Spectroscopic imaging and spatial localization using adiabatic pulses and applications to detect transmural metabolite distribution in the canine heart

ATP sensitive K(+) channels are critical for maintaining myocardial perfusion and high energy phosphates in the failing heart

Congestive heart failure (CHF) is associated with intrinsic alterations of mitochondrial oxidative phosphorylation which lead to increased myocardial cytosolic free ADP. ATP sensitive K(+) channels (KATP) act as metabolic sensors that are important for maintaining coronary blood flow (MBF) and in mediating the response of the myocardium to stress. Coronary adenosine receptors (AdR) are not normally active but cause vasodilation during myocardial ischemia. This study examined the myocardial energetic response to inhibition of KATP and AdR in CHF. CHF (as evidenced by LVEDP>20mmHg) was produced in adult mongrel dogs (n=12) by rapid ventricular pacing for 4weeks. MBF was measured with radiolabeled microspheres during baseline (BL), AdR blockade with 8-phenyltheophylline (8-PT; 5mg/kg iv), and KATP blockade with glibenclamide (GLB; 20μg/kg/min ic). High energy phosphates were examined with (31)P magnetic resonance spectroscopy (MRS) while myocardial oxygenation was assessed from the deoxymyoglobin signal (Mb-δ) using (1)H MRS. During basal conditions the phosphocreatine (PCr)/ATP ratio (1.73±0.15) was significantly lower than in previously studied normal dogs (2.42±0.11) although Mb-δ was undetectable. 8-PT caused ≈21% increase in MBF with no change in PCr/ATP. GLB caused a 33±0.1% decrease in MBF with a decrease in PCr/ATP from 1.65±0.17 to 1.11±0.11 (p<0.0001). GLB did not change the pseudo-first-order rate constant of ATP production via CK (kf), but the ATP production rate via CK was reduced by 35±0.08%; this was accompanied by an increase in Pi/PCr and appearance of a Mb-δ signal indicating tissue hypoxia. Thus, in the failing heart the balance between myocardial ATP demands and oxygen delivery is critically dependent on functioning KATP channels.

Myocytes oxygenation and high energy phosphate levels during hypoxia

Decrease of ambient oxygen level has been used in myocytes culture experiments in examining the responsiveness to stress secondary to hypoxia. However, none of these studies measure the myocytes oxygenation levels resulting in ambiguity as to whether there is insufficient oxygen delivery. This study examined the hypothesis that at a basal myocardial work state, adequate myocyte oxygenation would be maintained until extremely low arterial pO2 levels were reached. Myocyte pO2 values in normal dogs were calculated from the myocardial deoxymyoglobin (Mb- δ) levels using (1)H-spectroscopy (MRS) and were normalized to Mb-δ obtained after complete LAD occlusion. During Protocol 1 (n = 6), Mb-δ was measured during sequential reductions of the oxygen fraction of inspired gas (FIO2) from 40, 21, 15, 10, and 5%, while in protocol 2 (n = 10) Mb-δ was measured at FIO2 of 3%. Protocol 3 (n = 9) evaluated time course of Mb-δ during prolonged exposure to FIO2 of 5%. Myocardial blood flow (MBF) was measured with microspheres and high energy phosphate (HEP) levels were determined with (31)P-MRS. MVO2 progressively increased in response to the progressive reduction of FIO2 that is accompanied by increased LV pressure, heart rate, and MBF. Mb-δ was undetectable during FIO2 values of 21, 15, 10, and 5%. However, FIO2 of 3% or prolonged exposure to FIO2 of 5% caused progressive increases of Mb-δ which were associated with decreases of PCr, ATP and the PCr/ATP ratio, as well as increases of inorganic phosphate. The intracellular PO2 values for 20% reductions of PCr and ATP were approximately 7.4 and 1.9 mmHg, respectively. These data demonstrate that in the in vivo system over a wide range of FIO2 and arterial pO2 levels, the myocyte pO2 values remain well above the K(m) value with respect to cytochrome oxidase, and oxygen availability does not limit mitochondrial oxidative phosphorylation at 5% FIO2.

Phosphate metabolite concentrations and ATP hydrolysis potential in normal and ischaemic hearts

To understand how cardiac ATP and CrP remain stable with changes in work rate - a phenomenon that has eluded mechanistic explanation for decades - data from (31)phosphate-magnetic resonance spectroscopy ((31)P-MRS) are analysed to estimate cytoplasmic and mitochondrial phosphate metabolite concentrations in the normal state, during high cardiac workstates, during acute ischaemia and reactive hyperaemic recovery. Analysis is based on simulating distributed heterogeneous oxygen transport in the myocardium integrated with a detailed model of cardiac energy metabolism. The model predicts that baseline myocardial free inorganic phosphate (P(i)) concentration in the canine myocyte cytoplasm - a variable not accessible to direct non-invasive measurement - is approximately 0.29 mm and increases to 2.3 mm near maximal cardiac oxygen consumption. During acute ischaemia (from ligation of the left anterior descending artery) P(i) increases to approximately 3.1 mm and ATP consumption in the ischaemic tissue is reduced quickly to less than half its baseline value before the creatine phosphate (CrP) pool is 18% depleted. It is determined from these experiments that the maximal rate of oxygen consumption of the heart is an emergent property and is limited not simply by the maximal rate of ATP synthesis, but by the maximal rate at which ATP can be synthesized at a potential at which it can be utilized. The critical free energy of ATP hydrolysis for cardiac contraction that is consistent with these findings is approximately -63.5 kJ mol(-1). Based on theoretical findings, we hypothesize that inorganic phosphate is both the primary feedback signal for stimulating oxidative phosphorylation in vivo and also the most significant product of ATP hydrolysis in limiting the capacity of the heart to hydrolyse ATP in vivo. Due to the lack of precise quantification of P(i) in vivo, these hypotheses and associated model predictions remain to be carefully tested experimentally.

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.

Acute Effects of Febuxostat, a Nonpurine Selective Inhibitor of Xanthine Oxidase, in Pacing Induced Heart Failure

We investigated whether xanthine oxidase inhibition with febuxostat enhances left ventricular (LV) function and improves myocardial high energy phosphates (HEP) in dogs with pacing-induced heart failure (CHF). Febuxostat (2.2 mg/kg over 10 minutes followed by 0.06 mg/kg/min) caused no change of LV function or myocardial oxygen consumption (MVO2) at rest or during treadmill exercise in normal dogs. In dogs with CHF, febuxostat increased LV dP/dtmax at rest and during heavy exercise (P < 0.05), indicating improved LV function with no change of MVO2. Myocardial adenosine triphosphate (ATP) and phosphocreatine (PCr) were examined using 31P nuclear magnetic resonance spectroscopy in the open chest state. In normal dogs, febuxostat increased PCr/ATP during basal conditions and during high workload produced by dobutamine + dopamine (P < 0.05). PCr/ATP was decreased in animals with CHF; in these animals, febuxostat (given after completing basal and high workload measurements with vehicle) tended to increase PCr/ATP during basal conditions with no effect during catecholamine stimulation. Thus, febuxostat improved LV performance in awake dogs with CHF, but caused only a trend toward increased PCr/ATP in the open chest state. It is possible that the antecedent high workload condition prior to drug administration blunted the effect of febuxostat on HEP in the CHF animals. Alternatively, beneficial effects of febuxostat on LV performance in the failing heart may not involve HEP.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Myocardial oxygenation and high-energy phosphate levels during KATP channel blockade

Inhibition of ATP-sensitive K+ (KATP) channel activity has previously been demonstrated to result in coronary vasoconstriction with decreased myocardial blood flow and loss of phosphocreatine (PCr). This study was performed to determine whether the high-energy phosphate abnormality during KATP channel blockade can be ascribed to oxygen insufficiency. Myocardial blood flow and oxygen extraction were measured in open-chest dogs during KATP channel blockade with intracoronary glibenclamide, whereas high-energy phosphates were examined with 31P magnetic resonance spectroscopy (MRS), and myocardial deoxymyoglobin (Mb-delta) was determined with 1H MRS. Glibenclamide resulted in a 20 +/- 8% decrease of myocardial blood flow that was associated with a loss of phosphocreatine (PCr) and accumulation of inorganic phosphate. Mb-delta was undetectable during basal conditions but increased to 58 +/- 5% of total myoglobin during glibenclamide administration. This degree of myoglobin desaturation during glibenclamide was far greater than we previously observed during a similar reduction of blood flow produced by a coronary stenosis (22% of myoglobin deoxygenated during stenosis). The findings suggest that reduction of coronary blood flow with an arterial stenosis was associated with a decrease of myocardial energy demands and that this response to hypoperfusion was inhibited by KATP channel blockade.

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.

Effects of augmented delivery of pyruvate on myocardial high-energy phosphate metabolism at high workstate

This study was performed to determine whether the fall in myocardial high-energy phosphates (HEP) that occurs during high workstates can be ascribed to either inadequate glycolytic pyruvate generation and conversion to acyl-CoA or limitation of long-chain fatty acid transport into the mitochondria. This was tested by using infusions of either pyruvate or butyrate in anesthetized dogs. Pyruvate was used because it bypasses the glycolytic sequence of reactions, activates pyruvate dehydrogenase, and increases mitochondrial NADH concentration ([NADH(m)]) in isolated myocardium, whereas butyrate enters the mitochondria without need for transport by the rate-limiting, palmitoyl-carnitine transporter. Increasing blood pyruvate from 0.16 +/- 0.016 mM to >3 mM did not alter baseline HEP levels determined with (31)P nuclear magnetic resonance, but caused an increase in the rate-pressure product and a modest increase in myocardial oxygen consumption (MVO(2)). Infusion of dobutamine + dopamine (each 20 microg x kg(-1) x min(-1) iv) increased MVO(2) and caused decreases of myocardial phosphocreatine (PCr)/ATP. Pyruvate partially reversed the decrease of HEP levels produced by catecholamine stimulation, whereas butyrate had no effect. Neither pyruvate nor butyrate caused an increase of MVO(2) during catecholamine infusion. Deoxymyoglobin was not detected by (1)H magnetic resonance spectroscopyy in any group. The data demonstrate that carbon substrate availability to the mitochondria is not the only cause of the reduction of PCr/ATP that occurs at high workstates. Supplemental pyruvate (but not butyrate) attenuated the reduction of PCr/ATP during the high workstates; this may have resulted from direct effects on intermediary metabolism or from other effects such as the free radical scavenging activity of pyruvate.

Mitochondrial ATPase and high-energy phosphates in failing hearts

This study examined high-energy phosphates (HEP) and mitochondrial ATPase protein expression in hearts in which myocardial infarction resulted in either compensated left ventricular remodeling (LVR) or congestive heart failure (CHF). The response of HEP (measured via (31)P magnetic resonance spectroscopy) to a modest increase in the cardiac work state produced by dobutamine-dopamine infusion and pacing (if needed) was examined in 17 pigs after left circumflex coronary artery ligation (9 with LVR and 8 with CHF) and compared with 7 normal pigs. In hearts with LVR, the baseline phosphocreatine (PCr)-to-ATP ratio decreased, and calculated ADP increased; these changes were most severe in hearts with CHF. HEP levels did not change in normal or LVR hearts during dobutamine-dopamine infusion. However, in hearts with CHF, the PCr-to-ATP ratio decreased further, and free ADP increased. The mitochondrial protein levels of the F(0)F(1)-ATPase subunits were normal in hearts with compensated LVR. However, in failing hearts, the alpha-subunit decreased by 36%, the beta-subunit decreased by 16%, the oligomycin sensitivity-conferring protein subunit decreased by 40%, and the initiation factor 1 subunit decreased by 41%. Thus in failing hearts, reductions in mitochondrial F(0)F(1)-ATPase protein expression are associated with increased myocardial free ADP.

Myocardial creatine kinase kinetics and isoform expression in hearts with severe LV hypertrophy

Left ventricular (LV) hypertrophy (LVH) results in a fetal shift in myocardial creatine kinase (CK) expression. Because CK plays an important role in intracellular energy production, transport, and utilization, this study was performed to characterize changes in CK expression and CK flux in severe pressure-overload LVH. Ascending aortic banding in 8-wk-old dogs resulted in LVH with a 92% increase in relative LV mass. In LVH hearts, CK-M isoform mRNA was decreased by 40% (P = 0.05) and protein was decreased by 50% (P < 0.01), whereas mitochondrial CK protein was decreased by 22% (P < 0.05). CK-B isoform mRNA was undetectable in normal hearts but was prominently expressed in LVH (P < 0.01); CK-B protein was increased by more than 10-fold in LVH (P < 0.01). Despite these changes, total CK activity was normal in LVH. Myocardial CK flux was examined using (31)P magnetic resonance spectroscopy magnetization transfer. The CK forward rate constant was similar in normal and LVH hearts at baseline and did not change in either group during dobutamine treatment. In hearts with LVH, the CK forward flux rate was reduced by approximately 60% (P < 0.05) and decreased further during dobutamine. Thus, although pressure-overload LVH caused alterations of expression of both CK mRNA and protein levels, LV performance and oxygen consumption in response to dobutamine were normal. However, myocardial free ADP was increased in LVH hearts. This finding suggests that the CK alterations result in a need for higher ADP levels to maintain ATP synthesis in the hypertrophied heart.

Advances in human cardiac 31P-MR spectroscopy: SLOOP and clinical applications

Phosphorus magnetic resonance spectroscopy (31P-MRS) has revealed a lot about the biochemistry of physiological and pathological processes in the heart. Nevertheless, until today, cardiac 31P-MRS has not had any clinical impact, albeit some pioneering studies demonstrated that 31P-MRS can indeed provide diagnostic information. In this paper, the development of techniques for human cardiac 31P-MRS over the past decade is reviewed, and the requirements for a reliable clinical measurement protocol are discussed. Spatial localization with optimal pointspread function (SLOOP) is a new method to achieve spatial localization and absolute quantitation. Its properties are detailed, and preliminary findings in patients with dilated cardiomyopathy or myocardial infarction are presented.

High-energy phosphate metabolism and creatine kinase in failing hearts: a new porcine model

BACKGROUND

This study aimed to create a pig model of heart failure secondary to severe aortic stenosis and to examine the relationship between the alterations in myocardial high-energy phosphate (HEP) metabolism and protein expression of creatine kinase (CK) isoforms.

METHODS AND RESULTS

Sixteen pigs with left ventricular hypertrophy (LVH) secondary to ascending aortic banding and 10 normal pigs (N) were studied. Myocardial protein levels of CK isoforms (Western blot), HEP levels, and CK kinetics ((31)P MR spectroscopy) were measured under basal conditions. Nine of the 16 animals with LVH developed congestive heart failure (CHF), as evidenced by ascites (100 to 2000 mL). LV weight/body weight ratio (g/kg) was 2.18+/-0.15 in N hearts, 3.04+/-0.14 in hearts with LVH (P<0.01), and 4.23+/-0.36 in hearts with CHF (P<0.01 versus LVH). Right ventricle weight/body weight ratio and LV end-diastolic pressure were significantly higher in hearts with CHF (each P<0.01 versus N or LVH). Myocardial phosphocreatine/ATP ratios and the CK forward flux rates were decreased in LVH hearts, most severely in hearts with CHF. CK-M/beta-actin ratios were 2.21+/-12 (N), 1.69+/-0.15 (LVH), and 1.39+/-0.27 (CHF, P<0.05 versus N). CK-mitochondria (CK-Mt)/beta-actin ratios were 1.40+/-0.09 (N), 1.24+/-0.09 (LVH), and 1.02+/-0.08 (CHF, P<0.05 versus N or LVH). The severity of the reduction of CK flux rate was linearly related to the severity of the decrease of CK-Mt/beta-actin (r=0.68, P<0.01).

CONCLUSIONS

In this new model of heart failure/hypertrophy, the abnormal myocardial HEP metabolism is related to the decreased CK-Mt protein level, which in turn is related to the severity of the hypertrophy.

Myocardial oxygenation and high-energy phosphate levels during graded coronary hypoperfusion

This study was performed to determine the myocyte PO(2) required to sustain normal high-energy phosphate (HEP) levels in the in vivo heart. In 10 normal dogs, myocyte PO(2) values were calculated from the myocardial deoxymyoglobin resonance (Mb-delta) intensity determined with (1)H-NMR spectroscopy during sequential flow reductions produced by a hydraulic occluder that decreased coronary perfusion pressure to approximately 60, 50, and 40 mmHg and, finally, during total occlusion. Myocardial blood flow was measured with microspheres, and HEP levels were determined with (31)P magnetic resonance spectroscopy. During control conditions, Mb-delta was undetectable. Myocardial blood flow was 1.11 +/- 0.06 ml. min(-1). g(-1) during basal conditions and decreased with sequential graded occlusions to 0.78 +/- 0.05, 0.58 +/- 0.03, and 0.38 +/- 0.04 ml. min(-1). g(-1), respectively; blood flow during total occlusion was 0.07 +/- 0.02 ml. min(-1). g(-1). Reductions of blood flow caused progressive increases of Mb-delta, which were associated with decreases of phosphocreatine (PCr), ATP, and the PCr-to-ATP ratio, as well as progressive increases of the P(i)-to-PCr ratio. There was a strong linear correlation between normalized blood flow and Mb-delta (R(2) = 0.89, P < 0.01). Reductions of HEP and PO(2) were also highly correlated (although nonlinearly); with the assumption that myoglobin was 90% saturated with O(2) during basal conditions and 5% saturated during total coronary occlusion, the intracellular PO(2) values for 20% reductions of PCr and ATP were approximately 4. 4 and approximately 0.9 mmHg, respectively. The data indicate that O(2) availability plays an increasing role in regulation of oxidative phosphorylation when mean intracellular PO(2) values fall below 5 mmHg in the in vivo heart.

Noninvasive measurements of transmural myocardial metabolites using 3-D (31)P NMR spectroscopy

A completely noninvasive three-dimensional (3-D) static magnetic field magnitude spatially localized (31)P spectroscopy technique has been developed and applied to study the in vivo canine myocardium at 9.4 T. The technique incorporates both Fourier series windows and selective Fourier transform methods utilizing all three orthogonal gradients for 3-D phase encoding. The number of data acquisitions for each phase-encoding step was weighted according to the Fourier coefficients to define cylindrical voxels. Spatially localized (31)P spectra can be generated for voxels of desired location within the field of view as a postprocessing step. The quality of localization was first demonstrated by using a three-compartment phantom. The technique was then applied to in vivo canine models and yielded (31)P cardiac spectra with an excellent signal-to-noise ratio. The in vivo validation experiments, using an implanted 2-phosphoenolpyruvate-containing marker, demonstrated that the technique is capable of measuring at least two transmural layers of left ventricular myocardium representing the subepicardium and subendocardium.

Detection and quantification of free cytosolic inorganic phosphate and other phosphorus metabolites in the beating mouse heart muscle in situ

The aim of this study was the quantification of inorganic phosphate (Pi) and other phosphorus metabolites by (31)P NMR spectroscopy in the mouse heart muscle in situ, beating at around 600 min(-1). Male adult Quacker-bush mice (mean weight 32 +/- 7 g) were anaesthetized, ventilated and placed in a temperature-controlled animal holder. A purpose-built (31)P NMR surface coil was positioned against the exposed left ventricular myocardium. Partial signal overlap of Pi with 2,3-DPG from chamber blood was minimized using a DEPTH pulse sequence (180 degrees -90 degrees -180 degrees -180 degrees -acq.). Quantification of phosphorus metabolites was performed using an external standard positioned directly above the surface coil. We report for the mouse myocardium in situ an intracellular free [Pi] of <0.4 mM, pH of 7.32 +/- 0.1, free [Mg2+] of 0.41 +/- 0.1 mM, free [ADP] of 13 +/- 1.5 microM, [ATP] of 5 +/- 0.5 mM and [PCr] of 14 +/- 1.5 mM. The phosphorylation ratio (ATP/ADP Pi) was 1005 +/- 200 mM (-1) for a PCr/ATP ratio of 2.7 +/- 0.3. It was concluded that the detection of free [Pi] in the mouse myocardium in situ can be greatly enhanced using a DEPTH pulse sequence. Quantification of compounds using an external standard positioned directly above the surface coil gave comparable results to estimations using internal ATP that was quantified enzymatically. The close agreement between the external and internal methods indicates that ATP is 100% NMR visible in the mouse heart in situ.

Flow requirements in ventricular fibrillation: An in vivo nuclear magnetic resonance analysis of the left ventricular high-energy phosphate pool

STUDY OBJECTIVE

We sought to determine whether flow rates of approximately 60% of normal values are sufficient to preserve the left ventricular myocardial high-energy phosphate pool during ventricular fibrillation (VF).

METHODS

Mixed-breed swine (weight 22. 4+/-2.5 kg) were anesthetized with alpha-chloralose, placed in a state of VF, and perfused with extracorporeal circulation at a target flow of 50 mL.kg(-1).min(-1). In vivo whole-wall (average of left ventricular wall) and spatially localized phosphorous-31 nuclear magnetic resonance (NMR) spectra were acquired at baseline and during VF.

RESULTS

Mean flow during VF was 58+/-20 mL.kg(-1). min(-1) (+/-SD; 95% confidence interval, 44 to 71) or about 60% of baseline cardiac output (n=13). Whole-wall adenosine triphosphate (ATP) decreased during perfused VF (P <.05), whereas creatine phosphate (CP) remained unchanged from baseline. With spatially localized NMR, the ratios of CP/ATP were similar at baseline in all layers (endocardium --> epicardium) of the left ventricular wall. However, during perfused VF, subepicardial CP/ATP ratios increased by 14% to 40% compared with baseline values, whereas subendocardial CP/ATP ratios remained unchanged (1% to 3% increase). An additional 4 animals perfused at 72+/-10 mL.kg(-1).min(-1) (+/-SD; 95% confidence interval, 56 to 92) during VF had preservation of CP and ATP levels.

CONCLUSION

Flow levels equivalent to 60% of baseline cardiac output were insufficient to maintain normal high-energy phosphate levels in the in vivo fibrillating myocardium. At this level of flow, myocardial high-energy phosphate loss is nonhomogeneous within the left ventricular wall.

An efficient MR phosphorous spectroscopic localization technique for studying ischemic heart

To obtain the spatially resolved (31)P spectroscopic image from myocardium during an acute myocardium ischemia at a high signal-to-noise ratio (SNR) in a very limited time window, we have exploited the spatial variation of the radiofrequency (RF) field produced by a single loop transmit/receive (TR) RF coil along its axis for spatial discrimination. By incrementally lengthening the duration of a square RF excitation pulse, the positional information can be systematically encoded as harmonics of various orders in MR signal. In the in vivo open-chest animal experiment, this RF coil was surgically sutured onto the epicardial surface of the left ventricular (LV) wall over the region perfused by the left anterior descending coronary artery. Using only 17 encoding steps, we have obtained one-dimensional (31)P spectroscopic images from both a multiple-layer phosphor phantom and an in vivo LV myocardium. In the animal study, the cardiac gating is used with respiratory synchronization. The MR data were only collected during the end diastole phase of the cardiac cycle (cardiac and respiratory synchronized) with an effective sequence repetition time (TR) of 6 seconds (to ensure the complete relaxation of the phosphorous magnetization). The total acquisition time for a complete experiment is about 10 minutes. Prior to the CSI reconstruction process, the raw data matrix was zero-filled in the spatial dimension. The spatially resolved metabolite map exhibited all the metabolite peaks including creatine phosphate and adenosine triphosphate. At the layer of endocardium, two peaks corresponding to 2, 3-diphosphoglycerate, which is contained in the erythrocytes, were clearly seen in the LV wall. Also, the method allows compensation in both volume and coil sensitivity variations for the resulting spectra. All results have demonstrated that it is an efficient nuclear magnetic resonance method capable of obtaining high-quality (31)P spectroscopic images with both excellent spatial localization and SNR in the research of cardiac ischemia. J. Magn. Reson. Imaging 1999;10:892-898.

Transmural metabolic heterogeneity at high cardiac work states

This study compared the transmural distribution of high-energy phosphate (HEP) depletion during oxidative stress induced by pacing- and dobutamine-induced tachycardia in myocardium perfused by a flow-limiting coronary stenosis. Myocardial blood flow (MBF) was measured with radioactive microspheres. Creatine phosphate (CrP), ATP, and P(i) were measured with transmurally localized (31)P NMR spectroscopy. In normal dogs a hydraulic occluder was used to produce a left anterior descending coronary artery stenosis, which maintained constant flow measured with a Doppler probe. Tachycardia was induced by rapid pacing (200 beats/min, n = 11) or by dobutamine infusion (20 micrograms . kg(-1). min(-1) iv, n = 13) to produce a similar heart rate. In the presence of stenosis, pacing and dobutamine caused similar reductions of subendocardial (Endo)-to-subepicardial (Epi) MBF ratios (0.66 +/- 0.06 vs. 0.63 +/- 0.08, respectively). Stenosis plus pacing caused a decrease of the CrP-to-ATP ratio (CrP/ATP) in Endo from 2.00 +/- 0.07 to 1.65 +/- 0. 08 (P < 0.05) with no significant change in Epi. Stenosis plus dobutamine caused HEP changes across the left ventricular wall, which were most marked in the outer myocardial layer (Epi CrP/ATP decreased from 2.33 +/- 0.11 to 1.67 +/- 0.12; Endo CrP/ATP decreased from 1.99 +/- 0.09 to 1.64 +/- 0.12). Thus HEP changes during oxidative stress that are produced by pacing parallel the pattern of hypoperfusion and are most severe in the subendocardium. In contrast, in response to inotropic stimulation, the transmural metabolic changes did not correspond to the pattern of the hypoperfusion.

Oxygen delivery does not limit cardiac performance during high work states

This study tested the hypothesis that the loss of myocardial high-energy phosphates (HEP), which occurs during high cardiac work states [J. Zhang, D. J. Duncker, Y. Xu, Y. Zhang, G. Path, H. Merkle, K. Hendrich, A. H. L. From, R. Bache, and K. Uğurbil. Am. J. Physiol. 268: (Heart Circ. Physiol. 37): H1891-H1905, 1995], is not the result of insufficient intracellular O(2) availability. To evaluate the state of myocardial oxygenation, the proximal histidine signal of deoxymyoglobin (Mb-delta) was determined with (1)H nuclear magnetic resonance spectroscopy (MRS), whereas HEP were examined with (31)P MRS. Normal dogs (n = 11) were studied under basal conditions and during combined infusion of dobutamine and dopamine (20 micrograms . kg(-1). min(-1) iv each), which increased rate-pressure products to >50,000 mmHg. beats. min(-1). Creatine phosphate (CP) was expressed as CP/ATP, and myocardial myoglobin desaturation was normalized to the Mb-delta resonance present during total coronary artery occlusion. This Mb-delta resonance appeared at 71 parts per million downfield from the water resonance. CP/ATP decreased from 2. 22 +/- 0.12 during the basal state to 1.83 +/- 0.09 during the high work state (P < 0.01), whereas DeltaP(i)/CP increased from 0 to 0.21 +/- 0.04 (P < 0.01). Despite these HEP changes, Mb-delta remained undetectable. In contrast, when a coronary stenosis was applied to produce a similar decrease in CP/ATP, Mb-delta reached 0.38 +/- 0.10 of the value present during total coronary occlusion. These data demonstrate that Mb-delta is readily detected in vivo during limitation of coronary blood flow sufficient to cause a decrease of myocardial CP/ATP. However, similar HEP changes that occur at high work states in the absence of coronary occlusion are not associated with a detectable Mb-delta resonance. The findings support the hypothesis that the myocardial HEP changes observed at high work states are not due to inadequate O(2) availability to the mitochondria and emphasize the limitations of interpreting HEP alterations in the absence of knowing the level of myocyte oxygenation.

Myocardial oxygenation during high work states in hearts with postinfarction remodeling

BACKGROUND

Postinfarction left ventricular remodeling (LVR) is associated with reductions in myocardial high-energy phosphate (HEP) levels, which are more severe in animals that develop overt congestive heart failure (CHF). During high work states, further HEP loss occurs, which suggests demand-induced ischemia. This study tested the hypothesis that inadequate myocyte oxygen availability is the basis for these HEP abnormalities.

METHODS AND RESULTS

Myocardial infarction was produced by left circumflex coronary artery ligation in swine. Studies were performed in 20 normal animals, 14 animals with compensated LVR, and 9 animals with CHF. Phosphocreatine (PCr)/ATP was determined with 31P NMR and deoxymyoglobin (Mb-delta) with 1H NMR in myocardium remote from the infarct. Basal PCr/ATP tended to be decreased in postinfarct hearts, and this was significant in animals with CHF. Infusion of dobutamine (20 microg x kg-1 x min-1 IV) caused doubling of the rate-pressure product in both normal and LVR hearts and resulted in comparable significant decreases of PCr/ATP in both groups. This decrease in PCr/ATP was not associated with detectable Mb-delta. In CHF hearts, rate-pressure product increased only 40% in response to dobutamine; this attenuated response also was not associated with detectable Mb-delta.

CONCLUSIONS

Thus, the decrease of PCr/ATP during dobutamine infusion is not the result of insufficient myocardial oxygen availability. Furthermore, in CHF hearts, the low basal PCr/ATP and the attenuated response to dobutamine occurred in the absence of myocardial hypoxia, indicating that the HEP and contractile abnormalities were not the result of insufficient oxygen availability.

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.

Relationships between myocardial bioenergetic and left ventricular function in hearts with volume-overload hypertrophy

BACKGROUND

Left ventricular (LV) hypertrophy secondary to volume overload can result in alterations in myocardial bioenergetics and LV dysfunction. This study examined whether bioenergetic abnormalities contribute to the pump dysfunction.

METHODS AND RESULTS

Severe mitral regurgitation (MR) was produced in 10 dogs by disruption of the chordal apparatus. Hemodynamics and ventricular function were examined 11.7 months later under baseline conditions and during treadmill exercise. Myocardial high-energy phosphates were measured by using magnetic resonance spectroscopy at rest, during coronary vasodilation with adenosine, and during oxidative stress induced by rapid pacing and dobutamine. Chronic MR caused a 30% increase in LV mass and a 65% increase in LV volume. In MR animals, the hemodynamic and LV function were normal at rest, but abnormalities developed during beta-blockade and exercise. Myocardial creatine phosphate-to-ATP ratios were significantly lower in each layer across the LV wall in MR hearts than normal hearts. Myocardial blood flow and coronary reserve were normal in MR hearts. Moreover, hyperperfusion did not correct the abnormal bioenergetics. Despite altered bioenergetics at rest, the MR hearts tolerated rapid pacing and dobutamine infusion well.

CONCLUSIONS

In volume-overloaded LV hypertrophied hearts, alterations in myocardial high-energy phosphate levels do not induce abnormal mechanical performance at rest but may be related to a decreased contractile reserve during exercise.

Spatial localization with modified Fourier series windows. Application to the transmural 13C-nuclear magnetic resonance analysis of the in vivo myocardium

RATIONALE AND OBJECTIVES

A modified Fourier Series Window (FSW) method is introduced that provides a simple, reasonably accurate, solution to spatial localization for layers in nuclear magnetic resonance (NMR) spectroscopy. This method was developed because signal originating from spins immediately adjacent to the coil plane can leak into standard FSW localized spectra obtained from more distal layers.

METHODS

The B1 profile of the surface coil and a priori sample knowledge were used to generate modified FSW coefficients that largely compensate for contamination from proximal layers. These modified coefficients were used to acquire spatially localized spectra from a three-layered phantom containing inorganic phosphate, phosphocreatine, and pyrophosphate. Spatially localized spectra also were obtained from the open-chest canine myocardium.

RESULTS

The modified Fourier Series Window approach was validated using a quality assurance phantom. This method was then applied to the transmural analysis of 13C-containing metabolites in the in vivo canine myocardium during 3-(13)C sodium pyruvate infusion (n = 10). In vivo NMR spectra were characterized by resonances corresponding to the C2, C3, and C4 carbons of glutamic acid and to endogenous triacyglycerides and surface lipids. A transmural trend in metabolites could be observed under these conditions. This trend, however, was likely to result from the interference of surface lipids in the epicardial layer.

CONCLUSIONS

The authors demonstrate that the modified Fourier Series Window method can be applied in situations where signal-to-noise considerations limit the applicability of more sophisticated spatial localization methods. In addition, the authors report a slight gradient in the endogenous triacylglyceride resonance, which is likely to originate from the presence of surface lipids.

In vivo phosphorus spectroscopy of human skin

Skin 31P MRS measurements might detect metabolic damage from irradiation, chemotherapy, or ischemia. Although rat and cadaver data have demonstrated this potential (C.D. Cuono, et al., Plast. Reconstr. Surg. 81, 1-11 (1988), H.W. Klein, et al., Ann. Plast. Surg. 20, 547-551 (1988)), few studies of in vivo phosphorus human skin spectra have been published (A. Zemtsov, et al., J. Dermatol. Surg. Oncol. 15, 1207-1211 (1989), A Zemtsov, et al., J. Am. Acad. Dermatol. 30, 959-965 (1994)), and those likely reflect underlying muscle as much as skin. To separate 31P skin and muscle spectra, we have developed a unique two-layer "flotation" phantom for mapping coil sensitivity and an associated semiempirical two-power RF depth-resolved technique. Phantom and method have been applied in a study of 17 normal volunteers to obtain human in vivo 31P skin spectra uncompromised by muscle contamination and to quantitate ratios of major phosphometabolites. Skin results consistently showed low ratios of phosphocreatine (PCr) to adenosine triphosphate (ATP), high levels of phosphomonoester (PME), P(i), and phosphodiester (PDE) relative to PCr, and demonstrated a shift in pH toward greater alkalinity, compared to that with simultaneous muscle results.

Effect of left ventricular hypertrophy secondary to chronic pressure overload on transmural myocardial 2-deoxyglucose uptake. A 31P NMR spectroscopic study

BACKGROUND

This study tested the hypothesis that 31P nuclear magnetic resonance (NMR)-detectable 2-deoxyglucose (2DG) uptake is increased in chronically pressure-overloaded hypertrophied left ventricular myocardium.

METHODS AND RESULTS

31P NMR spectroscopy was used to determine the transmural distribution of high-energy phosphate levels and 2-deoxyglucose-6-phosphate (2DGP) accumulation during intracoronary infusion of 2DG (15 mumol.kg body wt-1.min-1) in eight normal dogs and in eight dogs with severe left ventricular hypertrophy (LVH) produced by ascending aortic banding. The ratio of LV weight to body weight was 8.25 +/- 0.65 g/kg in the LVH group compared with 4.35 +/- 0.11 g/kg in the normal group (P < .01). Myocardial ATP content was decreased by approximately 40% and phosphocreatine (PCr) by approximately 60% in LVH hearts. ATP values were transmurally uniform in LVH and normal hearts, whereas PCr was lower in the subendocardium (Endo) than the subepicardium (Epi) of both groups. The PCr/ATP ratio was lower in LVH hearts (1.72 +/- 0.05, 1.64 +/- 0.07, and 1.53 +/- 0.10 in Epi, midwall, and Endo, respectively) compared with normal hearts (2.36 +/- 0.05, 2.09 +/- 0.06, and 1.96 +/- 0.06; each P < .01 normal versus LVH). Arterial blood levels of glucose, insulin, and free fatty acids were comparable between groups, whereas arterial lactate and norepinephrine levels were significantly higher in the LVH group. 2DG infusion did not affect systemic hemodynamics or myocardial high-energy phosphate or inorganic phosphate levels in either group. At the end of 60 minutes of 2DG infusion, there was no detectable accumulation of 2DGP in the normal hearts. However, seven of the eight LVH hearts showed time-dependent accumulation of 2DGP, which was linearly related to the severity of hypertrophy (r = .90 for subendocardial 2DGP versus LV weight/body weight). A transmural gradient of 2DGP was present, with greatest accumulation in the subendocardium (3.3 +/- 1.6, 5.8 +/- 2.3, and 7.9 +/- 2.2 mumol/g in Epi, midwall, and Endo of the LVH hearts, respectively; P < .05 Epi versus Endo).

CONCLUSIONS

The pressure-overloaded hypertrophied left ventricle demonstrated increased accumulation of 2DGP detected with 31P NMR spectroscopy. Accumulation of 2DGP was positively correlated with the degree of hypertrophy and was most marked in the subendocardium.

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.

Sodium pentobarbital versus alpha-chloralose anesthesia. Experimental production of substantially different slopes in the transmural CP/ATP ratios within the left ventricle of the canine myocardium

BACKGROUND

Transmural analyses of the creatine phosphate (CP)/ATP ratio in various lamina of the canine myocardium have previously revealed significant variations in the CP/ATP ratio, with the subendocardial layer displaying a decreased ratio relative to the subepicardial layer. Without exception, these results were obtained under sodium pentobarbital anesthesia. These findings have been interpreted to imply that the normal endocardium may be operating in the oxygen-limited domain or that there are transmurally varying set points for the regulation of oxidative phosphorylation.

METHODS AND RESULTS

In this work, we examine the effect of the anesthetic regimen on the transmural CP/ATP ratio within the left ventricular wall of the canine myocardium using spatially localized 31P-nuclear magnetic resonance (NMR) and an open-chest model. Two anesthetics were compared, alpha-chloralose and sodium pentobarbital. Under sodium pentobarbital, the CP/ATP ratio ranged from 1.92 +/- 0.06 to 2.51 +/- 0.08 from endocardium to epicardium, resulting in a transmural slope in the CP/ATP ratio of 0.149 +/- 0.047 (n = 22). Under alpha-chloralose, CP/ATP ratios ranged from 2.18 +/- 0.05 to 2.32 +/- 0.06, with a transmural slope of 0.035 +/- 0.018 (n = 38). Thus, the transmural slope in CP/ATP ratio was nearly four times greater with sodium pentobarbital than with alpha-chloralose, and the difference in these slopes was statistically significant (P = .029). No difference was observed in average CP/ATP obtained from the entire wall with either anesthetic.

CONCLUSIONS

These results demonstrate that the transmural trend in CP/ATP ratio previously reported in the myocardium is likely to be a direct reflection of the sodium pentobarbital anesthetic regimen, not truly reflecting the trend in the normal unanesthetized animal. Moreover, since the transmural variation in CP/ATP ratio was greatly reduced with alpha-chloralose, it appears unlikely that the endocardium in the normal unanesthetized heart is operating in the oxygen-limited domain. These results also point to the importance of the anesthetic regimen in biochemical analysis, indicate the necessity of increased caution in directly translating results obtained under anesthesia, and demonstrate the unique power of in vivo NMR to extract such subtle biochemical information.

31P-nuclear magnetic resonance studies of chronic myocardial ischemia in the Yucatan micropig

In this work, an x-irradiation/high fat/high cholesterol diet-induced atherogenic model was invoked to examine the effects of severe diffuse atherosclerosis on myocardial metabolism in the in vivo porcine heart. This model was studied using spatially localized 31P-nuclear magnetic resonance (NMR) to monitor pH and the levels of inorganic phosphate, phosphomonoesters, creatine phosphate, and adenosine triphosphate as a function of workload transmurally in control swine and in animals suffering from chronic ischemic heart disease. These preliminary studies revealed that the development of severe atherosclerosis and the accompanying chronically diseased state produce changes in high energy phosphates and that increases in rate pressure products result in demonstrable signs of ischemia in the myocardium which span the entire left ventricular wall. Ischemic changes include a global increase in inorganic phosphate and corresponding decreases in creatine phosphate, ATP, and pH. Importantly, changes in intracellular pH are noted with even the slightest increase in workload suggesting that these diseased hearts display elevated glycolytic activity. By challenging these animals with increased cardiac workload, we directly visualize how the chronically compromised heart responds to severe oxygen challenges in a clinically relevant model of this situation.

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.

Bioenergetic abnormalities associated with severe left ventricular hypertrophy

Transmurally localized 31P-nuclear magnetic resonance spectroscopy (NMR) was used to study the effect of severe pressure overload left ventricular hypertrophy (LVH) on myocardial high energy phosphate content. Studies were performed on 8 normal dogs and 12 dogs with severe left ventricular hypertrophy produced by banding the ascending aorta at 8 wk of age. Spatially localized 31P-NMR spectroscopy provided measurements of the transmural distribution of myocardial ATP, phosphocreatine (CP), and inorganic phosphate (Pi); spectra were calibrated from measurements of ATP content in myocardial biopsies using HPLC. Blood flow was measured with microspheres. In hypertrophied hearts during basal conditions, ATP was decreased by 42%, CP by 58%, and the CP/ATP ratio by 32% in comparison with normal. Increasing myocardial blood flow with adenosine did not correct these abnormalities, indicating that they were not the result of persistent hypoperfusion. Atrial pacing at 200 and 240 beats per min caused no change in high energy phosphate content in normal hearts but resulted in further CP depletion with Pi accumulation in the inner left ventricular layers of the hypertrophied hearts. These changes were correlated with redistribution of blood flow away from the subendocardium in LVH hearts. These findings demonstrate that high energy phosphate levels and the CP/ATP ratio are significantly decreased in severe LVH. These abnormalities are proportional to the degree of hypertrophy but are not the result of persistent abnormalities of myocardial perfusion. In contrast, depletion of CP and accumulation of Pi during tachycardia in LVH are closely related to the pacing-induced perfusion abnormalities and likely reflect subendocardial ischemia.

Transaminase reaction rates, transport activities and TCA cycle analysis by post-steady state 13C NMR

In this work, we present the post-steady state analysis of the TCA cycle and a closed form solution to the rate of label washout from the C4 carbon of glutamic acid through the transaminases and the malate-aspartate shuttle and then through alpha-ketoglutarate dehydrogenase. We demonstrate using a model of this problem that the rate of label washout depends not only on the flux through alpha-ketoglutarate dehydrogenase, but most importantly on the activity of the malate-aspartate shuttle as determined by the forward and reverse fluxes through the transaminases and by the rate of transport of glutamate and alpha-ketoglutarate across the mitochondrial membrane.

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.

Spatially localized 31P NMR measurements of longitudinal relaxation rates in the canine myocardium

FLAX-ISIS spatial localization was combined with inversion recovery to enable the measurement of spatially localized T1 values. This approach was applied to the transmural determination of creatine phosphate longitudinal relaxation times in the canine myocardium. By examining five voxels spanning the left myocardial wall, we observed that transmural T1 values for creatine phosphate ranged from 3.61 +/- 0.20 in the endocardium to 4.00 +/- 0.20 in the epicardium at 4.7 Tesla. As such, the canine myocardium exhibits no transmural variation in the T1 values of creatine phosphate. This simple approach can be extended to enable the in vivo measurement of transmural enzymatic rates.

Improved cardiac gating and ventilation timing in animal experiments

In this work we present an improved solution for cardiac gating and ventilation timing in in vivo spectroscopic and imaging experiments. This approach makes use of a pulse conditioner to extract primary timing signals from noisy and fluctuating input signals. A timing controller is then used to set all additional timing outputs. Full feedback control is utilized in this timing controller to optimize both magnetic resonance and respiratory timing. Feedback control also helps ensure that timing errors will not occur during acquisition.

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.

Water-suppressed one-dimensional 1H NMR spectroscopic imaging of myocardial metabolites in vivo

Excitation by conventional B1 pulses in surface coil chemical-shift imaging experiments yields resonance intensities that vary greatly with position. B1-compensated semi-selective pulses overcome much of this problem. These pulses are employed for excitation and refocusing in 1H NMR spectroscopic localization in the intact heart.

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.

Hepatic D-galactosamine toxicity studied with localized in vivo 31P magnetic resonance spectroscopy in intact rats

Spatially resolved 31P magnetic resonance spectroscopy (MRS) at 4.7 T was applied to noninvasively assess liver phosphorus metabolites in a biochemically well-characterized model of hepatotoxicity induced by injection of a sublethal dose of D-galactosamine (galN). A newly developed hybrid method based on spectral localization with B0 and B1 gradients was employed to obtain multivoxel spectra in intact anesthesized rats. Spatially localized in vivo spectra were recorded 0 to 26 h after galN injection of female rats. In response to galN exposure, diphosphodiester peaks ascribed to UDP-hexosamines became detectable by 4 h and persisted up to 26 h. A metabolite coresonating with inorganic phosphate increased rapidly in intensity by 2 h after galN and returned to baseline by 18 h; this resonance was shown not to be Pi and was assigned to galN-1-phosphate by subsequent high resolution MRS experiments on extracts prepared from these livers. These results confirmed in vivo the metabolic perturbations described previously for this model of hepatotoxicity following biochemical studies based on classical extraction methods. Unlike the in vitro studies, however, these noninvasive experiments provided additional information on the time course of metabolic alterations on the same animal.

Spectroscopic imaging using variable angle excitation from adiabatic plane-rotation pulses

The technique of chemical-shift (spectroscopic) imaging has been implemented using a variable angle adiabatic plane-rotation pulse for small angle excitation over the entire sensitive volume of a surface coil. Optimum signal-to-noise ratio per unit time is obtained in applications where the pulse repetition rate is fast relative to 1/5 T1.

NMR determination of myocardial pH in vivo: separation of tissue inorganic phosphate from blood 2,3-DPG

Phosphorus NMR can measure myocardial tissue pH from the chemical shift of inorganic phosphate (Pi) in isolated buffer-perfused hearts, but in vivo the Pi peak originating from the myocardium is obscured by the resonance of 2,3-diphosphoglycerate (DPG) in the blood, making pH difficult to determine. Taking advantage of the fact that most of the interfering DPG is within the cardiac chambers and is rapidly flowing out of the sensitive volume of our coil, we developed a pulse sequence which would separate myocardial Pi signal from interfering DPG. We tested this method on a flow phantom and in living rat heart, using exogenous glycerol phosphate as a blood-pool marker. The results indicated that signal from moving and nonmoving substances could be separated, and derived values for myocardial pH and PCr/Pi ratio were consistent with previous estimates. This method should be useful for studying myocardial acid-base physiology with NMR.