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Ann Thorac Surg 2002;73:1216-1221
© 2002 The Society of Thoracic Surgeons
a Department of Anesthesiology, Weill Medical College at Cornell University and the Hospital for Special Surgery, New York, New York, USA
b Department of Pharmacology, Weill Medical College at Cornell University and the Hospital for Special Surgery, New York, New York, USA
c Department of Physiology and Biophysics, Weill Medical College at Cornell University and the Hospital for Special Surgery New York, New York, USA
d Department of Anesthesiology, Memorial Sloan-Kettering Cancer Center, New York, New York, USA
e Department of Critical Care Medicine, Memorial Sloan-Kettering Cancer Center, New York, New York, USA
f Department of Medicine, Columbia University New York, New York, USA
Accepted for publication November 19, 2001.
* Address reprint requests to Dr Heerdt, 525 East 68th St, Lasdon 2, Box 50, New York, NY 10021 USA
e-mail: pmheerd{at}mail.med.cornell.edu
| Abstract |
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Methods. Left ventricular tissue from 20 failing and 21 LVAD-supported hearts, catagorized as ischemic (ICM) or dilated (DCM) cardiomyopathy and four nonfailing hearts were studied. Myocyte mitochondrial ultrastructure was assessed by high-performance liquid chromatography determination of cardiolipin, a specific lipid component of the inner membrane, and its three major molecular species: L4, L3O, and L2O2.
Results. Both failing and LVAD-supported hearts exhibited a reduction in cardiolipin content that was independent of the type of cardiomyopathy. However, in failing/ICM hearts, there was a 25% increase in the L4/L3O ratio and a 70% increase in the L4/L2O2 ratio, indicating a change in cardiolipin composition. These alterations were normalized by LVAD support. In sharp contrast, molecular species ratios in DCM hearts were the same as those in nonfailing hearts regardless of whether LVAD support had been used or not.
Conclusions. These data demonstrate LVAD-induced reverse remodeling of myocyte cardiolipin composition in ICM but not DCM hearts.
| Introduction |
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LVAD support also improves indices of myocyte mitochondrial function [11]. The specific lipid component of mitochondrial membranes is cardiolipin (diphosphatidylglycerol), the presence of which is essential for oxidative ATP formation and substrate transport [1214]. Other data suggest that in addition to total cardioplipin content, the relative concentrations of the molecular species tetralinoleoyl-cardiolipin (L4, approximately 80% of total), trilinoleoyl-oleoyl-cardiolipin (L3O, approximately 15% of total), and dilinoleoyl-dioleoyl-cardiolipin (L2O2, 3% of total) are related to mitochondrial ultrastructure (ie, density of cristae membranes) [15]. We hypothesized that the reverse remodeling produced by LVAD-induced reductions in LV pressure and volume load may lead to alterations in myocyte mitochondrial ultrastructure. To test this hypothesis, cardiolipin content and composition were measured in myocardium harvested from patients with chronic heart failure, from LVAD-supported patients, and nonfailing control hearts. To assess the influence of underlying disease, data were subdivided by the etiology of failure broadly categorized as ischemic (ICM) or dilated (DCM) cardiomyopathy based upon clinical history.
| Material and methods |
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Analysis of myocyte diameter
For a subset of hearts (four nonfailing, nine failing, 11 LVAD), tissue samples were fixed in 10% buffered formalin, embedded in paraffin, and mounted on glass slides. As previously described [8], samples were then prepared with Massons Trichrome stain and images viewed on a Nikon microscope with a MTI 3CCD digital camera at 20x magnification. Digitally acquired images were analyzed using Image Pro Plus V3.0 by an examiner blinded to the heart condition. For diameter measurements, two orthogonal diameters were obtained per myocyte and then averaged. Only sections containing fibers cut in cross-section were analyzed. The diameters of 50 myocytes per slide were measured and then averaged. For ICM hearts, only regions remote from gross infarct zones were analyzed.
Data anlaysis
To determine differences between nonfailing, failing, and LVAD groups, data were first compared by analysis of variance and the Newman-Keuls test without regard to the etiology of failure. Analysis was then repeated with data subdivided by ICM or DCM. To lessen the potentially confounding variable of time dependence [17], nonfailing and failing heart data were compared with LVAD data obtained in patients supported for
40 days, as previously described [8, 17]. Correlation between cardiolipin content/composition and patient age or duration of LVAD support was determined by regression analysis after application of curve-fitting techniques as required (Igor Pro; Wavemetrics, Inc, Eugene, OR). For all comparisons, p less than or equal to 0.05 was considered significant. Data are presented as mean ± standard error.
| Results |
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| Comment |
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In some heart failure patients, LVAD support leads to a reversal of chamber enlargement and reduction in LV mass that is accompanied by improvement in global pump function [13, 5]. Additional studies of isolated myocytes and intact isometric LV trabeculae have demonstrated increased contractile function and an enhanced inotropic response to ß-adrenergic stimulation after LVAD [68], indicating that augmented LV pump function is not simply the result of changes in size and geometry. At the cellular level, isolated myocytes show LVAD-induced improvement in cytosolic Ca2+ transients (increased peak, accelerated decay) [6], and molecular studies have demonstrated an upregulation of SERCA2a mRNA, which is accompanied by increased protein production and enhanced SR Ca2+ uptake [7]. Previous data suggest that LVAD support can also improve mitochondrial coupling in CHF hearts [11]. We therefore hypothesized that hemodyanamic unloading of the failing human LV with a mechanical assist device would alter cardiolipin composition within the cardiomyocyte.
In relation to nonfailing hearts, failing hearts exhibited an approximate 25% reduction in total cardioplipin content regardless of whether failure was from ICM or DCM. After LVAD support of more than 40 days, total cardiolipin in both ICM and DCM hearts was further reduced to roughly 60% of the level measured in nonfailing hearts. Given that there is an increase in myocyte size in failure that can regresses during LVAD support [2, 8, 16] and possibly complicate interpretation of total cardiolipin content due to simultaneous changes in mitochondrial size, in a subset of patients, total cardiolipin was normalized to myocyte diameter. This analysis magnified the impact of heart failure on total cardiolipin (normalized value less than half that of nonfailing hearts) but eliminated the further decline suggested by LVAD support. These data indicate that heart failure has a marked effect on cardiolipin content of myocytes and that this effect persists even when the heart is unloaded to the point where it is required to little, if any, work and myocyte size regresses. Whereas the present study does not shed light on the mechanism(s) behind abnormal mitochondrial ultrastructure in heart failure, our data suggest that the fundamental abnormality is not reversed by LVAD support and are consistent with the possibility of mitochondrial atrophy.
In contrast, assessment of mitochondrial membrane remodeling based upon alterations in cardiolipin composition (ie, shifts in the relative proportions of the individual cardiolipin molecular species) indicated an LVAD-induced change in ICM but not DCM hearts. Of particular note was the finding that in failing hearts with ICM, there was a 25% increase in the L4/L3O ratio and a 70% increase in the L4/L2O2 ratio. These changes were not present after LVAD support, indicating normalization of cardiolipin composition. In sharp contrast, although total cardiolipin content in DCM hearts declined in a manner similar to that in ICM tissue, molecular species ratios in DCM hearts were the same as those in nonfailing hearts regardless of whether LVAD support had been used or not. These data suggest that elevation of the L4/L3O ratio in the ICM/failing hearts most likely represents a higher demand on oxidative ATP synthesis in those parts of the myocardium that survived the ischemic insult(s). Subsequent normalization of L4/L3O with LVAD treatment, therefore, appears to be the result of reduced myocardial energy demand and "relaxed" mitochondrial stress. Importantly, the marked differences between ICM and DCM hearts was apparent despite the relatively nonspecific differentiation of the pathologic states based entirely upon clinical history.
Although multiple changes in the structure and function of failing myocytes after LVAD have been described, to our knowledge, these are the first data demonstrating specific molecular alterations related more to the underlying disease than heart failure in general. This observation assumes particular importance in the context of the present study because when all failing hearts were pooled, changes in molecular species ratios within ICM hearts were masked by the lack of change in DCM hearts. However, results of the current study should be interpreted in the context of certain limitations. Of particular importance is the fact that although previous studies have provided data to firmly link mitochondrial cardiolipin to multiple aspects of cellular respiration, the specific relationships in failing myocardium remain to be established. In a previous study, Lee and colleagues [11] demonstrated a modest LVAD-induced improvement in mitochondrial respiratory function due to a decrease in state 4 respiration. Tissue samples from 7 LVAD-supported patients included in the study of Lee and colleagues were also analyzed for cardiolipin content and composition and suggested an association between shifts in molecular species ratios and oxidative function. However, because we do not have similar data in either nonfailing or failing hearts, we cannot conclude that LVAD-induced alterations in cardiolipin content or the ratios of individual molecular species reflect specific functional changes. Finally, the relatively small sample size and incomplete clinical history for the nonfailing hearts has potential impact. Although these hearts were not exhibiting clinical signs of failure at harvest, clearly, the circumstances leading to donor demise and the pharmacotherapy used to maintain viability until organ harvest may have influenced study results. Thus, whereas there was little variation in cardiolipin content and composition within the nonfailing group (as indicated by the small standard errors in Table 2), we cannot say with absolute certainty what the effects of failure or LVAD support are in relation to "normal." Nonetheless, the data clearly demonstrate differences between failing and LVAD-supported hearts.
In summary, the present study demonstrates that: (1) heart failure produces significant changes in myocardial cardiolipin content regardless of etiology but alters cardiolipin composition only in hearts with ischemic cardiomyopathy; and (2) LVAD support does not restore cardiolpin content of failing hearts but does normalize cardiolipin composition in those with ischemic cardiomyopathy. These data are consistent with disease-specific reverse remodeling of myocyte mitochondrial membranes after LVAD support.
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