Transmural metabolite distribution in regional myocardial ischemia as studied with 31P NMR

RF pulse methods for use with surface coils: Frequency-modulated pulses and parallel transmission

The first use of a surface coil to obtain a 31P NMR spectrum from an intact rat by Ackerman and colleagues initiated a revolution in magnetic resonance imaging (MRI) and spectroscopy (MRS). Today, we take it for granted that one can detect signals in regions external to an RF coil; at the time, however, this concept was most unusual. In the approximately four decade long period since its introduction, this simple idea gave birth to an increasing number of innovations that has led to transformative changes in the way we collect data in an in vivo magnetic resonance experiment, particularly with MRI of humans. These innovations include spatial localization and/or encoding based on the non-uniform B1 field generated by the surface coil, leading to new spectroscopic localization methods, image acceleration, and unique RF pulses that deal with B1 inhomogeneities and even reduce power deposition. Without the surface coil, many of the major technological advances that define the extraordinary success of MRI in clinical diagnosis and in biomedical research, as exemplified by projects like the Human Connectome Project, would not have been possible.

Imaging at ultrahigh magnetic fields: History, challenges, and solutions

Following early efforts in applying nuclear magnetic resonance (NMR) spectroscopy to study biological processes in intact systems, and particularly since the introduction of 4 T human scanners circa 1990, rapid progress was made in imaging and spectroscopy studies of humans at 4 T and animal models at 9.4 T, leading to the introduction of 7 T and higher magnetic fields for human investigation at about the turn of the century. Work conducted on these platforms has provided numerous technological solutions to challenges posed at these ultrahigh fields, and demonstrated the existence of significant advantages in signal-to-noise ratio and biological information content. Primary difference from lower fields is the deviation from the near field regime at the radiofrequencies (RF) corresponding to hydrogen resonance conditions. At such ultrahigh fields, the RF is characterized by attenuated traveling waves in the human body, which leads to image non-uniformities for a given sample-coil configuration because of destructive and constructive interferences. These non-uniformities were initially considered detrimental to progress of imaging at high field strengths. However, they are advantageous for parallel imaging in signal reception and transmission, two critical technologies that account, to a large extend, for the success of ultrahigh fields. With these technologies and improvements in instrumentation and imaging methods, today ultrahigh fields have provided unprecedented gains in imaging of brain function and anatomy, and started to make inroads into investigation of the human torso and extremities. As extensive as they are, these gains still constitute a prelude to what is to come given the increasingly larger effort committed to ultrahigh field research and development of ever better instrumentation and techniques.

A novel pharmacological strategy by PTEN inhibition for improving metabolic resuscitation and survival after mouse cardiac arrest

Sudden cardiac arrest (SCA) is a leading cause of death in the United States. Despite return of spontaneous circulation, patients die due to post-SCA syndrome that includes myocardial dysfunction, brain injury, impaired metabolism, and inflammation. No medications improve SCA survival. Our prior work suggests that optimal Akt activation is critical for cooling protection and SCA recovery. Here, we investigate a small inhibitor of PTEN, an Akt-related phosphatase present in heart and brain, as a potential therapy in improving cardiac and neurological recovery after SCA. Anesthetized adult female wild-type C57BL/6 mice were randomized to pretreatment of VO-OHpic (VO) 30 min before SCA or vehicle control. Mice underwent 8 min of KCl-induced asystolic arrest followed by CPR. Resuscitated animals were hemodynamically monitored for 2 h and observed for 72 h. Outcomes included heart pressure-volume loops, energetics (phosphocreatine and ATP from (31)P NMR), protein phosphorylation of Akt, GSK3β, pyruvate dehydrogenase (PDH) and phospholamban, circulating inflammatory cytokines, plasma lactate, and glucose as measures of systemic metabolic recovery. VO reduced deterioration of left ventricular maximum pressure, maximum rate of change in the left ventricular pressure, and Petco2 and improved 72 h neurological intact survival (50% vs. 10%; P < 0.05). It reduced plasma lactate, glucose, IL-1β, and Pre-B cell colony enhancing factor, while increasing IL-10. VO increased phosphorylation of Akt and GSK3β in both heart and brain, and cardiac phospholamban phosphorylation while reducing p-PDH. Moreover, VO improved cardiac bioenergetic recovery. We concluded that pharmacologic PTEN inhibition enhances Akt activation, improving metabolic, cardiovascular, and neurologic recovery with increased survival after SCA. PTEN inhibitors may be a novel pharmacologic strategy for treating SCA.

Development of functional imaging in the human brain (fMRI); the University of Minnesota experience

The human functional magnetic resonance imaging (fMRI) experiments performed in the Center for Magnetic Resonance Research (CMRR), University of Minnesota, were planned between two colleagues who had worked together previously in Bell Laboratories in the late nineteen seventies, namely myself and Seiji Ogawa. These experiments were motivated by the Blood Oxygenation Level Dependent (BOLD) contrast developed by Seiji. We discussed and planned human studies to explore imaging human brain activity using the BOLD mechanism on the 4 Tesla human system that I was expecting to receive for CMRR. We started these experiments as soon as this 4 Tesla instrument became marginally operational. These were the very first studies performed on the 4 Tesla scanner in CMRR; had the scanner become functional earlier, they would have been started earlier as well. We were aware of the competing effort at the Massachusetts General Hospital (MGH) and we knew that they had been informed of our initiative in Minneapolis to develop fMRI. We had positive results certainly by August 1991 annual meeting of the Society of Magnetic Resonance in Medicine (SMRM). I believe, however, that neither the MGH colleagues nor us, at the time, had enough data and/or conviction to publish these extraordinary observations; it took more or less another six months or so before the papers from these two groups were submitted for publication within five days of each other to the Proceedings of the National Academy of Sciences, USA, after rejection by Nature in our case. Thus, fMRI was achieved independently and at about the same time at MGH, in an effort credited largely to Ken Kwong, and in CMRR, University of Minnesota in an effort led by myself and Seiji Ogawa.

Transmural distribution of metabolic abnormalities and glycolytic activity during dobutamine-induced demand ischemia

The heterogeneity across the left ventricular wall is characterized by higher rates of oxygen consumption, systolic thickening fraction, myocardial perfusion, and lower energetic state in the subendocardial layers (ENDO). During dobutamine stimulation-induced demand ischemia, the transmural distribution of energy demand and metabolic markers of ischemia are not known. In this study, hemodynamics, transmural high-energy phosphate (HEP), 2-deoxyglucose-6-phosphate (2-DGP) levels, and myocardial blood flow (MBF) were determined under basal conditions, during dobutamine infusion (DOB: 20 microg x kg(-1) x min(-1) iv), and during coronary stenosis + DOB + 2-deoxyglucose (2-DG) infusion. DOB increased rate pressure products (RPP) and MBF significantly without affecting the subendocardial-to-subepicardial blood flow ratio (ENDO/EPI) or HEP levels. During coronary stenosis + DOB + 2-DG infusion, RPP, ischemic zone (IZ) MBF, and ENDO/EPI decreased significantly. The IZ ratio of creatine phosphate-to-ATP decreased significantly [2.30 +/- 0.14, 2.06 +/- 0.13, and 2.04 +/- 0.11 to 1.77 +/- 0.12, 1.70 +/- 0.11, and 1.72 +/- 0.12 for EPI, midmyocardial (MID), and ENDO, respectively], and 2-DGP accumulated in all layers, as evidenced by the 2-DGP/PCr (0.55 +/- 0.12, 0.52 +/- 0.10, and 0.37 +/- 0.08 for EPI, MID, and ENDO, respectively; P < 0.05, EPI > ENDO). In the IZ the wet weight-to-dry weight ratio was significantly increased compared with the normal zone (5.9 +/- 0.5 vs. 4.4 +/- 0.4; P < 0.05). Thus, in the stenotic perfused bed, during dobutamine-induced high cardiac work state, despite higher blood flow, the subepicardial layers showed the greater metabolic changes characterized by a shift toward higher carbohydrate metabolism, suggesting that a homeostatic response to high-cardiac work state is characterized by more glucose utilization in energy metabolism.

Oxidative capacity in failing hearts

Although high-energy phosphate metabolism is abnormal in failing hearts [congestive heart failure (CHF)], it is unclear whether oxidative capacity is impaired. This study used the mitochondrial uncoupling agent 2,4-dinitrophenol (DNP) to determine whether reserve oxidative capacity exists during the high workload produced by catecholamine infusion in hypertrophied and failing hearts. Left ventricular hypertrophy (LVH) was produced by ascending aortic banding in 21 swine; 9 animals developed CHF. Basal myocardial phosphocreatine (PCr)/ATP measured with 31P NMR spectroscopy was decreased in both LVH and CHF hearts (corresponding to an increase in free [ADP]), whereas ATP was decreased in hearts with CHF. Infusion of dobutamine and dopamine (each 20 microg. kg-1. min-1 iv) caused an approximate doubling of myocardial oxygen consumption (MVO2) in all groups and decreased PCr/ATP in the normal and LVH groups. During continuing catecholamine infusion, DNP (2-8 mg/kg iv) caused further increases of MVO2 in normal and LVH hearts with no change in PCr/ATP. In contrast, DNP caused no increase in MVO2 in the failing hearts; the associated decrease of PCr/ATP suggests that DNP decreased the mitochondrial proton gradient, thereby causing ADP to increase to maintain adequate ATP synthesis.

Nicorandil improves myocardial high-energy phosphates in postinfarction porcine hearts

1. Nicorandil is a potent vasodilator combining the effects of a nitrate with an ATP-sensitive potassium channel (K(ATP)) opener. Because the postinfarct remodelled heart has increased vulnerability to subendocardial hypoperfusion, it is possible that the vasodilator effects of nicorandil could cause transmural redistribution of blood flow away from the subendocardium. Alternatively, the K(ATP) channel opening effects of nicorandil could exert a beneficial effect on mitochondrial respiration. Consequently, the present study was performed to examine the effect of nicorandil on energy metabolism in the postinfarct heart. 2. Studies were performed in swine in which myocardial infarction produced by proximal left circumflex coronary artery ligation had resulted in left ventricular remodeling. [(31)P] nuclear magnetic resonance spectroscopy (MRS) was used to examine the myocardial energy supply/demand relationship across the left ventricular wall while the transmural distribution of blood flow was examined with radioactive microspheres. Data were obtained during baseline conditions and during infusion of nicorandil (100 microg, i.v., followed an infusion of 25 microg/kg per min). 3. Nicorandil caused coronary vasodilation with a preferential increase in subepicardial flow; however, subendocardial flow also increased significantly. Nicorandil had no significant effect on the rate-pressure product or myocardial oxygen consumption. The ratio of phosphocreatine (PCr)/ATP determined with MRS was abnormally depressed in remodelled hearts (2.01 +/- 0.11, 1.85 +/- 0.10 and 1.59 +/- 0.11 for subepicardium, midwall and subendocardium, respectively) compared with normal (2.22 +/- 0.11, 2.01 +/- 0.15 and 1.80 +/- 0.09, respectively). Nicorandil had no effect on the high-energy phosphate content of normal hearts. However, nicorandil increased the PCr/ATP ratio in the subendocardium of remodelled hearts from 1.59 +/- 0.11 to 1.87 +/- 0.10 (P < 0.05). 4. Although nicorandil caused modest redistribution of blood flow away from the subendocardium of the postinfarct left ventricle, this was associated with an increase of the PCr/ATP ratio towards normal. These results suggest that nicorandil exerts a beneficial effect on energy metabolism in the subendocardium of the postinfarct remodelled left ventricle.

Myocardial creatine kinase kinetics in hearts with postinfarction left ventricular remodeling

This study examined whether alterations in myocardial creatine kinase (CK) kinetics and high-energy phosphate (HEP) levels occur in postinfarction left ventricular remodeling (LVR). Myocardial HEP and CK kinetics were examined in 19 pigs 6 wk after myocardial infarction was produced by left circumflex coronary artery ligation, and the results were compared with those from 9 normal pigs. Blood flow (microspheres), oxygen consumption (MVO2), HEP levels [31P magnetic resonance spectroscopy (MRS)], and CK kinetics (31P MRS) were measured in myocardium remote from the infarct under basal conditions and during dobutamine infusion (20 micrograms. kg-1. min-1 iv). Six of the pigs with LVR had overt congestive heart failure (CHF) at the time of study. Under basal conditions, creatine phosphate (CrP)-to-ATP ratios were lower in all transmural layers of hearts with CHF and in the subendocardium of LVR hearts than in normal hearts (P < 0.05). Myocardial ATP (biopsy) was significantly decreased in hearts with CHF. The CK forward rate constant was lower (P < 0.05) in the CHF group (0.21 +/- 0.03 s-1) than in LVR (0.38 +/- 0.04 s-1) or normal groups (0.41 +/- 0.03 s-1); CK forward flux rates in CHF, LVR, and normal groups were 6.4 +/- 2.3, 14.3 +/- 2.1, and 20.3 +/- 2.4 micromol. g-1. s-1, respectively (P < 0.05, CHF vs. LVR and LVR vs. normal). Dobutamine caused doubling of the rate-pressure product in the LVR and normal groups, whereas CHF hearts failed to respond to dobutamine. CK flux rates did not change during dobutamine in any group. The ratios of CK flux to ATP synthesis (from MVO2) under baseline conditions were 10.9 +/- 1.2, 8. 03 +/- 0.9, and 3.86 +/- 0.5 for normal, LVR, and CHF hearts, respectively (each P < 0.05); during dobutamine, this ratio decreased to 3.73 +/- 0.5, 2.58 +/- 0.4, and 2.78 +/- 0.5, respectively (P = not significant among groups). These data demonstrate that CK flux rates are decreased in hearts with postinfarction LVR, but this change does not limit the response to dobutamine. In hearts with end-stage CHF, the changes in HEP and CK flux are more marked. These changes could contribute to the decreased responsiveness of these hearts to dobutamine.

Preservation of canine myocardial high-energy phosphates during low-flow ischemia with modification of hemoglobin-oxygen affinity

Conventional approaches for the treatment of myocardial ischemia increase coronary blood flow or reduce myocardial demand. To determine whether a rightward shift in the hemoglobin-oxygen saturation curve would reduce the metabolic and contractile effects of a myocardial oxygen-supply imbalance, we studied the impact of a potent synthetic allosteric modifier of hemoglobin-oxygen affinity, a 2-[4-[[(3,5-disubstituted anilino)carbonyl]methyl] phenoxy] -2-methylproprionic acid derivative (RSR13), during low-flow ischemia. Changes in myocardial high-energy phosphate levels and pH were studied by 31P nuclear magnetic resonance (NMR) spectroscopy in 12 open-chest dogs randomized to receive RSR13 or vehicle control during a reversible reduction of left anterior descending (LAD) coronary artery blood flow. Changes in cardiac metabolites and regional ventricular function studied by pressure segment-length relations were also investigated in additional animals before and after RSR13 administration during low-flow LAD ischemia. The intravenous administration of RSR13 before ischemia resulted in a substantial increase in the mean hemoglobin p50 and attenuated the decline in cardiac creatine phosphate/adenosine triphosphate (PCr/ATP), percent PCr, and pH during ischemia without a change in regional myocardial blood flow, heart rate, or systolic blood pressure. RSR13 given after the onset of low-flow ischemia also improved cardiac PCr/ATP ratios and regional function as measured by fractional shortening and regional work. Thus, synthetic allosteric reduction in hemoglobin-oxygen affinity may be a new and important therapeutic strategy to ameliorate the metabolic and functional consequences of cardiac ischemia.

Measurement of myocardial pH by saturation transfer in man

Myocardial pH has been shown in animal models to be a sensitive indicator of ischemia. In vivo measurement in humans using 31p magnetic resonance spectroscopy is complicated by the overlap of blood 2,3-diphosphoglycerate peaks with the P(i) peak used for pH measurement. A "saturation transfer" method combined with spatial presaturation of skeletal muscle signal is presented which can obtain spectra from the heart free of contamination of 2,3-DPG signal in which intracellular P(i) resonance can be clearly observed. Application to a group of six normal subjects found that the chemical shift of the intracellular inorganic phosphate peak was 4.95+/-0.06 relative to the phosphocreatine peak. This is equivalent to a pH of 7.11+/-0.05.

Three-dimensional 31P magnetic resonance spectroscopic imaging of regional high-energy phosphate metabolism in injured rat heart

The purpose of this study was to measure the spatially varying 31P MR signals in global and regional ischemic injury in the isolated, perfused rat heart. Chronic myocardial infarcts were induced by occluding the left anterior descending coronary artery eight weeks before the MR examination. The effects of acute global low-flow ischemia were observed by reducing the perfusate flow. Chemical shift imaging (CSI) with three spatial dimensions was used to obtain 31P spectra in 54-microl voxels. Multislice 1H imaging with magnetization transfer contrast enhancement provided anatomical information. In normal hearts (n = 8), a homogeneous distribution of high-energy phosphate metabolites (HEP) was found. In chronic myocardial infarction (n = 6), scar tissue contained negligible amounts of HEP, but their distribution in residual myocardium was uniform. The size of the infarcted area could be measured from the metabolic images; the correlation of infarct sizes determined by histology and 31P MR CSI was excellent (P < 0.006). In global low-flow ischemia (n = 8), changes of HEP showed substantial regional heterogeneity. Three-dimensional 31P MR CSI should yield new insights into the regionally distinct metabolic consequences of various forms of myocardial injury.

Functional and bioenergetic consequences of postinfarction left ventricular remodeling in a new porcine model. MRI and 31 P-MRS study

BACKGROUND

The underlying mechanisms by which left ventricular remodeling (LVR) leads to congestive heart failure (CHF) are unclear. This study examined the functional and bioenergetic abnormalities associated with postinfarction ventricular remodeling in a new, large animal model.

METHODS AND RESULTS

Remodeling was induced by circumflex coronary artery ligation in young pigs. LV mass, volume, ejection fraction (EF), the ratio of scar surface area to LV surface area, and LV wall stresses were calculated from magnetic resonance imaging anatomic data and simultaneously measured LV pressure. Hemodynamics, transmural blood flow, and high-energy phosphates (spatially localized 31P-nuclear magnetic resonance) were measured under basal conditions, during hyperperfusion induced by pharmacological vasodilation with adenosine, and during pyruvate infusion (11 mg/kg per minute IV). Six of 18 animals with coronary ligation developed clinical CHF while the remaining 12 animals had LV dilation (LVR) without CHF. The results were compared with 16 normal animals. EF decreased from 55.9 +/- 5.6% in normals to 34.6 +/- 2.3% in the LVR group (P < .05) and 24.2 +/- 2.8% in the CHF group (P < .05 versus LVR). The infarct scar was larger in CHF hearts than in LVR hearts (P < .05). In normals, LV myocardial creatine phosphate (CP)/ATP ratios were 2.10 +/- 0.10, 2.06 +/- 0.16, and 1.92 +/- 0.12 in subepicardium (EPI), mid myocardium (MID), and subendocardium (ENDO), respectively. In LVR hearts, the corresponding ratios were decreased to 1.99 +/- 0.13, 1.80 +/- 0.14, and 1.57 +/- 0.15 (ENDO P < .05 versus normal). In CHF hearts, CP/ATP ratios were 1.41 +/- 0.14, 1.33 +/- 0.15, and 1.25 +/- 0.15; (P < .05 versus LVR in EPI and MID). The calculated myocardial free ADP levels were significantly increased only in CHF hearts.

CONCLUSIONS

Bioenergetic abnormalities in remodeled myocardium are related to the severity of LV dysfunction, which, in turn, is dependent on the severity of the initiating myocardial infarction.

Bioenergetic consequences of left ventricular remodeling

BACKGROUND

Left ventricular (LV) remodeling is associated with LV dysfunction and decrease of coronary flow reserve. The underlying mechanisms responsible for these alterations are unclear. Changes in myocardial high-energy phosphate levels may be associated with these alterations.

METHODS AND RESULTS

Twelve dogs with LV remodeling secondary to discrete necrosis produced by transmyocardial DC shock were compared with 8 normal dogs. LV mass and end-diastolic volume were measured by magnetic resonance imaging 7 days before and 12.9 +/- 1.3 months after DC shock. Transmurally localized 31P nuclear magnetic resonance spectra from five layers across the LV wall were obtained simultaneously with transmural blood flow measurements (microspheres) under basal conditions and during pacing at 200 and 240 beats per minute. LV mass and end-diastolic volume were significantly increased after DC shock (33% and 26%, respectively, each P < .01). Under basal conditions, the subendocardial creatine phosphate (CP)/ATP ratio was significantly lower in remodeled LV compared with the control group (1.71 +/- 0.09 versus 2.04 +/- 0.09, P < .05). The subendocardial CP/ATP ratio was inversely correlated with both the increase in LV mass and LV end-diastolic volume (r = -.77 and r = -.70, P < .01 and P < .05, respectively). In remodeled myocardium, pacing induced a significant increase in LV end-diastolic pressure (from 8 +/- 1 to 20 +/- 3 mm Hg, P < .05), which was accompanied by a significant decrease of subendocardial/subepicardial (Endo/Epi) blood flow ratio (from 1.01 +/- 0.10 to 0.63 +/- 0.11, P < .05) and a significant decrease in subendocardial CP/ATP ratio (from 1.78 +/- 0.07 to 1.61 +/- 0.10, P < .05) and increase of delta P(i)/ATP ratio (from 0 to 0.24 +/- 0.05, P < .01). The decrease in subendocardial CP/ATP ratio was correlated with the decrease in Endo/Epi blood flow ratio (r = .79, P < .05).

CONCLUSIONS

These results demonstrate that alterations in myocardial high-energy phosphate levels are correlated with the extent of LV remodeling. In remodeled hearts, pacing-induced tachycardia produces further changes of myocardial high-energy phosphate levels in the subendocardium that appear to be related to ventricular dysfunction and redistribution of blood flow away from the subendocardium.

Effects of dobutamine on myocardial blood flow, contractile function, and bioenergetic responses distal to coronary stenosis: implications with regard to dobutamine stress testing

To determine the effects of dobutamine stimulation on myocardium distal to a coronary stenosis, transmural spatially localized phosphorus 31 nuclear magnetic resonance measurements of myocardial high-energy phosphate compounds (adenosine triphosphate and phosphocreatine), inorganic phosphate, and blood flow and systolic wall thickening were made in 8 open-chested dogs. Data were collected under (1) control conditions, (2) after the application of a moderate coronary stenosis, (3) during infusion of dobutamine with continuing stenosis, and (4) after the release of the stenosis with continuing dobutamine. Stenosis was associated with concordant reductions of subendocardial blood flow, wall thickening, and high-energy phosphate, and mild elevation of inorganic phosphate; subepicardial measurements were essentially unchanged. During dobutamine infusion, blood flow increased in all myocardial layers. Wall thickening returned to control values in the subendocardium and increased nonsignificantly in the subepicardium. Additional loss of high-energy phosphate occurred only in the subepicardium. The data suggest that improved contractile function associated with dobutamine infusion resulted from the inotropic effects of dobutamine and was made possible by the improved blood flow it produced. The data indicate that measurements of blood flow and contractile function do not reliably predict the transmural myocardial metabolic responses to inotropic perturbations in the hypoperfused heart. Taken together, the present findings yield insights with regard to the interpretation of diagnostic dobutamine stimulation testing with single photon emission tomography, radionuclide angiography, and echocardiography.

Transmural distribution of 2-deoxyglucose uptake in normal and post-ischemic canine myocardium

This investigation was performed to determine (i) whether 31P spatially localized 31P NMR spectroscopy could be utilized to determine the transmural distribution of 2-deoxyglucose (2DG) uptake in the in vivo canine heart and (ii) whether transmural 2DG uptake would be affected by a preceding ischemic insult. 2DG was infused and the accumulation of 2-deoxyglucose-6-phosphate (2DGP) was monitored (by means of spatially localized 31P NMR) in control hearts, in pharmacologically hyperperfused hearts, and in hearts subjected to four (5 min) occlusions of the left anterior descending coronary artery. Myocardial blood flow was measured with radioactive microspheres. In control hearts, subendocardial (ENDO) 2DGP contents were significantly higher than those in the subepicardium (EPI) being 3.8 +/- 0.3 and 2.2 +/- 0.2 mumol/g, respectively; the ENDO/EPI ratio of 2DGP was 1.70 +/- 0.21. During hyperperfusion blood flow increased approximately four-fold but 2DGP accumulation was not altered. ATP levels in post-ischemic myocardium were significantly decreased (ENDO more than EPI) and 2DGP accumulation in each layer was increased (p < 0.01 vs control); however, the ENDO/EPI ratio of 2DGP was not altered. 2DG infusion induced a marked elevation of blood insulin and norepinephrine levels. These data demonstrate that in the presence of high blood levels of 2DG and insulin: (i) 2DGP accumulation can be measured in the in vivo canine heart; (ii) in normal hearts 2DG uptake is more pronounced in the inner layers of the left ventricular wall (this transmural 2DG uptake gradient is not due to subendocardial hypoperfusion); and (iii) 2DG uptake is greater in the post-ischemic heart but the ENDO/EPI gradient of 2DG uptake is not altered indicating that the more severe ischemic insult in the subendocardium does not result in a disproportionate increase in 2DG uptake in that region of the myocardium. Although 2DG uptake patterns in this model most probably reflect those of glucose (at comparable glucose and insulin levels), quantitative extrapolations with regard to the rate of glucose uptake are not possible from the present data.

In vivo human myocardial metabolism during aerobic exercise by phosphorus-31 nuclear magnetic resonance spectroscopy

A few studies have been made in vivo on human myocardial energy metabolism. Hence, no discussion has taken place on metabolism during exercise or of training effects on metabolism. We examined human myocardial energy metabolism at rest and during exercise, and also training effects on the metabolism by phosphorus-31 nuclear magnetic resonance (31P NMR)-spectroscopy. Six sedentary male students (Cont) and six male long distance runners (Tr) were the subjects. Energy metabolism data were obtained from myocardium during rest and exercise by the region selection method using 31P NMR. Rotation of the legs while riding a bicycle, which was fitted with an ergometer we had made ourselves for NMR, imposed given exercise intensities. The heart rate was measured in a stationary phase during exercise. Although the heart rate at rest in the Tr group was significantly lower [Tr, 52.5 (SD 3.1) beat.min-1; Cont, 67.1 (SD 2.9) beat.min-1], no significant difference was observed in myocardial energy metabolism using the 31P NMR method [Tr, phosphocreatine/beta-adenosine 5'-triphosphate (PCr/beta-ATP); 1.51 (SD 0.02); Cont, 1.51 (SD 0.01)]. When NMR measurements were investigated at two different intensities of exercise, heart rates in the Cont group were significantly higher by about 20 beat.min-1 than those in the Tr group at both exercise intensities, while no difference in energy metabolism was observed between the groups or between rest and exercise [Tr, 75.9 (SD 3.6), 88.3 (SD 3.7) beat.min-1; PCr/beta-ATP 1.51 (SD 0.03), 1.51 (SD 0.03); Cont, 95.9 (SD 2.4), 115.1 (SD 3.5) beat.min-1, PCr/beta-ATP 1.51 (SD 0.01), 1.51 (SD 0.04)].(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial bioenergetic abnormalities in a canine model of left ventricular dysfunction

OBJECTIVES

The purpose of this study was to assess high energy phosphate compound metabolism in remodeled left ventricular myocardium.

BACKGROUND

The development of heart failure several years after myocardial infarction is often unexplained. Certain abnormalities of remodeled myocardium suggest that structural changes occurring in viable myocardium after discrete myocardial damage may contribute to the later appearance of heart failure. Whether these abnormalities alter metabolism in the surviving muscle and thereby possibly contribute to ventricular dysfunction is unknown.

METHODS

High energy phosphate compound metabolism was assessed using spatially localized phosphorus-31 nuclear magnetic resonance spectroscopy. Eleven dogs with documented left ventricular dysfunction, resulting from infarction produced by transmyocardial direct current shock, were compared with eight normal dogs. Analyses were performed at baseline and during coronary hyperperfusion induced by intravenous adenosine. Myocardial blood flow was measured with radioactive microspheres.

RESULTS

The creatine phosphate/adenosine triphosphate (CP/ATP) ratio was significantly reduced in the left ventricular dysfunction group in both the subepicardium ([mean +/- SE] 1.94 +/- 0.08 vs. 2.32 +/- 0.13, p = 0.019) and the subendocardium (1.71 +/- 0.07 vs. 2.05 +/- 0.07, p = 0.004). Intravenous adenosine produced significant coronary hyperemia in both groups but was less marked in dogs with left ventricular dysfunction. The improvement in myocardial perfusion was accompanied by a significant increase in the subendocardial CP/ATP ratio (from 1.71 +/- 0.07 to 1.92 +/- 0.08, p = 0.01) in dogs with left ventricular dysfunction.

CONCLUSIONS

An abnormal transmural distribution of high energy phosphate compounds is evident in remodeled myocardium. This abnormality may be related in part to mismatch of oxygen delivery and demand.

Transmural saturation transfer analysis of the creatine kinase system in the mammalian heart

31P NMR spatial localization and saturation transfer techniques were combined to enable the transmural measurement of the forward creatine kinase (CK) rate (ATP:creatine N-phosphotransferase, EC 2.7.3.2.) in the in vivo canine myocardium. Five epicardial towards endocardial regions of the left ventricle (LV) were simultaneously examined using spatially localized voxels. Although intraleft ventricular CP/ATP ratios were constant, the pseudo first order rate constant (k') and the forward creatine kinase rate (Rf) displayed a 61% variation across the LV wall. Because CK levels and calculated [ADP], [CP] and pH are transmurally invariant in the normal left ventricle, the observed changes in the Rf could not be explained by changes in the absolute levels of these substrates and of creatine kinase. In addition, because myocardial oxygen consumption rates are known to be higher in the endocardium, these results imply that forward creatine kinase rates are not directly related to oxidative phosphorylation rates.

Responses of myocardial high energy phosphates and wall thickening to prolonged regional hypoperfusion induced by subtotal coronary stenosis

The response of the myocardium to prolonged or chronic ischemia may differ from the well documented changes that occur acutely subsequent to the onset of hypoperfusion. Therefore, we have examined in an instrumented canine model and using spatially localized spectroscopy to achieve transmural differentiation, the myocardial HEP and Pi levels as well as wall thickening in situ during prolonged ischemia induced by sustained coronary artery stenosis. The results demonstrate that subtotal coronary artery occlusion causes immediate and transmurally inhomogeneous decreases in the myocardial HEP content and increase in the Pi/CP ratio; however, during prolonged mild hypoperfusion, metabolic changes occur which lead to statistically significant recovery of CP (but not ATP) and disappearance of Pi despite the persistence of reduced blood flow and oxygen supply. Upon release of the occlusion, the previously ischemic muscle recovered blood flow, and some (but not all) of its preischemic contractile function without parallel changes in the HEP levels. It is concluded that normal HEP and Pi levels cannot be equated with either the absence of underperfusion or insensitivity of NMR spectroscopy to ischemia. Rather, it is imperative that both functional and spectroscopic measurements are performed simultaneously to distinguish between ischemic myocardium which is adapted versus unadapted to the hypoperfusion.

31P magnetic resonance spectroscopy in dilated cardiomyopathy and coronary artery disease. Altered cardiac high-energy phosphate metabolism in heart failure

BACKGROUND

The purpose of this work was to further define the value of cardiac 31P magnetic resonance (MR) spectroscopy for patients with coronary artery disease and dilated cardiomyopathy.

METHODS AND RESULTS

Blood-corrected and T1-corrected 31P MR spectra of anteroseptal myocardium were obtained at rest using image-selected in vivo spectroscopy localization, a selected volume of 85 +/- 12 cm3, and a field strength of 1.5 T. Nineteen volunteers had a creatine phosphate (CP)/ATP ratio of 1.95 +/- 0.45 (mean +/- SD) and a PDE/ATP ratio of 1.06 +/- 0.53; in four patients with left anterior descending coronary artery (LAD) stenosis, six patients with chronic anterior wall infarction, and four patients with chronic posterior wall infarction, CP/ATP and phosphodiester (PDE)/ATP ratios did not differ from those in volunteers. Twenty-five measurements of 19 patients with dilated cardiomyopathy yielded a CP/ATP of 1.78 +/- 0.51 and a PDE/ATP of 0.98 +/- 0.56 (p = NS versus volunteers). When these patients were grouped according to the severity of heart failure, however, CP/ATP was 1.94 +/- 0.43 in mild (p = NS versus volunteers) and 1.44 +/- 0.52 in severe DCM (p < 0.05), respectively. No correlation was found between CP/ATP and left ventricular ejection fraction or fractional shortening, but correlation of CP/ATP with the New York Heart Association (NYHA) class was significant (r = 0.60, p < 0.005). Six patients with dilated cardiomyopathy were studied repeatedly before and after 12 +/- 6 weeks of drug treatment leading to clinical recompensation with improvement of the NYHA status by 0.8 +/- 0.3 classes. Concomitantly, CP/ATP increased from 1.51 +/- 0.32 to 2.15 +/- 0.27 (p < 0.01), whereas PDE/ATP did not change significantly.

CONCLUSIONS

Cardiac high-energy phosphate metabolism at rest is normal in LAD stenosis and chronic myocardial infarction in the absence of heart failure. The CP/ATP ratio has low specificity for the diagnosis of dilated cardiomyopathy. However, CP/ATP correlated with the clinical severity of heart failure and may improve during clinical recompensation.

31P NMR spectroscopy of the human heart at 4 T: detection of substantially uncontaminated cardiac spectra and differentiation of subepicardium and subendocardium

31P NMR spectroscopy of the human heart was undertaken at 4 T to investigate whether spectra localized exclusively to the myocardium can be obtained. Utilizing a Fourier series window approach to spectroscopic imaging, we find that at least two layers across the anterior left ventricle wall can be detected, with voxel sizes of about 8 cm3.

Complementarity of magnetic resonance spectroscopy, positron emission tomography and single photon emission tomography for the in vivo investigation of human cardiac metabolism and neurotransmission

The three techniques allowing the noninvasive study of cardiac metabolism, namely magnetic resonance spectroscopy (MRS), positron emission tomography (PET) and single photon emission computed tomography (SPET), all use external detection with stable or radioactive isotopes. These techniques yield different information. PET is quantitative and very sensitive, and therefore only tracer amounts of molecules need to be injected. It allows neurotransmitters and receptors to be studied and a global view of metabolism (oxygen consumption, glucose and fatty acid utilization) to be obtained. SPET also has good sensitivity, but uses gamma-emitting isotopes of heteroatoms. Their longer half-lives allow follow-up for hours or days. MRS is based on stable elements with high (hydrogen 1, phosphorus 31, fluorine 19...) or low (carbon 13, Deuterium) natural abundance. It has very low sensitivity and only millimolar concentrations of substrates can be detected, but various parts of metabolism can be studied. The in vivo measurement of myocardial concentration of substances has many problems that are common to all three techniques (measurement of the volume, measurement of the quantity of each molecule, resolution, partial volume effect, improvement of the signal-to-noise ratio, movement of the organ). The complementarity of the techniques is illustrated by their applications to the study of cardiac metabolism. For instance, the energy metabolism can be studied by 31P-MRS, which detects the high-energy compounds ATP and phosphocreatine, and 13C-MRS yields information on the tricarboxylic acid cycle activity. PET and SPET allow the utilization of fatty acids, the normal fuels of the heart, to be studied. During ischaemia, PET with 18F-fluorodeoxyglucose (18FDG) can determine the glucose consumption and 1H-MRS shows the increase in lactic acid, reflecting anaerobic glycolysis. Comparison of the use of acetate labelled with 11C for PET or 13C for MRS shows the potentials and limitations of each technique. Myocardial perfusion can be evaluated directly with various PET tracers or indirectly with thallium 201 or various technetium-99m-labelled tracers by SPET. No MRS marker of perfusion is so far clinically available. Mainly SPET and PET are used clinically for the investigation of ischaemic heart disease as well as cardiomyopathies, but some initial results using 31P-MRS are being obtained.

Quantitation of the extent of acute myocardial infarction by phosphorus-31 nuclear magnetic resonance spectroscopy

Phosphorus-31 nuclear magnetic resonance (P-31 NMR) spectroscopy is able to identify alterations in myocardial high energy phosphate metabolism associated with acute infarction. It was hypothesized that the extent of acute myocardial infarction could be quantitated from changes in the tissue content of inorganic phosphate (Pi), phosphocreatine (PCr) and adenosine triphosphate (ATP) derived from P-31 NMR spectra. Nine isolated, perfused rat hearts were studied at 121.5 MHz. After baseline spectra were obtained, varying locations of either the right or the left coronary artery were occluded without removing the heart from the spectrometer. Spectra were then collected during regional ischemia at 15 and 45 min after occlusion. Phosphate metabolites were quantitated from the baseline and 45-min regional ischemia spectra, times at which the metabolites are at steady state for the normal and ischemic conditions. The heart was removed from the spectrometer, perfused for a total duration of 2 h and sectioned into 2-mm thick slices for triphenyltetrazolium chloride staining. Percent infarct was determined by manual tracing of magnified, digitized images of the stained sections. Coronary blood flow, heart rate and blood pressure were monitored throughout the experiment. Significant linear relations were found between percent infarct (by triphenyltetrazolium chloride staining) and the percent change of beta-ATP (r = -0.74), Pi (r = 0.83) and the PCr/Pi ratio (r = -0.71) at 45 min after coronary occlusion. Coronary flow was also found to correlate significantly with percent infarct (r = -0.70). These results are applicable to in vivo P-31 NMR studies of acute infarction where the volume of interest may include both normal and acutely infarcted myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Hexagonal surface gradient coil for localized MRS of the heart

A surface gradient coil designed to obtain localized magnetic resonance spectroscopy of the heart in vivo is described. Images and 31P spectra from phantoms and both pig and dog hearts in vivo are shown. The coil was used in conjunction with a rf surface coil to obtain 31P spectra from transmural sections of the left ventricular wall of a dog heart in vivo to demonstrate differences between normal and ischemic tissue.

Transmural distribution of extracellular purines in isolated guinea pig heart

The purine adenosine appears to be involved in regulation of coronary vascular tone. Little is known concerning the levels and distribution of adenosine and related purines in the extracellular fluid of the heart. We have measured epicardial and endocardial levels of adenosine, inosine, hypoxanthine, AMP, and IMP in isolated constant flow perfused guinea pig hearts by using a recently developed technique with porous nylon sampling discs. Venous effluent purine levels were also measured. Concentrations of all purines measured, excluding IMP, were significantly higher in endocardial fluid samples than in epicardial fluid samples (P less than 0.05). Conversely, IMP levels were significantly lower in endocardial than in epicardial samples. The magnitude of the endocardial/epicardial ratios for adenosine, inosine, hypoxanthine, AMP, and IMP were approximately 12:1, 4:1, 5:1, 4:1, and 1:2, respectively. To assess cellular damage, lactate dehydrogenase activity was measured in all fluid samples and was not significantly different in endocardial and epicardial fluid. These data support the existence of significant transmural gradients for extracellular purine levels in crystalloid perfused guinea pig hearts. Transmural differences in vasoactive adenosine levels may be partially due to the greater endocardial oxygen consumption and metabolism and may be involved in maintaining relatively high subendocardial blood flows in the face of high intramyocardial pressures.

Simultaneous cardiac mechanics and phosphorus-31 NMR spectroscopy during global myocardial ischemia and reperfusion in the intact dog

To investigate the high-energy phosphate metabolic correlates of left ventricular (LV) dysfunction during the onset and recovery from severe, global myocardial ischemia in vivo, seven preinstrumented closed-chest dogs had ECG-gated phosphorus-31 (31P) NMR-spectroscopy (NMR-S) studies performed and LV micromanometer and sonomicrometer data measured before, during, and every 5 min following severe occlusive global myocardial ischemia. Ischemic LV + dP/dtmax fell from 2396 +/- 576 mm Hg/s at baseline to 2185 +/- 478 mm Hg/s (p less than 0.05) and did not normalize until after 30 min of reperfusion. LV ejection fraction (EF) decreased significantly (0.32 +/- 0.07 EF units to 0.12 +/- 0.13 EF units; p less than 0.05) and did not recover by 30 min of reperfusion (0.27 +/- 0.09 units; P less than 0.05 vs baseline). Simultaneous 31P NMR-S studies demonstrated excellent beta-ATP signal-to-noise (10 +/- 4:1). Myocardial acidosis occurred during global ischemia (delta pH = -0.22 +/- 0.23 units; p less than 0.05), with recovery at 30 min of reperfusion. Inorganic phosphate/phosphocreatine ratio (Pi/PCr) increased significantly during ischemia (0.46 +/- 0.07 to 0.61 +/- 0.07; P less than 0.05), with delayed normalization of this ratio at 30 min of reperfusion. beta-ATP peak area did not change during ischemia. Pi/PCr and LV contractility (+dP/dtmax) were significantly correlated at baseline (r = -0.70) and during global ischemia (r = -0.78; p less than 0.01), but not during recovery (r = 0.006; p = NS). Therefore, the simultaneous evaluation of high-fidelity hemodynamic data and topical 31P NMR-S can be performed in the intact state.

Regional myocardial metabolism of high-energy phosphates during isometric exercise in patients with coronary artery disease

BACKGROUND

The maintenance of cellular levels of high-energy phosphates is required for myocardial function and preservation. In animals, severe myocardial ischemia is characterized by the rapid loss of phosphocreatine and a decrease in the ratio of phosphocreatine to ATP.

METHODS

To determine whether ischemic metabolic changes are detectable in humans, we recorded spatially localized phosphorus-31 nuclear-magnetic-resonance (31P NMR) spectra from the anterior myocardium before, during, and after isometric hand-grip exercise.

RESULTS

The mean (+/- SD) ratio of phosphocreatine to ATP in the left ventricular wall when subjects were at rest was 1.72 +/- 0.15 in normal subjects (n = 11) and 1.59 +/- 0.31 in patients with nonischemic heart disease (n = 9), and the ratio did not change during hand-grip exercise in either group. However, in patients with coronary heart disease and ischemia due to severe stenosis (greater than or equal to 70 percent) of the left anterior descending or left main coronary arteries (n = 16), the ratio decreased from 1.45 +/- 0.31 at rest to 0.91 +/- 0.24 during exercise (P less than 0.001) and recovered to 1.27 +/- 0.38 two minutes after exercise. Only three patients with coronary heart disease had clinical symptoms of ischemia during exercise. Repeat exercise testing in five patients after revascularization yielded values of 1.60 +/- 0.20 at rest and 1.62 +/- 0.18 during exercise (P not significant), as compared with 1.51 +/- 0.19 at rest and 1.02 +/- 0.26 during exercise before revascularization (P less than 0.02).

CONCLUSIONS

The decrease in the ratio of phosphocreatine to ATP during hand-grip exercise in patients with myocardial ischemia reflects a transient imbalance between oxygen supply and demand in myocardium with compromised blood flow. Exercise testing with 31P NMR is a useful method of assessing the effect of ischemia on myocardial metabolism of high-energy phosphates and of monitoring the response to treatment.

Correlation between transmural high energy phosphate levels and myocardial blood flow in the presence of graded coronary stenosis

Spatially localized nuclear magnetic resonance spectroscopy was used to investigate with transmural differentiation the response of myocardial high energy phosphate compounds and inorganic orthophosphate (Pi) to graded reductions in coronary blood flow caused by sustained coronary stenosis. In an open-chest model, localized 31P nuclear magnetic resonance spectra from five layers across the left ventricular wall were obtained simultaneously with transmural blood flow measurements during control conditions and during sustained graded reductions in intracoronary pressure. Both the blood flow, and high energy phosphate and Pi contents displayed transmural heterogeneity in response to decreases in intracoronary pressure. The subendocardial creatine phosphate (CP) level remained unchanged as blood flow was reduced to approximately 0.7 ml/min/g wet wt and decreased precipitously beyond this critical flow level. The relation between CP and flow in the midmyocardium and especially in the subepicardium was more complex. Subepicardial CP content did not correlate well with blood flow; however, in cases in which a coronary stenosis resulted in subendocardial hypoperfusion but subepicardial flow was near or above normal, a close correlation was present between subepicardial and subendocardial CP levels. ATP levels in all layers remained unaltered until blood flow was severely reduced. These results demonstrate that 1) the myocardial high energy phosphate and Pi levels at any transmural layer are not generally determined by O2 and blood flow limitation under basal conditions; 2) during subtotal coronary occlusion, increased oxygen extraction is able to meet myocardial needs until a critical level of stenosis is reached; 3) below a critical flow level, subendocardial CP and Pi contents are closely correlated with absolute subendocardial blood flow; and 4) in the presence of a coronary stenosis, subepicardial CP and Pi contents may change even in the absence of perfusion deficit secondary to loss of subendocardial function.

Measurement of ATP synthesis rates by 31P-NMR spectroscopy in the intact myocardium in vivo

The ability to measure ATP synthesis rates using 31P-NMR spectroscopy is demonstrated in the normal, ischemic, and postischemic myocardium in vivo. Cardiopulmonary bypass (CBP) was employed to induce 20 min of global myocardial ischemia, and to conduct magnetization transfer measurements during the ischemic episode and following reperfusion and return to normal circulation. For the first few minutes of ischemia, transfer of magnetization from ATP gamma to Pi was extensive and the resultant fractional reduction (delta M/M0) in the Pi resonance intensity reached approximately 100%. Subsequent to reperfusion and stabilization off CPB and on normal circulation, both the fractional reduction and the spin-lattice relaxation time, T1*, of the Pi resonance were determined when ATP gamma spins were saturated. Under these conditions, the unidirectional ATP synthesis rate was 0.41 +/- 0.09 (SEM, N = 4) mumol/s/g wet wt. The data suggest that in the canine myocardium in vivo, glycolytic enzymes mediate a very rapid exchange between Pi and ATP gamma-phosphates during early phases of ischemia; in the postischemic reperfused myocardium, however, the glycolytic contribution to the unidirectional Pi----ATP rate measured by NMR in vivo is relatively small compared to that observed in glucose-perfused, postischemic rat hearts.

In vivo phosphorus-31 NMR spectroscopy of abnormal myocardial high-energy phosphate metabolism during cardiac stress in hypertensive-hypertrophied non-human primates

To study the functional and metabolic correlates of left ventricular hypertrophy [LVH] in non-human primates, 7 hypertensive baboons [papio anubis] with 4.6 +/- 0.1 years of hypertension produced by a two-kidney one-clip model, and echocardiographically documented concentric LVH underwent serial phosphorus-31 [P-31] NMR Spectroscopy studies at rest and during inotropic cardiac stress produced by dobutamine infusion [5 micrograms/kg/minute]. Responses in LVH baboons were compared to those in 5 normotensive, sex and weight-matched control animals. The ratio of P-31 NMR-S derived inorganic phosphates [Pi] to phosphocreatine [PCr] was significantly greater at rest in LVH baboons [0.53 +/- 0.06 versus controls = 0.41 +/- 0.17; P less than 0.05]. With dobutamine drug stress, the Pi/PCr ratio rose significantly in LVH baboons [0.77 +/- 0.15 versus 0.56 +/- 0.16; P less than 0.05 at 15 minutes]. Despite hemodynamic recovery, the 5 minute post-dobutamine Pi/PCr ratio remained elevated compared to baseline in LVH baboons only [0.78 +/- 0.16 versus 0.53 +/- 0.06; P less than 0.05]. In pre-instrumented baboons [n = 5], the 'transfer function' of cardiac work [heart rate x LV end-systolic pressure x + dp/dt max] versus Pi/PCr ratio was abnormally shifted rightward and downward [r = 0.80] with LVH as compared to the linearly increasing response in controls. We conclude that in vivo P-31 NMR Spectroscopy studies during dobutamine stress demonstrate reduced PCr stores, delayed metabolic recovery following cessation of inotropic stress, and an abnormal rightward shift in the 'transfer function' in LVH baboons.

Epicardial and endocardial localized 31P magnetic resonance spectroscopy: evidence for metabolic heterogeneity during regional ischemia

Previous studies have noted that myocardial blood flow and high energy phosphates are heterogeneous across the myocardial wall during ischemia. In order to determine whether differences in metabolites between the subendocardium and subepicardium could be detected using 31P magnetic resonance spectroscopy, the Fourier series window (FSW) experiment was implemented on a porcine model of graded regional ischemia. FSW experiments using a planar phantom showed a 46% improvement in localization to the subendocardium compared to a one-pulse experiment. Animal studies of graded ischemia demonstrated a gradient in the phosphocreatine to inorganic phosphate ratio in the myocardium that paralleled the gradient in blood flow. These studies demonstrate the ability of spatially localized 31P magnetic resonance spectroscopy to detect regional changes in myocardial high energy phosphates localized to the subepicardium and subendocardium.