectroscopy (MRS) before implantation and was correlated with myocardial function measured with thermodilution in heart transplant patients. No significant correlation was observed between the cardiac index of heart transplant patients during the first hours after transplantation and the phosphocreatine/ATP (r = .13, P = .54), inorganic phosphate/ATP (r = .26, P = .21), phosphomonoesters/ATP (r = .02, P = .92), or phosphocreatine/inorganic phosphate (r = .16, P = .44) ratio or the intracellular pH (r = .06, P = .78) at the time of reperfusion. However, 1 week after transplantation, a significant correlation was observed between the cardiac index and the phosphocreatine/ATP (r = .49, P = .01), phosphomonoesters/ATP (r = .45, P = .02), and phosphocreatine/inorganic phosphate (r = .40, P = .05) ratios at the time of reperfusion. In contrast, the inorganic phosphate/ATP (r = .10, P = .63) ratio and pH (r = .31, P = .13) at the time of reperfusion showed a poor correlation with the cardiac index 1 week after transplantation.

CONCLUSIONS

Functional recovery after human heart transplantation is related to the metabolic condition of the hypothermic donor heart.

Multiparameter monitoring and analysis of in vivo ischemic and hypoxic heart

We describe a unique in vivo technique which addresses the multifactorial function of the heart, i.e., simultaneous measurement of myocardial ion transport (two mini-electrode systems to measure K+e and Ca2+e), energy metabolism (NADH fluorescence to measure NADH redox state), and coronary flow (laser-Doppler perfusion) using a multiprobe assembly (MPA) which contains transducers for all measurements. The MPA (which is 6 mm in diameter) was applied to the external surface of the heart in an open chest dog model. To test MPA function, myocardial ischemia was produced by application of a balloon occluder to the left anterior descending coronary (LAD) artery, and hypoxia was produced by changing the inspired O2-N2 ratio until the PaO2 was 20-30 torr. The MPA simultaneously monitored changes in ion flux, heart metabolism, and tissue perfusion during pathophysiological intervention.

Metabolic regulation of in vivo myocardial contractile function: multiparameter analysis

To gain insight into the mechanisms of myocardial regulation as it relates to the interaction of mechanical and metabolic function and perfusion, intact animal models were instrumented for routine physiological measurements of mechanical function and for measurements of metabolism (31P NMR, NADH fluorescence (redox state)) and perfusion (2H NMR and Laser doppler techniques). These techniques were applied to canine and cat models of volume and/or pressure loading, hypoxia, ischemia and cardiomyopathic states. Data generated using these techniques indicate that myocardial bioenergetic function is quite stable under most loading conditions as long as the heart is not ischemic. In addition, these data indicate that there is no universal regulator and that different biochemical regulators appear to mediate stable function under different physiological and pathophysiological conditions: for example; during hypoxia, NADH redox state appears to play a regulatory role; and in pressure loading, ADP, phosphorylation potential and free energy of ATP hydrolysis as well as NADH redox state appear to be regulatory.

Cardiac contractile dysfunction during mild coronary flow reductions is due to an altered calcium-pressure relationship in rat hearts

Coronary artery stenosis or occlusion results in reduced coronary flow and myocardial contractile depression. At severe flow reductions, increased inorganic phosphate (Pi) and intracellular acidosis clearly play a role in contractile depression. However, during milder flow reductions the mechanism(s) underlying contractile depression are less clear. Previous perfused heart studies demonstrated no change of Pi or pH during mild flow reductions, suggesting that changes of intravascular pressure (garden hose effect) may be the mediator of this contractile depression. Others have reported conflicting results regarding another possible mediator of contractility, the cytosolic free calcium (Cai). To examine the respective roles of Cai, Pi, pH, and vascular pressure in regulating contractility during mild flow reductions, Indo-1 calcium fluorescence and 31P magnetic resonance spectroscopy measurements were performed on Langendorff-perfused rat hearts. Cai and diastolic calcium levels did not change during flow reductions to 50% of control. Pi demonstrated a close relationship with developed pressure and significantly increased from 2.5 +/- 0.3 to 4.2 +/- 0.4 mumol/g dry weight during a 25% flow reduction. pH was unchanged until a 50% flow reduction. Increasing vascular pressure to superphysiological levels resulted in further increases of developed pressure, with no change in Cai. These findings are consistent with the hypothesis that during mild coronary flow reductions, contractile depression is mediated by an altered relationship between Cai and pressure, rather than by decreased Cai. Furthermore, increased Pi and decreased intravascular pressure may be responsible for this altered calcium-pressure relationship during mild coronary flow reductions.

Nuclear magnetic resonance spectroscopy of excised human hearts

BACKGROUND

Phosphorus nuclear magnetic resonance spectroscopy has been proposed as a method of studying the metabolism of the myocardium in patients. Little is known about 31P nuclear magnetic resonance spectroscopy of diseased human hearts.

METHODS

Two donor hearts meeting the requirements for heart transplantation and 11 diseased hearts were removed during a transplantation procedure and were studied in a horizontal 2.35 T superconducting magnet. Spectra were obtained at 0 degrees C about 30 minutes after the excision. The areas of the inorganic phosphate peak (Pi) and of the phosphocreatine peak (PCr) were summed and expressed as a ratio with respect to the area of the beta ATP peak.

RESULTS

The ratio (Pi + PCr)/beta ATP was found to be significantly lower in five hearts with a myocardial infarct (0.77 (0.18)) than in hearts with dilated cardiomyopathy (1.25 (0.29)) and in normal hearts (1.69 (0.11)). The area of the phosphodiester peak was expressed as a ratio with respect to the area of the beta ATP peak: no differences were found between the three groups.

CONCLUSIONS

These results suggest that the phosphocreatine concentration is lower in ischaemic heart disease than in dilated cardiomyopathy and that the phosphodiester peak is probably not useful in distinguishing between these two types of heart disease.

Adenosine release and high energy phosphates in intact dog hearts during norepinephrine infusion

Cardiac adenosine release is thought to depend on the oxygen supply/demand ratio, and this effect may be mediated by changes in high energy phosphate concentrations. Previous studies supporting this hypothesis have been done primarily in isolated hearts. We tested this hypothesis in intact dog hearts. Anesthetized, open-chest dogs were placed in a 4.7-T magnet where 31P nuclear magnetic resonance spectra were acquired via a surface coil over the heart at 2-minute intervals (60 scans, 2-second interpulse delay). Coronary sinus flow was shunted through a flow probe and returned via a jugular vein. After a control period, intracoronary norepinephrine was infused (12 micrograms/min) for 16 minutes and plasma samples were taken every 5 minutes. The phosphocreatine/ATP peak area ratio was used as an index of high energy phosphate changes. During norepinephrine infusion, arterial pressure, heart rate, coronary sinus flow, oxygen consumption, and adenosine release all increased significantly. Adenosine release peaked at 5 minutes but remained elevated after 15 minutes. There was a transient fall in the phosphocreatine/ATP ratio (9.2 +/- 3.1%, p less than 0.05) during the first 7 minutes, but the ratio returned to control levels by 9 minutes. The oxygen supply/consumption ratio increased after 5 minutes of norepinephrine infusion and then returned to control levels. We conclude that during norepinephrine infusion in vivo, persistent adenosine release can occur with only small transient changes in high energy phosphate concentrations and with no decrease in the oxygen supply/demand ratio.

In vivo 31P nuclear magnetic resonance findings on heterotopically allografted hearts in rats treated with a novel immunosuppressant, FK506

Administration of FK506 for 15 days at daily doses of 3.2 mg/kg p.o., 10 mg/kg p.o., 0.32 mg/kg i.m., or 1 mg/kg i.m. to heart-allografted rats resulted in a significant prolongation of graft survival time. The best graft acceptance was obtained in the 1 mg/kg i.m. group: all six grafts survived longer than 50 days, and two of them, indefinitely. The 31P nuclear magnetic resonance (NMR) technique was utilized to investigate in vivo the energy metabolism of grafts. The ratios of inorganic phosphate (Pi)/phosphocreatine (PCr) and PCr/ATP were useful parameters for monitoring cardiac insufficiency after transplantation. The mean ratios of Pi/PCr and PCr/ATP in syngeneic grafts were 0.38 +/- 0.11 and 1.88 +/- 0.42, respectively. In the control allografts, a rapid increase in the Pi/PCr ratio and a decrease in the PCr/ATP ratio were found from day 5. During the period of FK506 administration, increased Pi/PCr and decreased PCr/ATP ratios were also observed in all groups. The changes in these ratios were related with FK506 dosage. The results suggest that FK506 has a side-effect on graft metabolism. The metabolism tended to improve upon cessation of the drug in all grafts, but it worsened again in 3-3 1/2 weeks in the rats treated with 3.2 mg/kg p.o., 10 mg/kg p.o., or 0.32 mg/kg i.m. This seemed to be due to graft rejection.(ABSTRACT TRUNCATED AT 250 WORDS)

Regeneration of myocardial phosphocreatine in pigs despite continued moderate ischemia

The effects of 1 hour of mild and moderate reductions in coronary blood flow on myocardial high-energy phosphate levels were evaluated. Thirty anesthetized pigs were instrumented with left anterior descending arterial and venous catheters, crystals for instantaneous wall thickness, and a fluid-filled occluder. Measurement of myocardial blood flow was performed with microspheres, and a series of myocardial biopsies also was performed. In 10 pigs, overall coronary blood flow was lowered by 22%, with a fall in subendocardial-to-subepicardial flow ratio from 1.11 to 0.54 and in wall thickening from 33% to 15%. Subendocardial flow fell 48%. Coronary blood flow and thickening were constant during 1 hour of ischemia. Phosphocreatine (mumol/g wet wt) in the subendocardial third of the ischemic zone fell from 7.6 to 3.8 at 5 minutes of ischemia (p less than 0.005 versus control) and returned to normal (7.9) at 60 minutes (p = NS), despite ongoing ischemia. Subendocardial ATP (mumol/g wet wt) fell slowly from 4.3 and leveled off at 2.1 at 60 minutes of ischemia (p less than 0.001 versus control). Similar regeneration of phosphocreatine was found in seven additional pigs, with a 43% transmural reduction in coronary blood flow and a 66% reduction in subendocardial flow. No significant changes in ATP and phosphocreatine were noted in two different control groups (n = 13 pigs). The regeneration of phosphocreatine despite ongoing ischemia and low ATP levels was not related to changes in myocardial oxygen demand or consumption, or in regional function during the period of ischemia. This may reflect 1) a successful downregulation of the energy needs of the ischemic myocardium to maintain cell viability, or 2) a metabolic abnormality in the ability of the cells to produce ATP primarily or by use of phosphocreatine.

Relationship between myocardial metabolites and contractile abnormalities during graded regional ischemia. Phosphorus-31 nuclear magnetic resonance studies of porcine myocardium in vivo

The mechanisms responsible for changes in myocardial contractility during regional ischemia are unknown. Since changes in high-energy phosphates during ischemia are sensitive to reductions in myocardial blood flow, it was hypothesized that myocardial function under steady-state conditions of graded regional ischemia is closely related to changes in myocardial high-energy phosphates. Therefore, phosphorus-31 nuclear magnetic resonance spectroscopy was employed in an in vivo porcine model of graded coronary stenosis. Simultaneous measurements of regional subendocardial blood flow, high-energy phosphates, pH, and myocardial segment shortening were made during various degrees of regional ischemia in which subendocardial blood flow was reduced by 16-94%. During mild reductions in myocardial blood flow (subendocardial blood flow = 83% of nonischemic myocardium), only the ratio of phosphocreatine to inorganic phosphate (PCr/Pi), Pi, and [H+] were significantly changed from control. PCr, ATP, and PCr/ATP were not significantly reduced from control with mild reductions in blood flow. Changes in myocardial segment shortening were most closely associated with changes in PCr/Pi (r = 0.94). Pi and [H+] were negatively correlated with segment shortening (r = -0.64 and -0.58, respectively) and increased over twofold when blood flow was reduced by 62%. Thus, these data demonstrate that PCr/Pi is sensitive to reductions in myocardial blood flow and closely correlates with changes in myocardial function. These data are also consistent with a role for Pi or H+ as inhibitors of myocardial contractility during ischemia.

Hypoxia as a risk factor for doxorubicin-induced cardiotoxicity: a NMR evaluation

Previous studies suggested that one possible mechanism of doxorubicin (DXR)-induced cardiomyopathy involves the depletion of high-energy phosphate stores. In this study, we used 31P nuclear magnetic resonance to assess the high-energy phosphate content in Langendorff perfused rat hearts. Hearts were perfused in normoxic conditions (spontaneous flow) or in partially hypoxic conditions obtained by perfusing at 50% of the spontaneous flow. DXR was used at the subtoxic conditions of 50 mg/l for 15 min and at the cardiotoxic concentration of 100 mg/l for 60 min. Left ventricular pressure (dP/dt), heart rate, myocardial ATP and PCr levels and PCr/ATP ratio were measured. We found that, in normoxic conditions, DXR (50 mg/l, 15 min) does not impair cellular high-energy phosphate metabolism. However, in mild hypoxic conditions, DXR induces a significant decrease in PCr/ATP ratio, due to a decrease in PCr and to a simultaneous increase in ATP. Similar results are obtained after 60 min perfusion with the cardiotoxic dose of DXR. This study suggests that hypoxia may represent a risk factor for the development of DXR-induced acute cardiotoxicity.

Characterization of high-energy phosphate compounds during reperfusion of the irreversibly injured myocardium using 31P MRS

Phosphorus-31 magnetic resonance spectroscopy (MRS) was used to monitor regional changes in high-energy phosphorus compounds and intracellular pH during 60 min of acute regional ischemia (acute occlusion of left anterior descending artery) and reperfusion in open-chest cats using a 1.2-cm two-turn coil sutured to the myocardium. During the 60-min ischemic phase, phosphocreatine (PCr) intensity was reduced to 47 +/- 4.9% (mean +/- SE) of control (p less than 0.01) by 15 min postocclusion while adenosine triphosphate (ATP) intensity decreased more slowly with the decrease (66 +/- 5.6%) achieving significance (p less than 0.05) only at 60 min postocclusion. Inorganic phosphate (Pi) increased to a maximum of 397 +/- 42% of control (p less than 0.01) while the pH decreased progressively from 7.36 +/- 0.02 to 6.02 +/- 0.14 (p less than 0.01). After release of occlusion PCr intensity recovered to 86 +/- 12% of the initial control value at 15 min postreperfusion but showed a subsequent downward trend to 79 +/- 8.8%. The ATP did not recover but tended to decline further during reperfusion. The Pi intensity decreased to 260 +/- 38% of control while the pH increased to 7.01 +/- 0.23 by 15 min postreperfusion. Thus, the reperfused irreversibly injured myocardium is characterized by persistent depletion of PCr and ATP and elevation of Pi. Phosphorus-31 MRS provides a nondestructive method for characterizing the reperfused irreversibly damaged myocardium.

Rates of glycolysis and glycogenolysis during ischemia in glucose-insulin-potassium-treated perfused hearts: A 13C, 31P nuclear magnetic resonance study

The effects of 11.7 mM glucose, insulin, and potassium (GIK) on metabolism during ischemia were investigated in the perfused guinea pig heart using magnetic resonance spectroscopy. Intracellular metabolites, primarily glycogen and glutamate, were labeled with 13C by addition of [1-13C]glucose to the perfusate during a normoxic, preischemic period. 13C and 31P NMR spectroscopy was used to observe the metabolism of 13C-labeled metabolites simultaneously with high-energy phosphorus metabolites and pH. The extent of acidosis and the rate and amount of labeled lactate accumulation during ischemia were the same in control (3 mM glucose + insulin) and GIK-treated hearts. In contrast, the rate of labeled glycogen mobilization during ischemia in GIK-treated hearts was one third the rate observed in control hearts. These observations suggest that GIK decreased the rate of glycogenolysis during ischemia without affecting the rate of glycolysis. We propose that glucose contributed as a glycolytic substrate to a greater extent during ischemia in GIK-treated hearts than in hearts perfused with 3 mM glucose and insulin. The glycogen-sparing effect of GIK demonstrated in these studies could delay the onset of ischemic damage in a clinical setting by prolonging the availability of glycolytic substrate necessary for production of high-energy phosphate.

Monitoring the bioenergetics of cardiac allograft rejection using in vivo P-31 nuclear magnetic resonance spectroscopy

Monitoring human cardiac allograft rejection is currently accomplished by endomyocardial biopsy. Available noninvasive methods for identifying rejection have lacked the necessary sensitivity or specificity, or both, for routine clinical application. In vivo phosphorus-31 (P-31) nuclear magnetic resonance (NMR) spectroscopy has been used for monitoring phosphorus metabolism in both animal models and humans. In the present study this technique was employed as a noninvasive means to assess the bioenergetic processes that occur during cardiac allograft rejection in a rat model. Brown Norway rat hearts were transplanted subcutaneously into the anterior region of the neck of Lewis rat recipients (allografts). Control isografts employed Lewis donors and recipients. Phosphocreatine to inorganic phosphate (PCr/Pi), phosphocreatine to beta-adenosine triphosphate (PCr/ATP beta), beta-adenosine triphosphate to inorganic phosphate (ATP beta/Pi) ratios and pH of the transplanted hearts were monitored using surface coil P-31 NMR spectroscopy (at 4.7 tesla) daily for 7 days. To allow recovery from the compromise induced by the surgical procedure, the measurements obtained on day 2 were taken as a baseline. PCr/Pi was unchanged or increased in the isografts but decreased continually in allografts, with the difference becoming significant by day 4 when compared with levels in day 2 allografts (p less than 0.005) and by day 3 when compared with levels in the isograft group (p less than 0.05). PCr/ATP beta in isografts did not change throughout the study; however, allografts demonstrated a significant decrease as early as day 3 (p less than 0.01), although a significant difference between isografts and allografts did not become manifest until day 4 (p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanics of rat myocardium revisited: investigations of ultra-thin cardiac muscles under high energy demand

Disregarding the influence of thickness on elevated strength of isolated preparations inevitably leads to erroneous tension-frequency relations, especially in the range of high frequencies. Thus, much of the confusion in interpreting the atypical negative staircase phenomenon of the rat heart is due to this. In view of the fact that the rat has become the preferred laboratory animal in cardiological research, it was imperative to reinvestigate force-frequency relations using ultra-thin preparations of the rat right ventricle (d less than 0.1 mm). Contrary to popular opinion, it could be demonstrated that the rat myocardium shows a positive staircase in the range of physiological heart rates. An increase in tension is still attainable even at frequencies up to 600 min-1. The interval-strength relations exhibit a minimum at frequencies of 60-120 min-1, being shifted to higher frequencies with increasing diameter, vanishing completely for thick preparations (d greater than 1.0 mm). At high extracellular Ca++ concentration the positive staircase even of ultra-thin muscles is flattened. However, it can be reinforced when the strength, and thus the energy expenditure, is reduced by lowering the extension. The same is true for contractions under hypoxia. From these findings it seems probable that many investigations on isolated heart muscles of the rat, as well as other species, are objectionable when done under high energy demand, as diffusion problems will certainly limit any rise in contractility.

Evaluation of the hereditary Syrian hamster cardiomyopathy by 31P nuclear magnetic resonance spectroscopy: improvement after acute verapamil therapy

The relation between metabolic and functional derangement in various cardiomyopathies has not been well characterized. This information was specifically sought in a spontaneous cardiomyopathic model. Metabolic and hemodynamic parameters were obtained in glucose-perfused beating hearts of 180-200-day-old cardiomyopathic Syrian hamsters and age-matched healthy animals. This period in the cardiomyopathic hamster lifetime is intermediary between the necrotic phase and the appearance of heart failure. We used 31P nuclear magnetic resonance spectroscopy to analyze energy metabolites and intracellular pH. Cardiomyopathic hamsters had significantly higher mole fraction values for inorganic phosphate, lower phosphocreatine mole fraction as well as lower phosphocreatine/inorganic phosphate and adenosine triphosphate/inorganic phosphate ratios. Analysis of pH indicated the presence of regions of increased acidity within the heart of myopathic hamsters. Cardiomyopathic hamsters also had significantly lower left ventricular pressure, coronary flow, and myocardial oxygen consumption. Separate groups of normal and myopathic hamsters were given verapamil for 24 hours (one injection of 4 mg/kg s.c. followed by 1.2 g/l in drinking water). Verapamil-treated myopathic hamsters had evidence of markedly improved mitochondrial function when compared with untreated animals. Left ventricular pressure and coronary flow rose to normal levels. Replacing glucose by pyruvate in the perfusate of myopathic hamsters results in a marked increase in left ventricular pressure, coronary flow, and oxygen consumption with a moderate rise in phosphocreatine. Thus, 180-200-day-old cardiomyopathic hamster heart is characterized by evidence of decreased mitochondrial function, by areas of increased acidity within the heart, and by reduced left ventricular function.(ABSTRACT TRUNCATED AT 250 WORDS)