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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Circulation. Author manuscript; available in PMC 2010 April 14.
Published in final edited form as:
PMCID: PMC2743337
NIHMSID: NIHMS126391

Reduced myocardial creatine kinase flux in human myocardial infarction: An in vivo phosphorus magnetic resonance spectroscopy study

Abstract

Background

Energy metabolism is essential for myocellular viability. The high-energy phosphates, ATP and phosphocreatine (PCr), are reduced in human myocardial infarction (MI), reflecting myocyte loss and/or decreased intracellular ATP generation by creatine kinase (CK), the heart's prime energy reserve. The pseudo-first-order CK rate-constant, k, measures intracellular CK reaction kinetics and is independent of myocyte number within sampled tissue. CK flux is defined as the product of [PCr] and k. CK flux and k have never been measured in human MI.

Methods and Results

Myocardial CK metabolite concentrations, k, and CK flux were measured noninvasively in fifteen patients 7-weeks to 16-years post-anterior MI using phosphorus magnetic resonance spectroscopy. In patients, mean myocardial [ATP] and [PCr] were 39-44% lower than in fifteen control subjects (PCr=5.4±1.2 in MI vs. 9.6±1.1 μmol/g.wet.wt. in controls, P<0.001; ATP=3.4±1.1 in MI vs. 5.5±1.3 μmol/g.wet.wt., P<0.001). The myocardial CK rate-constant, k, was normal in MI (0.31±0.08s-1) compared to controls (0.33±0.07 s-1), as was PCr/ATP (1.74±0.27 in MI vs. 1.87±0.45). However, CK flux was halved in MI (to 1.7±0.5 μmol[g.s]-1 vs. 3.3±0.8, P<0.001).

Conclusions

These first observations of CK kinetics in prior human MI demonstrate that CK ATP supply is significantly reduced due to substrate depletion, likely attributable to myocyte loss. That k and PCr/ATP are unchanged in MI, is consistent with the preservation of intracellular CK metabolism in surviving myocytes. Importantly, the results support therapies that primarily ameliorate the effects of tissue and substrate loss post-MI and that reduce energy demand, rather than those that increase energy transfer or workload in surviving tissue.

Keywords: myocardial infarction, creatine kinase, adenosine triphosphate, metabolism, magnetic resonance spectroscopy, energy supply

Introduction

Adenosine triphosphate (ATP) is essential for normal cardiac function, including myofibrillar contraction, ion transport, and myocyte viability1. ATP is generated at the myofibrils by the creatine kinase (CK) reaction, which transfers a high-energy phosphoryl group from phosphocreatine (PCr) to adenosine diphosphate (ADP)2. The movement of phosphoryl groups through CK, or CK flux, serves as the primary energy reserve in cardiac muscle and acts as both a spatial and temporal ATP buffer2,3. The CK flux is defined as the product of the PCr concentration with the pseudo-first-order forward reaction rate constant, k (s-1). This rate constant can be interpreted as the fraction of the PCr pool turning over to create ATP each second. k is a measure of the intracellular function of the CK reaction and as such, is independent of the number of myocytes within the studied sample.

Conventional quantitative, phosphorus (31P) magnetic resonance spectroscopy (MRS) provides noninvasive in vivo measures of cardiac concentrations of the high energy phosphates ATP and PCr4-8. Although CK flux can be measured by saturation transfer 31P MRS methods9-14, these were impractical in humans until recently, because of the inefficiency of the saturation transfer protocol, the need to provide spatial localization for a noninvasive clinical setting, and the need to combine the protocol with noninvasive metabolite concentration measurements in the same exam. We recently introduced the Four Angle Saturation Transfer (FAST) method to measure reaction rate constants with about an order-of-magnitude faster scan-time than the standard method15. FAST enables direct noninvasive localized measurements of the myocardial CK pseudo-first order rate constant, k, in addition to the CK flux when it is combined with concentration-referenced metabolite quantification methods4-7 performed during the same MRS exam. Using this technique, we reported a 50% reduction in CK flux in patients with non-ischemic dilated cardiomyopathy and mild-to-moderate chronic heart failure (CHF)16 and a 65% decrease in patients with pressure-overload left ventricular hypertrophy and CHF in the absence of coronary artery disease17.

Studies in patients with prior infarction using conventional quantitative 31P MRS demonstrate significant reductions in cardiac PCr and ATP concentrations as compared to concentrations in healthy controls5. This decrease may be due to myocyte loss and/or decreased mitochondrial high-energy phosphoryl generation and subsequent transfer, via CK flux, to myofibrils in surviving myocytes. We therefore used the FAST saturation transfer technology to test the hypothesis that CK flux and/or k is reduced in patients with prior myocardial infarction (MI) as compared with healthy controls, and report here for the first time, CK flux, its determinants k and [PCr], as well as [ATP], in the anterior left ventricular (LV) wall of these patients.

Methods

Study protocol

All human studies were approved by The Johns Hopkins Institutional Review Board for human investigation. All subjects gave informed consent following explanation of the study and protocol. Fifteen patients 31-83 years of age (mean, 57 ±16yr; median 57yr; 4 women) with a history of anterior, anterior-septal, anterior-lateral, or anterior-apical MI and echocardiographic anterior wall motion abnormalities were studied at rest on a GE 1.5T MRI/MRS system (General Electric Healthcare Technologies, Waukesha WI), 7 wk-16 yr post-MI (mean 6.4 ±6.0 yrs; median, 2.4 yr; n=14, one MI of unknown age). All of the patients were candidates for prophylactic implantable cardioverter defibrillators. Fifteen healthy subjects 31-60 years old (mean 41 ±7 yr, P =0.002 vs patients; median 41yr; 3 women) with no history of heart disease, hypertension or diabetes served as controls.

Owing to the duration of the MRS exam, a separate MRI exam that incorporated cine, myocardial tagging18, and late gadolinium enhancement (LGE)19 MRI protocols was performed on all patients within 24 hours of the MRS exam, to evaluate LV function and to determine MI location and size. The cine images were acquired using steady-state free precession (repetition time, TR =3.8 ms; echo time, TE =1.6 ms; flip-angle α =45°; field-of-view, FOV =36-40 cm; 8-mm slices; 2567×160 points; 40 ms temporal resolution). Short-axis LGE images were acquired 10-15 min after a bolus injection of 0.2 mmol/kg gadodiamide (Omniscan, Amersham Health, Princeton NJ), using an inversion recovery fast gradient-echo pulse sequence (TR =5.4 ms; TE =1.3 ms; inversion time, TI =150-250 ms to null the normal myocardial signal; 36-40 cm FOV with 256×192 points and 8-mm slices; number of excitations, NEX =2; α = 20°).

MRS studies were performed with subjects oriented prone on a 6.5 cm 31P receive/25 cm 31P transmit surface coil set. Conventional saturation transfer MRS requires two fully-relaxed acquisitions (TR[dbl greater-than sign]T1, the longitudinal relaxation time), with and without the γ-phosphate of ATP (γ-ATP) saturated, followed by a measurement of the T1 of PCr with γ-ATP saturated, for a total of 7-8 long-TR acquisitions which are impractical for localized human heart studies15. In contrast, FAST measures k with just four efficient short-TR (TR[double less-than sign]T1) acquisitions: with, and without γ-ATP saturated, and with two different pulse flip-angles15. The complete patient cardiac MRS protocol16 thus comprises: (i) conventional scout proton (1H) MRI acquired with the scanner's body coil to position the anterior myocardium over the coil, and for shimming; (ii) acquisition of the four 31P FAST data sets localized by one-dimensional chemical shift imaging (1D CSI; thirty-two trans-axial 1-cm thick slices; TR = 1 s; NEX = 12 and α = 60°, and NEX = 24 and α = 15° with chemical selective saturation at ±2.7 ppm)15; (iii) acquisition of a fifth 31P 1D CSI set with saturation turned-off (α = 60°; NEX = 12; gated with TR~ 1s) for phosphate metabolite quantification; and (iv) acquisition of a sixth 1H 1DCSI data set with the 31P coil (α = 60°; NEX = 4; gated with TR ~2s) to provide a water concentration reference for metabolite quantification6. The total MRS exam time was about 70 min. After the patient MRS exam, steps (iii) and (iv) were repeated, fully-relaxed (TR = 4s for 1H; 8s for 31P), on a phosphate reference phantom to calibrate the ratio of phosphate to proton signal for determining concentration from steps (iii) and (iv)6.

Data Analysis

Cine MRI was processed with the GE scanner's CINETOOL software to obtain LV ejection fraction (EF), cardiac volume, and mass by standard methods. Infarct size, measured in grams, was determined from the size of the region exhibiting LGE, defined as the region exhibiting an elevated signal intensity, as compared to peak remote signal19. The peak remote signal was determined by tracing the endocardial and epicardial borders in each short axis cross-section, and defining an approximately 50 mm2 region of interest (ROI) within normal, artifact-free, remote myocardium. The areas of LGE with signal intensities greater than the peak remote signal in each involved slice were added to obtain the total infarct size19. In addition, the transmural fraction of tissue occupied by hyper-enhancing infarction that was present in the tissue sampled by 31P MRS, was estimated by comparing LGE images with the scout images acquired during 31P MRS. Each short-axis LGE image was divided into 12 sectors, and the two pairs of sectors adjacent to the anterior right ventricular insertion site were used to assess the transmurality of the infarct in the antero-septal region sampled by 31P MRS as identified in the scout images.

The forward CK pseudo-first order rate constant, k in s-1, was calculated from the saturation spillover-corrected Equations 5, 6, and 9 of Ref. (15) based on the amplitude of the PCr signal in spectra acquired from MRS protocol steps (ii)-(iii) as a function of depth through the chest and anterior myocardium. These k values are directly comparable to those published earlier employing the same technique15-17. We also report rates, k*, that are corrected in accordance with the latest numerical analysis of errors due to spillover irradiation20:

k=k(k20.0534k0.0534)(0.2025Q2+0.0098Q0.2585),
(1)

where Q is the ratio of PCr measured in the FAST experiment with control saturation, and that measured with no saturating radiation whatsoever.

The 31P MRS protocol provided sufficient data to enable the metabolite concentrations, [PCr] and [ATP], where square brackets denote tissue concentrations in units of μmol/g wet wt. of tissue, to be determined noninvasively in two ways16,17. First, the concentrations were calculated from the ratio of the corresponding metabolite peak areas acquired in step (iii), to the water signal from step (iv), multiplied by the 1H/31P calibration factor, and the cardiac tissue water proton concentration6. The latter was taken as 86 mol/kg of tissue wet wt. based on a 77% water content, the same as in MI21 (edema resolves by 4-5 weeks after MI22). Second, concentrations were determined from the ratio of the saturation- and blood ATP-corrected metabolite signals, to the signal from the phosphate reference phantom4,5. For both methods, the metabolite signal areas were determined by Gaussian fitting and corrected for blood ATP and partial saturation using standard blood 2,3-diphosphoglycerate-to-ATP ratios and relaxation times23,24, assumed unchanged in these patients25. The two concentration estimates, which are not independent because they use the same 31P measurements, were then averaged to obtain a single value at each depth in the chest and anterior myocardium for each subject17. The forward CK flux, is then determined from the product, {k.[PCr]}, in μmol/g wet wt/s at each depth. Flux, k, and [PCr] values for the anterior myocardial wall are averaged from the 2-3 adjacent MRS slices intersecting the anterior myocardium, as identified from the corresponding scout MRI.

Results are presented as mean ± standard deviation (SD). Statistical significance was evaluated by two-tailed independent t-testing, or by paired t-testing where explicitly noted. Correlation coefficients for functional, morphologic, temporal and metabolic measures were calculated in the fifteen patients, and the significance of the correlations determined therefrom. A P<0.05 was considered significant.

The authors had full access to the data and take responsibility for its integrity. All authors have read and agree to the manuscript as written.

Results

Patients had evidence of LV remodeling post MI with mean LV EFs by MRI of 30 ±9%, mean LV end-diastolic (EDV) and end-systolic volumes (ESV) of 231 ±53 ml and 165 ±51 ml respectively, and LV masses of 142 ±58 g. Cine MRI and myocardial tagging confirmed the presence of anterior wall dysfunction (akinesis, dyskinesis) in all patients. Mean infarct mass by LGE MRI was 48 ±23 g, corresponding to 29 ±12% of the total LV mass.

Figure 1 shows typical scout MRI and localized 31P saturation transfer MRS results from a healthy control subject (Fig. 1b), and from a patient with a 12 year old anterior-apical MI, along with short-axis LGE images in which the infarct is rendered bright from the mid LV extending distally to the apex (Fig. 1c). In the 31P spectra (Fig 1b, c), the reduction in PCr height with γ-ATP saturated (right) as compared with control irradiation (left), is proportional to the CK pseudo-first-order rate constant, k. In the control subject (Fig 1b), this reduction corresponds to more than one third of the PCr turning over per second, with k = 0.38 s-1. Myocardial [PCr] was measured at 10.4 μmol/g wet wt, yielding a forward CK flux of 3.9 μmol(g.s)-1. The same k was measured in spectra from 2 slices in the MI patient (Fig. 1c). However, the lower signal-to-noise ratio in the patient spectra reflects a reduced metabolite concentration of 6.9 μmol/g for myocardial [PCr]. Consequently, the forward flux for generating ATP via CK is reduced to 2.6 μmol(g.s)-1 in this subject.

Figure 1
Typical MRI and localized 31P saturation transfer MRS results from a healthy control subject (M, 37 yrs; a, b), and a patient (M, 31 yrs) with a 12-year old anterior-apical MI of mass 38g, representing 31% of the LV (c-e). (a) Scout axial 1H MRI used ...

The measured 31P MRS PCr/ATP ratio, [PCr], k, and CK flux averaged across the anterior myocardium of MI patients and controls, are plotted in Fig. 2. Mean [PCr] shows a highly significant reduction in the anterior myocardium of MI patients to 56% of that in healthy subjects (P <0.001). [ATP] is similarly reduced to 61% (P <0.001 vs controls). If treated separately, the water and the phosphate reference calculations of [PCr] and [ATP] yield results for control subjects that do not differ significantly (P =0.1, paired t-test) as noted previously15, and both methods separately yield similar highly significant reductions in [PCr] and [ATP] to 47-66% in MI (P<0.001 vs controls). Despite the reduction in [PCr] and [ATP], neither the average PCr/ATP ratio, nor k, in MI patients differ statistically significantly from those of healthy subjects, although PCr/ATP trends lower. Consequently, the CK flux, which is the product of the reduced [PCr] and k, is also halved in MI compared with that in control subjects (P < 0.001). The mean metabolite concentrations, ratios, reaction rates, and CK fluxes are summarized in Table 1.

Figure 2
The average myocardial PCr/ATP ratio, [PCr], k, and CK flux (left to right) measured by 31P MRS in the anterior myocardium of MI patients (filled triangles) and controls (open circles). Crosses denote mean values with error bars (±1 SD; *P <0.001 ...
Table 1
Summary of 31P MRS findings

We also compared metabolic measures in adjacent superficial and deeper myocardial slices and found the same results, with an approximately 50% reduction in [PCr] at both levels in MI patients. Thus, in MI patients, [PCr] was 6.5 ±1.4 and 5.0 ±1.8 μmol/g in the superficial and deeper myocardial slices respectively, vs 10.5 ±1.7 and 9.2 ±1.8 μmol/g in controls (P<0.001). Nevertheless the CK reaction rate, k, in the same sections did not differ significantly between MI patients and controls (from superficial to deeper myocardium, k =0.32 ±0.16 and 0.36 ±0.2 s-1, respectively in MI vs 0.33 ±0.12 and 0.38 ±0.17 s-1 in controls). Thus CK flux was reduced in the anterior wall of patients with MI in proportion to the reduction in metabolite pools (PCr and ATP) with no significant reduction in k, interpreted as the fraction of PCr pool exchanging with ATP each second. Use of Eq. (1)20 for correcting k did not affect these results (k* in Table 1).

The fraction of hyper-enhancing tissue or infarct transmurality, present in the apical antero-septal region sampled by 31P MRS, as estimated from the LGE images, was 61±23%. As depicted in Fig. 3, this is comparable, although somewhat larger than the degree of [PCr] and [ATP] depletion (46±12%, P =0.04 and 41±20%, P =0.06, respectively).

Figure 3
The mean fraction of hyper-enhancing tissue or infarct transmurality, as % of antero-septal LV wall (black bar, left axis) in the apical antero-septal region sampled by 31P MRS, and the antero-septal LV [PCr] and [ATP] reductions, as % of mean values ...

Regional metabolic measures from the anterior wall generally did not correlate with global measures of cardiac mass, chamber size or EF. Cardiac [PCr] correlated modestly with LV EF (r =0.57, P =0.03; Fig. 4). No other significant correlations were found between anterior myocardial 31P MRS indices ([PCr], [ATP], PCr/ATP, k, CK flux) and global functional or morphologic MRI measures of lesion and chamber volumes (LV EF, LV EDV, LV ESV, lesion mass, lesion % of LV, lesion age) in patients, or 31P MRS indices with age in controls. However, trends at the 0.05 <P <0.1 level were evident in correlations between both [PCr] and CK flux with LV ESV (r = 0.46 and r =0.49, respectively), and between [ATP] and LV EF (r = 0.47) in patients.

Figure 4
The correlation between LV EF and anterior myocardial [PCr] (r =0.57, P =0.03). The line-of-best-fit to the data is computed using the method-of-least-squares.

Discussion

This is the first study of CK flux, the heart's prime energy reserve, in humans following MI. The primary finding is that there is a significant reduction in CK flux due to a decrease in [PCr] in these patients studied 7wk.-16yr. post-MI, but that the fraction of the PCr pool exchanging with ATP each second, k, is not changed, as compared with healthy controls. [ATP] is also decreased in patients with prior infarction, while the PCr/ATP ratio is not changed. These observations of reduced [ATP] and [PCr] with preserved k and PCr/ATP are consistent with metabolite loss in infarcted areas and preservation of nearly normal metabolism in residual non-infarcted tissues within the region interrogated by 31P MRS.

Our finding that PCr/ATP is not altered in patients with prior anterior MI is consistent with the first, albeit limited, human cardiac in vivo 31P MRS measurements in MI patients26, and several subsequent studies in larger patient groups27,28. However, reductions in cardiac PCr/ATP at the P <0.05 level are reported in some studies of MI5,29,30. The association of other conditions that may reduce myocardial PCr/ATP, such as dilated cardiomyopathy (DCM)23,27, LV hypertrophy (LVH)8,17,31-34 and/or CHF31,27,23,16, are possible confounding factors in these reports. More recently, myocardial PCr/ATP was reported to be 50% lower in the immediate border-zone of a porcine infarct model, as compared to that in remote regions of swine, suggesting that local energy defects may exist in the immediate peri-infarct region and contribute to local dysfunction14. Unlike human studies which must be performed noninvasively, the porcine myocardium was sampled with a small 31P detection coil applied directly to the infarct border, yielding much smaller sampling volumes (0.18 ml)14 than those presently achievable in human studies. Thus, at present, we can only conclude that bulk myocardial PCr/ATP in the relatively large sampling volume achievable in humans using present methodology is only minimally, if at all, reduced in the anterior myocardial wall of these patients with prior anterior MI.

Concentration measurements offer additional insight into cardiac metabolism, as an unchanged PCr/ATP ratio may be due to unaltered PCr and ATP concentrations, or to significant, but similar, changes in both metabolites. Our findings that [PCr] and [ATP] concentrations are significantly reduced but to similar extents in anterior MI, are consistent with mean reductions of 56% to 68% of normal values for [ATP] and [PCr], respectively, observed by noninvasive 31P MRS in patients with prior anterior MI and fixed defects on radionuclide images5; with measurements on biopsy specimens obtained from MI patients at surgery34; and with in vivo and in vitro canine studies7. It is important to note that these non-invasive techniques are measuring overall, bulk concentrations within the examined tissue, rather than direct intracellular concentrations. As such, the measurement of concentration, alone, as included here and in prior studies, cannot differentiate between reductions due to myocyte loss, reductions due to lower concentrations within individual surviving myocytes, or reductions arising from heterogeneous mixtures of infarcted and surviving myocytes with normal or abnormal metabolite concentrations.

This is the first report of the CK rate and flux in patients with prior MI. As noted above, the pseudo-first order forward reaction rate-constant k can be interpreted as the fraction of the PCr pool used to create ATP via the CK reaction each second, which is a measure of intracellular metabolic function. We have previously reported both k and CK flux, the product of k with [PCr], in the normal human heart at rest and stress16, in patients with nonischemic DCM with CHF16, and in those with pressure-overload LVH with and without CHF17. Those studies employed identical 31P MRS methods, are therefore directly comparable with the present work, and are summarized in Fig. 5. Like the present results for MI patients, no reduction in k was present in those patients with LVH who were not in CHF17, despite significant reductions in [PCr]. Thus, in at least two common cardiac conditions, bulk tissue PCr levels are depleted while the CK pseudo-first order rate constant k (Fig. 5a) is not changed. These new findings in patients with prior MI extend and are consistent with 31P MRS saturation transfer findings from a porcine model of MI13, which showed a significant reduction in [PCr] in all infarcted animals, but a reduction in the CK rate constant k at 6 weeks post-MI only in those animals with severe CHF, as evidenced by cyanosis and ascites13. None of our patients had severe CHF. Importantly, in both that animal study and now shown here for the first time in humans, low myocardial [PCr] significantly reduces forward CK flux–by nearly 50% in our study, and by 30% in the infarcted swine without CHF13.

Figure 5
(a) Comparison of FAST CK reaction rates, k, and (b) forward CK flux measured by FAST 31P MRS from current (hatched bars), and prior studies of normal subjects during a 200% dobutamine-induced increase in cardiac work load (“stress”)16 ...

The CK reaction is important to energy metabolism because of its ability to rapidly buffer ATP, and its putative role in shuttling high-energy phosphate in the form of PCr between the mitochondria, where ATP is produced by oxidative phosphorylation, and the myofibrils where it is consumed for contraction. The 50% reduction in CK flux to 1.7 ±0.5 μmol(g s)-1 (Table 1) observed here in patients with prior MI is comparable to values of 1.6 ±0.6 and 1.1 ±0.4 μmol(g s)-1 measured in CHF patients with non-ischemic global cardiomyopathies16,17. For those CHF patients it was noted that reductions to such levels may limit energy supply if CK were essential as a spatial/temporal energy buffer during periods of peak energy demand during the cardiac cycle and/or stress16,17.

In general, reductions in myocardial CK flux can be due to a loss of total enzyme activity, altered intracellular substrate levels or ratios, or to allosteric modifications of the enzyme. At this time there is no in vivo method for resolving bulk tissue changes in ATP and PCr concentrations due to myocyte loss, from those due to altered intracellular metabolite levels in surviving cells. However, k, as a measure of intracellular metabolic function, is not confounded by myocyte loss since it measures only the surviving cells that contribute to the 31P MRS signal. Thus, the observation that k, interpreted as the fraction of the intracellular PCr pool exchanging with ATP each second, is unaltered in patients with prior MI 7 wk-16 yr post-MI (Table 1), supports a hypothesis that intracellular CK reaction kinetics are essentially normal in the surviving myocytes. Our observation that myocardial PCr/ATP ratios are also preserved supports this view as well, considering that the observed macroscopic-level [PCr] and [ATP] reductions do not necessarily reflect depletion of intracellular [PCr] and [ATP] in surviving myocytes, and that intracellular ATP levels are highly regulated and maintained1. Therefore, the tissue reductions seen in [PCr] and [ATP] by 31P MRS are most likely attributable to a proportionate loss of myocytes, with the consequent macroscopic reduction in PCr substrate being responsible for the observed reduction in the CK flux in the region sampled.

Limitations

The main limitation in applying 31P MRS to relate metabolism and function in patients is its relatively low sensitivity and spatial resolution, for which higher field MRI/MRS scanners are a possible remedy35. To date this has generally limited clinical studies to anterior LV regions which are larger than those accessible to invasive studies in animal models, including the peri-infarct area, as noted above14. Indeed, additional signal contributions from surrounding normal myocardium may explain the difference between the 61% infarction estimate by MRI, and the 46% metabolite loss seen by 31P MRS (Fig. 3). Thus, the local reductions in metabolite concentrations and CK flux, are likely even larger than the changes reported here.

Note also that while it is possible to improve spatial resolution by signal averaging to improve the sensitivity, this inevitably reduces the number of different 31P MRS experiments that can be accommodated within a total exam time that is tolerable for most patients—about one hour. Loss of even one of the six spectral acquisitions in the present study would sacrifice a measure of either the concentration4,6 or the reaction kinetics15. For the present work separate MRI/MRS exams were performed to acquire all of the metabolic, functional and viability measures, so that a precise match between the MRI18,19 and the metabolic data was not feasible. Nevertheless, patients on average had large anterior MIs representing approximately 30% of the entire LV mass, and, based on scout MRI scans obtained in both studies, the infarcted region was interrogated by 31P MRS.

Because there was no significant correlation between [ATP], [PCr], PCr/ATP, or CK flux and age in controls, and because k was the same in both patients and controls and did not correlate with age, we do not expect that age confounds the primary findings. Indeed, none of the metabolic measures listed in Table 1 are altered by more than 8% if the youngest controls and oldest patients are excluded to render either no significant difference in age, or the same mean age for both groups. This includes the P values which do not change, except for [ATP] where P =0.002 in MI vs controls (for 8 patients 45 ±10 yrs vs 8 controls 45 ±7 yrs).

Clinical implications

These results demonstrate that the primary effect of MI on CK metabolism is reduced CK flux due to substrate depletion in the infarcted area, while the CK reaction rate constant remains intact. The substrate loss causes a reduction in the ATP delivered by CK that is comparable to that previously reported in dilated and hypertrophic cardiomyopathy. The findings that k and PCr/ATP are normal in patients 7-weeks or longer post-MI, are consistent with the hypothesis that CK metabolism is essentially intact in surviving myocytes. Importantly, the results support therapies that primarily ameliorate the effects of tissue loss on substrate depletion, and those that reduce energy demand in the post-infarcted heart, rather than those that affect energy transfer, which does not appear to be significantly depressed in surviving myocytes, or those that increase demand in the surviving tissue.

Acknowledgments

Funding sources: Supported by NIH grant 2RO1 HL56882 and by the Donald W. Reynolds Foundation.

Footnotes

Clinical Trial Registration Information: http://www.clinicaltrials.gov. Identifier: NCT00181233

Clinical Perspective: Contractile dysfunction in patients with myocardial infarction (MI) may be due to cell loss, abnormal metabolism in surviving tissue, or a mix of both. The creatine kinase (CK) reaction serves as the heart's major energy reserve, providing ATP via phosphocreatine (PCr) to fuel contractile function. The CK forward pseudo-rate-constant for generating ATP is a measure of intracellular metabolism and independent of the number of myocytes. To determine whether intracellular CK metabolism is preserved post-MI, we used, for the first time, a new magnetic resonance spectroscopy technique to directly measure the CK reaction rate constant, as well as the forward CK flux, the PCr and the ATP concentrations in patients with prior anterior MI. We find that the primary effect of MI on CK metabolism is a reduction in PCr and ATP which reduces the forward CK flux for generating ATP in the infarcted area. However, the CK reaction rate constant as well as the PCr/ATP ratio remain essentially intact. The results are consistent with metabolite loss in infarcted areas, but preservation of near-normal intracellular metabolism in residual non-infarcted tissue. Importantly, the results support therapies that primarily ameliorate the effects of tissue loss on metabolite depletion, and those that reduce energy demand post-MI, rather than those that affect energy transfer, which does not appear to be significantly depressed in surviving myocytes, or those that increase the energy demand on the surviving tissue.

Conflict of Interest Disclosures: Other than salary support from the above grants, the authors have no relevant financial conflicts of interest.

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