Myocardial Function, Energy Provision, and Carnitine Deficiency in Experimental Uremia
Veena Reddy*,
Sunil Bhandari and
Anne-Marie L. Seymour*
* Department of Biological Sciences, University of Hull, Hull, and Department of Renal Medicine, Hull and East Yorkshire Hospital NHS Trust, Kingston-upon-Hull, United Kingdom
Address correspondence to: Dr. Anne-Marie L. Seymour, Department of Biological Sciences, University of Hull, Hull, HU6 7RX, UK. Phone: +44-14824-65517; Fax: +44-14824-65458; E-mail: a.m.seymour{at}hull.ac.uk
Received for publication August 23, 2005.
Accepted for publication November 1, 2006.
Cardiac complications are the leading cause of mortality inpatients with chronic renal failure. Secondary carnitine deficiency,which is frequently observed in hemodialysis patients, has beenassociated with cardiac hypertrophy and heart failure and mayimpair myocardial fatty acid oxidation. In chronic kidney disease,impaired carnitine homeostasis also may affect myocardial metabolism.In this study, myocardial function and substrate oxidation inconjunction with carnitine deficiency were investigated in experimentalrenal failure. Uremia was induced in male Sprague-Dawley ratsvia a two-stage five-sixths nephrectomy. Cardiac function andsubstrate oxidation were assessed in vitro by means of isovolumicperfusion using 13C nuclear magnetic resonance at 3 and 6 wkof uremia. Renal impairment as assessed by serum creatininewas more severe initially and was associated with a significantdeficiency in serum free carnitine (43%; P < 0.001) and elevatedacyl carnitine/free carnitine ratio. Myocardial tissue carnitineconcentrations, however, were unaffected. A moderate degreeof cardiac hypertrophy (10 to 14%; P < 0.05) was observedin uremia without evidence of dysfunction or changes in myocardialsubstrate utilization. It is concluded that renal dysfunctionis associated with cardiac hypertrophy in the presence of normalmyocardial carnitine levels, despite a significant depletionin serum carnitine. This may be a factor in maintaining normalcardiac function and metabolism.
Cardiovascular complications are the leading cause of mortality,accounting for 50% of all deaths among patients with end-stagerenal failure in developed countries (1,2). The majority ofthese deaths are from cardiac causes (3). Indeed, the prevalenceof heart failure among these patients is as high as 40% andis responsible for 12% of the total deaths (4). At present,the underlying mechanisms of this cardiac dysfunction remainunclear.
Our previous studies, which used an experimental model of chronicuremia, identified 40% reduction in myocardial phosphocreatineand phosphocreatine/ATP ratio (5), in parallel with left ventricularhypertrophy (LVH) (6). These observations are supported by clinicalstudies of patients on peritoneal dialysis, with a negativecorrelation between phosphocreatine/ATP ratio and dialysis duration(7). As cardiac ATP production is tightly linked to substratemetabolism, one potential mechanism for the impaired energeticsmay be through alterations in the profile of energy provision.The hypertrophied heart is characterized by remodeling of theleft ventricle at cellular and structural levels, involvinga reexpression of the fetal metabolic phenotype and a downregulationof the adult phenotype. These changes result in a switch inthe profile of energy provision from fatty acid oxidation (8–10)toward a greater reliance on glucose metabolism (11,12). Theextent to which this may occur in the uremic myocardium, however,is unclear.
Secondary carnitine deficiency frequently is seen in uremicpatients, particularly in those who are on maintenance hemodialysistherapy (13). Carnitine plays a pivotal role in myocardial energymetabolism through (1) transport of long-chain fatty acyl intermediates,across the inner mitochondrial membrane for subsequent oxidation(9), and (2) regulation of carbohydrate metabolism by modulationof the intramitochondrial acetyl-CoA:CoA ratio (14). Thus, carnitinedeficiency, a characteristic of cardiac hypertrophy and heartfailure (15) may impair both fatty acid and carbohydrate oxidation.Here, we postulated that altered myocardial substrate utilization,in parallel with impaired carnitine metabolism and LVH in thesetting of uremia, may contribute to cardiac dysfunction inchronic kidney disease (CKD). Currently, little is known aboutthe role of metabolic remodeling on cardiac energy status andfunction in CKD. This study aimed to determine the extent ofmetabolic remodeling in the uremic heart, in parallel with serumand myocardial carnitine concentrations in experimental uremia,and, concurrently, assess their consequences on myocardial function.
Model of Chronic Uremia
All animal experiments conformed to the Guide for the Care andUse of Laboratory Animals published by the National Institutesof Health (NIH Publication No. 85-23, revised 1996). Chronicuremia was induced in male Sprague-Dawley rats (Charles River,Sussex, UK) that weighed 180 to 230 g, via a two-stage five-sixthsnephrectomy model, as described previously (5). Anesthesia wasinduced by an inhalation mixture of halothane (Merial, London,UK) and oxygen. In brief, a midline abdominal incision was madeand the left kidney exposed, renal vasculature was clamped,and approximately two thirds of the renal parenchyma was excised.One week later, rats were anesthetized again and the right kidneywas excised via a flank incision. Weight-matched control ratsunderwent similar procedures, except the kidneys were only decapsulatedand replaced intact.
Forty-eight hours later, rats were housed individually and pair-fedwith the control group. They had free access to water, and bodyweights were recorded weekly. BP was measured at 3 and 6 wkin conscious animals by tail plethysmography using an automatednoninvasive BP monitor (Harvard Apparatus, Eden Bridge, UK).An average of four to five measurements per rat during a 30-minperiod were recorded.
Isolated Heart Perfusions
Rats were studied 3 and 6 wk after surgery. After anesthesiawith sodium pentobarbitone (100 mg/kg body wt, intraperitoneally),hearts were excised and placed immediately in ice-cold Krebs-Henseleitbuffer. The aorta was cannulated and hearts were perfused inan isovolumic mode using a water-jacketed oxygenator (16), withKrebs-Henseleit buffer that contained 3% BSA (essentially fattyacid–free; Intergen, Purchase, NY) and the following components(in mM): 118 NaCl, 25 NaHCO3, 1.2 KH2PO4, 4.8 KCl, 1.2 MgSO4,1.25 CaCl2, 0.5 glutamine, 0.3 sodium palmitate, 5 glucose,1 sodium lactate, and 0.1 sodium pyruvate and 100 µU/mlinsulin (Sigma-Aldrich, Poole, UK) and equilibrated with 95%O2/5% CO2 (38°C, pH 7.4) (17).
Contractile function was monitored continuously via a fluid-filledballoon that was inserted into the left ventricle (17) and connectedto a physiologic pressure transducer (SensoNor, Horten, Norway)and a bridge amp that was linked to a two-channel MacLab/2esystem (AD Instruments, Hastings, England). Left ventricularend diastolic pressure was set to 5 mmHg by adjustment of balloonvolume. Coronary flow rate was adjusted to maintain a perfusionpressure of 65 mmHg.
After an initial 20-min equilibration period, perfusion mediumwas replaced with one that contained identical substrates andconcentrations as previous, except palmitate, glucose, and/orlactate were labeled with 13C ([U-13C] sodium palmitate, and[1-13C]glucose or [3-13C]lactate; Cambridge Isotopes, Andover,MA), and perfused for an additional 45 min. Heart rate, leftventricular developed pressure (LVDP), and myocardial oxygenconsumption (MVO2) were recorded every 10 min, and the meanvalues were determined. Rate pressure product (RPP) was calculatedfrom the product of heart rate and LVDP, and cardiac efficiencywas calculated from RPP divided by MVO2. At the end of the perfusionperiod, hearts were freeze-clamped and extracted with 6% perchloricacid for 13C nuclear magnetic resonance (NMR) analysis (18).
13C-NMR Spectroscopy
Cardiac tissue extracts were analyzed by proton decoupled 13CNMR spectroscopy using a JEOL JMN-LA400 FT NMR spectrometer(JEOL, Welwyn Garden City, UK), interfaced with a 9.4 Teslavertical bore superconducting magnet. 13C spectra were acquiredat 101 MHz using a WALTZ decoupling sequence at 25°C, with24,000 scans (pulse width 3.57 µs, interpulse delay 0.3s, sweep width 200 ppm). The relative contributions of palmitate,glucose, and lactate to the total acetyl CoA pool that enteredthe TCA cycle were determined using glutamate isotopomer analysis(19) from TCAcalc software developed by Dr. F.M.H. Jeffrey (Universityof Texas, Southwestern Medical Centre, Dallas, TX).
Isolation of Ventricular Myocytes
Ventricular cardiomyocytes were isolated from 3- and 6-wk controland uremic rats as described previously (20). In brief, afteranesthesia, hearts were excised and cannulated for retrogradeperfusion. After 10 min of noncirculating perfusion with bufferthat contained (in mM) 60 NaCl, 16 KCl, 3.25 MgSO4, 7 H2O, 1.2KH2PO4, 10 HEPES, 80 mannitol, 10 2,3-butanedione monoxime,20 taurine, 11 glucose, and 5 sodium pyruvate (pH 7.2), oxygenatedwith 100% O2, at 37°C, perfusion was switched to a circulatingmode with 30 ml of the same buffer, which contained 0.1% collagenase(type II; Worthington, Lakeland, NJ) and 0.5% BSA (fractionV, essentially fatty acid–free; Sigma, St. Louis, MO).CaCl2 was added in 5-µl increments to achieve a finalconcentration of 1 mM Ca2+. After 45 to 60 min, ventricles weredissected and agitated gently in 10 ml modified Krebs-Ringer-HEPESbuffer that contained (in mM) 120 NaCl, 2.6 KCl, 1.2 MgSO4,7 H2O, 10 HEPES, 1 CaCl2, 11 glucose, and 2 sodium pyruvateand 3% BSA (pH 7.4) to disperse the cells. The cell suspensionwas filtered through 25-µm nylon gauze (Miracloth; Calbiochem,La Jolla, CA) and centrifuged for 10 min at 300 rpm. The pelletedmyocytes were resuspended in 5 ml of the latter buffer, incubatedat 37°C, and gassed with 100% O2. Photomicroscopic measurementsof 40 ventricular cardiomyocytes for each heart were recordedusing a Leitz Laborlux S microscope (Leica, Milton Keynes, UK),with a x40 objective lens connected to a Cool SNAP-PRO camera,with Image-Pro Plus software (Media Cybergenetics, Marlow, UK).Only rod-shaped cardiomyocytes with clear sarcomere striationswere analyzed.
Serum Biochemistry and Metabolite Analysis
Blood samples were collected immediately after excision of thehearts. Hematocrit was determined using an ABL 70 blood gasanalyzer (Radiometer, Sussex, UK). Serum urea and electrolyteswere analyzed using a Beckman Coulter LX20 analyser (High Wycombe,UK) in the Department of Clinical Chemistry at Hull Royal Infirmary(UK). Serum free (FC) and total carnitine (TC) concentrationswere determined at the Department of Clinical Chemistry at SheffieldChildren’s Hospital (UK) using a Micromass Quattro LCtandem mass spectrometer. Concentrations of serum glucose, triglycerides,and free fatty acids were determined using spectrophotometricassay kits (no. 510, Sigma; no. 336, Boehringer Mannheim, Lewes,Sussex, UK; no. 1383175, Roche, Mannheim, Germany). Cardiactissue FC and TC concentrations were analyzed by tandem massspectrometry at the Department of Clinical Chemistry at NewcastleRoyal Victoria Hospital (UK).
Statistical Analyses
Results are expressed as means ± SEM. Statistical significancewas determined by unpaired t test and one-way ANOVA with posthoc comparison by Bonferroni test where appropriate. Bivariatecorrelation analysis was performed by Pearson correlation analysis.P < 0.05 was considered to be statistically significant.
Renal Failure Model
Characteristics of the experimental model are presented in Table 1.Body weights were similar between control and uremic rats at3 and 6 wk after surgery. However, heart weight, heart weight/bodyweight, and heart weight/tibia length ratios were increasedin uremic rats at both stages of uremia, indicating myocardialhypertrophy. The remnant kidney weights in the uremic groupwere comparable to the left kidney weights of controls by 3wk and were significantly heavier by 6 wk (18.8%; P < 0.05),suggesting progressive compensatory hypertrophy of the renalremnant. Diastolic, systolic, and mean arterial pressures wereelevated in uremic rats at all times. Both serum creatinineand urea were elevated significantly (P < 0.001) in uremicrats, although the degree of renal dysfunction was more pronouncedat the early stage of uremia. Hematocrit was lower in uremicrats (16%; P < 0.001) at 3 wk, although by 6 wk, it was comparablebetween the control and uremic groups. A significant inversecorrelation was observed between serum creatinine and hematocritin all groups (R = –0.57, P < 0.01; Figure 1).
Figure 1. Correlation between serum creatinine and hematocrit (two-tailed Pearson correlation coefficient, R = –0.566, P < 0.01).
Cardiomyocyte dimensions of control and uremic groups at 3 and6 wk after surgery are given in Table 2. These data confirmthe presence of cardiac hypertrophy suggested by the morphologicdata in Table 1. Cell length increased by 6.7% (P < 0.01)and 5.4% (P < 0.05) and cell width increased by 14.3% (P< 0.01) and 10.6% (P < 0.01) at 3 and 6 wk, respectively,after induction of uremia. Cell length/width ratio remainedunaltered. Induction of uremia did not modify serum glucose,triglyceride, or free fatty acid concentrations at 3 or 6 wk(Table 3).
Table 3. Serum metabolite concentrations in control and uremic groups at 3 and 6 wk after surgery
Serum and Tissue Carnitine Concentrations
Serum and cardiac tissue FC, acyl carnitine (AC), and TC concentrationsin control and uremic groups at 3 and 6 wk after surgery areshown in Figure 2. Serum FC concentration (Figure 2a) was reducedsignificantly in the uremic group at 3 wk (43%; P < 0.001)with a concomitant decrease in TC concentration (40%; P <0.01). However, at 6 wk, serum FC concentration remained significantlyreduced (18%; P < 0.01), whereas the reduction in serum TCdid not reach significance (13%; NS). Overall, these changesresulted in an increase in AC/FC ratio in uremia (0.35 ±0.04 versus 0.26 ± 0.02 [P < 0.05] and 0.30 ±0.03 versus 0.21 ± 0.01 [P < 0.01]) at 3 and 6 wk,respectively. A significant inverse correlation was observedbetween serum creatinine and serum FC and TC concentrationsin all groups, suggesting that the degree of FC and TC depletionmay parallel the severity of renal dysfunction (Figure 3). Despitesignificant serum carnitine deficiency, myocardial FC and TClevels were unaltered in uremia (Figure 2B).
Figure 2. Free (FC) and acyl carnitine (AC) concentrations in serum (a) and myocardial (b) tissue. (AC + FC = total carnitine [TC]). In serum: P < 0.01, TC concentration uremic versus control; **P < 0.01, ***P < 0.001 FC concentration uremic versus control.
Figure 3. Correlation between serum creatinine and (a) FC (R = –0.634 P < 0.01) and (b) TC (R = –0.610, P < 0.01) concentrations in all uremic and control groups (3 and 6 wk).
Function of Isolated Isovolumic Hearts
Mean heart rate, LVDP, and RPP of hearts from control and uremicrats are shown in Figure 4. Heart rates were similar in allgroups. At 3 and 6 wk, LVDP and, consequently, RPP were lowerin uremia, although not significantly different.
Figure 4. Mean cardiac functional parameters in control and uremic groups at 3 and 6 wk. (a) Heart rate. (b) Left ventricular developed pressure (LVDP). (c) Rate pressure product (RPP).
MVO2 was unchanged at 3 (23.5 ± 1.7 versus 21.6 ±1.8 µmol/min per g dry heart wt) and 6 wk (17.7 ±0.8 versus 16.9 ± 2.9) in control versus uremic hearts,respectively. Similarly, cardiac efficiency was comparable betweenthe two groups at both time points (1.4 ± 0.01 versus1.2 ± 0.01 x 103, 3 wk; 1.4 ± 0.2 versus 1.4 ±0.2 x 103, 6 wk). Therefore, there was no evidence of cardiacdysfunction in this model of uremia.
13C NMR Analysis
Three weeks after induction of uremia, glucose contributed 16.4± 1.5 versus 16.7 ± 1.5% (NS) to substrate oxidation,whereas palmitate contributed 26.3 ± 2.6 versus 29.9± 2.4% (NS) and lactate contributed 59.1 ± 1.2versus 52.7 ± 3.3% in control and uremic hearts, respectively(Figure 5a). Unlabeled substrates (exogenous pyruvate, endogenoustriglycerides, and glycogen) made up the remaining contribution.
Figure 5. (a) Contribution of glucose (n = 10), palmitate (n = 16), lactate (n = 6), and unlabeled substrates (n = 16) at 3 wk after surgery to substrate oxidation. (b) Contribution of glucose (n = 6), palmitate (n = 6), and unlabeled substrates (n = 6) at 6 wk after surgery to substrate oxidation.
At 6 wk, a small reduction in palmitate use and increase inglucose utilization was observed in the uremic hearts, althoughthese changes were NS (Figure 5b). Unlabeled substrates includinglactate contributed 47.9 ± 4.8 and 50.8 ± 1.5%(NS) in control and uremic hearts, respectively. Thus, no changein the profile of substrate utilization was observed at either3 or 6 wk.
This study has demonstrated that renal dysfunction is associatedwith cardiac hypertrophy, without any alterations in cardiacfunction or myocardial energy metabolism, in contrast to ouroriginal hypothesis. Although there is a significant depletionin serum FC and TC concentration, cardiac tissue carnitine contentremains unchanged, which may be sufficient to maintain fattyacid oxidation and thereby prevent early metabolic changes inthe hypertrophied uremic myocardium. This is the first studyto characterize myocardial energy provision over time in anexperimental uremic model.
Model of Uremia
The greater severity of uremia 3 wk after surgery is most probablydue to acute tubular necrosis in the renal remnant, resultingfrom temporary occlusion of renal vasculature during the surgicalprocedure. Partial or complete recovery from acute tubular necrosisand hypertrophy of the remnant kidney may account for the improvementin renal function by 6 wk. Similar findings of enhanced renalfunction over time have been made in an in vivo study usingthe same uremic model (21). Hypertension was evident from theearly phase of uremia and increased with progressive exposureto uremia (Table 1). Diastolic and mild systolic hypertensionin the 6-wk control group may be due to a sympathetic responsefrom restraining the rats, although any such effect should beconsistent between the two groups. BP assessment under anesthesiamight resolve this issue but could result in underestimationof the true values.
Reduced hematocrit at 3 wk also may be related to the severityof renal dysfunction. This assumption is supported by the inversecorrelation that was observed between serum creatinine and hematocrit(Figure 1). With acute deterioration in renal function, a markeddecrease in erythropoietin production can cause anemia. Alternatively,blood loss during surgery may cause anemia at this stage, whichis then replaced by erythropoiesis, resulting in an almost completerecovery of anemia by 6 wk.
Cardiac Hypertrophy
LVH is the most frequent cardiac adaptation in CKD, developingearly and progressing in prevalence and severity with deteriorationin renal function (22). It is evident in >75% of patientsat the time of initiation of renal replacement therapy. In ourstudy, a mild (10 to 14%) yet significant hypertrophy was observedfrom the early stages of uremia, consistent with previous observationsusing the same model (6). The myocyte data (Table 2) demonstratean increase in both cell length and cell width, consistent witheccentric hypertrophy (23). Hypertension per se leads to concentricenlargement of the ventricle, characterized by an increase incell width (wall thickness) during developing and compensatedphases, whereas volume overload leads to a proportional growthin cell length and width (24). Therefore, our findings indicatea developing adaptive hypertrophy secondary to volume overload(Table 2). Comparable body weights between control and uremicgroups suggest that volume overload was not overt, but we hypothesizethat a degree of volume overload could be present at the cellularlevel, through an osmotic effect of uremic toxins. Assessmentof tissue tonicity would assist in deciphering this.
Although the pathogenesis of LVH in renal disease remains unclear,anemia, together with volume overload and hypertension, is apotential contributory factor (25). Intervention studies havefound that correction of anemia with recombinant erythropoietincan reduce left ventricular dimensions (26,27), with partialregression of LVH and left ventricular dilation. In our study,recovery of anemia to near-normal levels by 6 wk was not associatedwith regression of hypertrophy, highlighting the involvementof other contributory factors in the development of cardiachypertrophy. Therefore, development of cardiac hypertrophy thatwas observed in this study arises from a combination of factors,including hypertension, volume overload, and, in the initialphase, anemia.
Uremia and Carnitine
Secondary carnitine deficiency in patients with end-stage renalfailure results from reduced dietary intake, impaired synthesisand handling by the kidney, and, most common, from chronic dialyticloss (28). The marked deficiency in serum FC at 3 and 6 wk (Figures 2and 3) most probably is related to the degree of renal dysfunction,because dietary and dialytic losses of carnitine have not playeda role here. Under normal conditions, more than >90% of thefiltered carnitine is reabsorbed (29), with FC being preferentiallyreabsorbed, resulting in four to eight times higher urinaryexcretion of carnitine esters (30). Thus, a deterioration inrenal function is associated with decreased carnitine clearanceand impaired excretion of AC. Studies of predialysis uremicpatients have found increased levels of FC and TC, with markedlyelevated concentrations of AC (31), resulting in elevated AC:FCratios. However, carnitine metabolism in the early stages ofrenal failure has not been assessed to date. Our study highlightsthat in early uremia, both serum FC and TC concentrations decreasein parallel with renal dysfunction (Figure 3), whereas AC:FCratio increases significantly. Reduced renal biosynthesis mayaccount for the carnitine deficiency in early uremia.
It is of interest that although there is pronounced serum FCand TC depletion in uremia, myocardial carnitine concentrationswere unchanged. Carnitine distribution in skeletal and cardiacmuscles accounts for 97% of total body carnitine, with 2% inthe liver and kidneys and the remainder in extracellular fluid(32). AC:FC ratio is considered normal when it is 0.40 (13),as is observed in this study. Thus, prolonged and progressiveserum carnitine deficiency may be required before any alterationsin myocardial carnitine content occur.
Cardiac Metabolism and Function
The profile of myocardial substrate oxidation was unalteredin uremia despite the presence of cardiac hypertrophy and significantserum carnitine deficiency (Figure 5). At 3 wk, lactate wasthe predominant substrate for oxidation in all hearts, whereaspalmitate and glucose contributed approximately 30 and 16% respectively.Contributions from unlabeled substrates (exogenous pyruvate,endogenous triglycerides, or glycogen), were minimal in bothgroups. At 6 wk, contributions from palmitate and glucose weresimilar to those at 3 wk in both groups. The remaining contribution(approximately 50%) was from unlabeled substrates. Our previousstudies on control animals of the same age demonstrated lactateas the predominant substrate of energy provision, accountingfor approximately 60 (33) and 49% (34). Because no alterationin substrate metabolism is observed between control and uremicgroups, it is most likely that the majority of the unlabeledsubstrate that is oxidized by control and uremic hearts at 6wk is lactate.
Previous studies of the hypertrophied heart have demonstrateda reexpression of the fetal metabolic phenotype, with a downregulationof fatty acid oxidation (8,35) and increase in glucose metabolism(11,12). However, it is unclear whether these metabolic adaptationsprecede the development and contribution to the severity ofhypertrophy. This study indicates that myocardial hypertrophyin uremia can occur without any metabolic alterations. In supportof this, recent clinical and experimental studies have demonstratedthat the switch in substrate selection occurs primarily in severeend-stage heart failure while remaining normal in mild to moderatehypertrophy (36–38). In addition, maintenance of the metabolicprofile that was observed here may result from sufficient tissuecarnitine concentrations to sustain fatty acid oxidation. Itis possible that the extent of metabolic remodeling in uremiamay be determined by the combination of the degree of cardiachypertrophy, carnitine deficiency, and cardiac workload (39).Here, cardiac hypertrophy in uremia was moderate (10 to 14%),serum carnitine deficiency was insufficient to deplete myocardialcarnitine concentrations (Figure 2), and cardiac workload waslow; therefore, substrate selection remained unaltered. Thesedata suggest that in early uremia, mild cardiac hypertrophydevelops without any alteration in cardiac function or metabolicprofile.
The absence of significant cardiac contractile dysfunction inour study (Figure 4) was a surprising finding in the face ofmany clinical studies that have demonstrated prevalent myocardialdysfunction in early and later stages of uremia (40,41). However,many patients with CKD, in addition to having risk factors forheart failure such as LVH, hypertension, and anemia, have anumber of comorbidities, including diabetes and ischemic heartdisease, which may cause further deleterious effects on themyocardium. Our study used relatively moderate workloads, underwhich cardiac dysfunction was not apparent (Figure 4). Increasingcardiac workload, via adrenergic stimulation, or increasingperfusion pressure may unmask or exacerbate any underlying cardiacdysfunction. Indeed, Raine et al. (5) observed impaired cardiaccontractility in isolated working hearts only under conditionsof increased workload. Similarly, in cardiomyocytes that wereisolated from uremic hearts, a depression in contractility andrates of contraction and relaxation were observed only afterstimulation with calcium and isoproterenol (42). Extending theperiod of uremia within the experimental setting may generatea more progressive decline in cardiac function, which wouldhelp us to identify the underlying mechanisms of cardiac dysfunctionin uremia, without the presence of confounding variables thatare present in the clinical setting.
This study has demonstrated that in early stages of chronicuremia, cardiac hypertrophy develops with no impact on cardiacfunction or alteration in myocardial metabolism. Maintenanceof normal tissue carnitine stores in the setting of significantserum carnitine depletion may sustain myocardial fatty acidoxidation, thereby preventing metabolic remodeling in the earlystages of cardiac hypertrophy in uremia.
This work was supported by the British Heart Foundation (grantFS/02/039).
We are grateful to Jenny Foster for invaluable technical supportand Dr. Ian Hanning of Hull Royal Infirmary for analyses ofserum urea and electrolytes.
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Abstract
Introduction
0.40 (13), as is observed in this study. Thus, prolonged and progressive serum carnitine deficiency may be required before any alterations in myocardial carnitine content occur. 
