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Articles |
1
Departments of Medicine and Cardiology and
2
Laboratory Medicine and Pathology, Hennepin County Medical Center, and University of Minnesota Medical School, Minneapolis, MN 55415.
a Author for correspondence: Hennepin County Medical Center, Clinical Laboratories MC 812, 701 Park Ave., Minneapolis MN 55415. Fax 612-904-4229; e-mail fred.apple{at}co.hennepin.mn.us
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Key Words: indexing terms: enzyme immunoassay • renal failure • heart function
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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cardiac evaluation group
Twenty-four patients were chosen from seven dialysis centers from
within the metropolitan area if they had no history of acute heart
disease and no regional wall motion abnormalities on a
preenrollment echocardiogram or multigated acquisition scan. After
informed consent, patients underwent history and physical examination,
12-lead electrocardiography, two-dimensional echocardiography, and
serial biochemical sampling for total CK, CK-MB mass, cTnT, cTnI, blood
urea nitrogen (BUN), and creatinine. Within 10 days of biochemical
sampling, the first three parts of the cardiac evaluation were
completed. These studies were performed on the same day and interpreted
by practitioners blinded to the patient's biochemical data. We
evaluated 19 patients in the outpatient setting and another 5 patients
during a hospitalization for noncardiac illness. M-mode echocardiogram
measurements were made of left ventricular (LV) posterior wall
thickness (normal reference range, 6–11 mm) and intraventricular
septal thickness (normal reference range, 6–13 mm) (25).
LV mass (normal reference range, 102–287 g) was calculated by the
method of Troy et al. (26). Blood samples were obtained
immediately before the dialysis procedure. All patients were dialyzed
three times per week on Monday–Wednesday–Friday or
Tuesday–Thursday–Saturday. A sample was obtained for each patient on
at least 2 consecutive dialysis days. In follow-up telephone interviews
6 months after the initial evaluation, histories were updated to
include subsequent hospitalizations or evaluations related to
cardiovascular disease. Two of the 24 patients were lost to follow-up.
skeletal muscle biopsy group
The five patients with chronic renal failure in the biopsy study
had biochemical sampling for total CK, CK-MB mass, cTnT, cTnI, BUN, and
creatinine. Skeletal muscle biopsies were taken from the brachial
radialis muscle at the time of hemodialysis vascular access placement
in three patients. The external oblique muscle was biopsied at the time
of renal transplantation in two patients. Serum samples were obtained
before biopsy.
serum biochemical markers
Blood samples were collected in evacuated tubes and centrifuged
immediately; sera were frozen at -40 °C until all were collected.
Total CK activity (reference values: men, 
300 U/L; women, 200 U/L)
was measured at 37 °C on an Ektachem 700 RX with a kinetic enzymatic
method (Johnson & Johnson, Rochester, NY). The lower limit of detection
was 20 U/L (27). CK-MB (reference value, 5 µg/L) was
measured by a mass immunoassay based on a monoclonal antibody that
recognizes CK-MB, but neither CK-MM nor CK-BB, on the Stratus II
Analyzer (Dade International, Miami, FL) (27). The
proportion of CK-MB to total CK was calculated as: % CK-MB =
(CK-MB/total CK) x 100 (reference value, 2.5%) (27).
cTnT (reference value, 0.2 µg/L) was quantified by a
first-generation commercial ELISA on the ES300 (CardiAC Troponin T;
Boehringer Mannheim, Indianapolis, IN) (27), using
streptavidin-coated tubes and two anti-cTnT antibodies. The capture
antibody (designated M7) is specific toward cTnT, but the horseradish
peroxidase-labeled second antibody (1B10) has ~10% cross-reactivity
with skeletal muscle troponin T, resulting in 2–4% assay
cross-reactivity with skeletal TnT. cTnT was also quantified by a
second-generation immunoassay, which uses the same capture antibody
(M7) and a detection antibody (M11.7) (Enzymun, ES300; Boehringer
Mannheim) that shows no cross-reactivity with skeletal troponin T
concentrations as much as 1000 µg/L (<0.005%) (24).
The lower limit of detection for both cTnT assays was 0.02 µg/L. In
only 18 of 24 of the study subjects were enough patient sera available
for reanalysis by the Enzymun assay.
cTnI (reference value, 0.8 µg/L) was measured by the prototype
immunoassay and the Food and Drug Administration-approved immunoassay
(Stratus II; Dade International) with the same two cTnI-specific
monoclonal antibodies as capture and labeled antibody. The specificity
of the antibodies (28) and the analytical performances of
the prototype (11) and production immunoassays
(29) have previously been reported. The detection limit in
serum of this immunoassay was 0.35 µg/L.
western blots
Biopsies were frozen immediately after collection and stored at
-80 °C until homogenized. Western blots were completed for cTnI and
cTnT. Frozen tissue (40–60 mg of healthy human myocardium, and healthy
and diseased skeletal muscle) was homogenized in 1 mL of potassium
phosphate buffer [200 mmol/L K2HPO4, 5
mmol/L ethyleneglycol-bis-(ß-aminoethyl
ether)-N,N,N',N-tetraacetic acid, 5 mmol/L
ß-mercaptoethanol, 100 mL/L glycerol, pH 7.4] in a Polytron
homogenizer (Brinkmann Instruments, Westbury, NY). The homogenates were
incubated at ambient temperature for 1 h and then centrifuged at
40 000 x g for 30 min. The total protein
concentration was determined by a modified Lowry method
(30) (Sigma Diagnostics, St. Louis, MO). Then 50 µg of
total protein from each homogenate were separated on two identical 12%
sodium dodecyl sulfate–polyacrylamide gel electrophoresis minigels for
1 h at 150 V on a MiniProtean electrophoresis apparatus (Bio-Rad,
Hercules, CA). After electrophoresis, the proteins were transferred
(Mini TransBlot electrophoresis transfer apparatus; Bio-Rad) to
nitrocellulose membranes in the cold for 1 h at 100 V in transfer
buffer (25 mmol/L Tris, 192 mmol/L glycine, 200 mL/L methanol, pH 8.3).
Nonspecific binding sites were blocked by incubating the membrane in 50
g/L nonfat dry milk, 20 mmol/L Tris base, 137 mmol/L NaCl, 0.1 mL/L
Tween 20, pH 7.6, buffer (TTBS) overnight at 4 °C. The membranes
were washed three times in TTBS and then incubated for 2 h with
the primary monoclonal antibodies [either monoclonal anti-cTnT (JS-2;
Lakeland Biomedical, Minneapolis, MN) or cTnI (11E12 [31];
Sanofi Diagnostics Pasteur, Marnes-la-Coquette, France)] diluted to 2
µg/mL in 10 g/L dry milk in TTBS. (Note: The monoclonal anti-cTnT
antibody, derived from hybridization of BALB/c mouse splenic B cells
immunized with human cTnT and myeloma line sp 2/0, exhibited minor
cross-reactivity with some human skeletal muscle troponin T isoforms,
according to Lakeland Biomedical). The membranes were washed three
times in TTBS and incubated for 1 h with a 1:3000 dilution of the
secondary horseradish peroxidase-labeled anti-mouse Ig antibody
(Amersham, Arlington Heights, IL). The membranes were again washed
three times in TTBS before a 1-min incubation with the chemiluminescent
substrate (ECL; Amersham). The substrate was drained off, and the
membranes were placed in a sealed plastic bag and exposed to x-ray film
(Fuji Rx; Fisher Scientific, Chicago, IL) for 1 min. Commercially
prepared molecular mass calibrators (Bio-Rad) and purified cTnT, cTnI,
fast skeletal TnT, and skeletal TnI (a gift from Spectral Diagnostics,
Toronto, Canada) were included in each run. The blot was imaged on a
Personal Densitometer SI (Molecular Dynamics, Sunnyvale, CA).
statistical analysis
Continuous variables were reported as means and ranges. Analysis
of variance was used for comparing repeated measures of cTnI, cTnT,
total CK, and CK-MB. Simple regression was used to analyze the
relationships between CK-MB and cTnI, CK-MB and cTnT, and first-
and second- generation cTnT assay results. Statistical
significance was defined as P <0.05.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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clinical evaluation group
The mean patient age was 49 years (range 30–73 years), and 75%
were male. Renal failure was caused by diabetes in 11 (46%) patients
and by chronic glomerulonephritis (n = 3), polycystic kidney
disease (n = 1), hypertensive nephrosclerosis (n = 6), focal
glomerulosclerosis (n = 2), and poststreptococcal
glomerulonephritis (n = 1). Patients had been on dialysis for an
average of 2 years (1–4 years). In addition to diabetes, risk factors
for cardiovascular disease included hypertension in 92%,
hyperlipidemia in 26%, tobacco use in 33%, and a positive family
history in 12% of patients. Their medications included erythropoietin,
antihypertensives, insulin, vitamins, and lipid-lowering drugs.
Clinical histories did not show past or current cardiac injury. Mean
blood pressures were 152 mm Hg (96–204 mm Hg) systolic and 83 mm Hg
(65–120 mm Hg) diastolic. By cardiac examination, 15 patients were
apparently healthy; 9 patients had a systolic murmur and 3 of these had
a fourth heart sound. No patient had a pericardial friction rub or
congestive heart failure.
Electrocardiograms completed within 10 days of biochemical sampling exhibited apparently healthy sinus rhythm (n = 24), LV hypertrophy (n = 14), and nonspecific ST and T wave changes (n = 9). Echocardiograms from this time demonstrated apparently healthy LV systolic function (n = 22) and mildly decreased LV function (n = 2). Two patients had small pericardial effusions. None had a regional wall-motion abnormality. LV hypertrophy was present in 21 patients. Mean LV M-mode echocardiogram measurements were: diastolic posterior wall thickness, 13 mm (9–18 mm); diastolic septal thickness, 17 mm (11–18 mm); mass, 397 g (277–499 g).
Follow-up telephone interviews were completed with 22 patients between 6 and 8 months after biochemical sampling. Four patients had undergone renal transplantation. No patient had a history of cardiovascular symptoms, evaluations, or hospitalizations. In the two patients who were not interviewed, concentrations of CK-MB, cTnT, and cTnI were normal.
biochemical data
Three consecutive serum samples were obtained from 19 patients,
and 2 consecutive samples were obtained from 5 patients for a total of
67 samples. Variations were not statistically significant during the
sampling days for any biochemical markers. The mean total CK for all
samples was 107 (20–399) U/L. A total CK measurement exceeding the
upper reference limit (300 U/L for men) was observed in one patient.
The mean CK-MB mass for all 67 samples was 3.6 (0–17.2) µg/L. At
least one sample had increased CK-MB in 7 of 24 (30%) patients (Fig. 1
); 16 of 19 (84%) samples measured above the normal reference
range. The mean CK-MB percentage for all samples was 4.5% (0–32%).
In 13 of 24 (54%) patients (Fig. 1
), CK-MB percentages for 32 of 62
(52%) samples were above the normal reference range.
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The mean ELISA cTnT for all samples was 0.6 (0–3.49) µg/L. For 17 of
24 (71%) patients in whom serum cTnT was measured by the
first-generation (ELISA) assay, cTnT concentrations were increased
(Fig. 1
) in 38 of 67 (57%) samples. The mean cTnI for all samples was
0.6 (0–13.2) µg/L. A single patient had an increased cTnI
concentration for all three measurements (Fig. 1
). An additional 10
predialysis samples for this patient obtained during a 4-month period
revealed chronic increases of CK-MB (mean, 5.4; range, 5.1–5.9
µg/L), ELISA cTnT (mean, 2.8; range, 2.75–3.12 µg/L), and cTnI
(mean, 11.0; range, 9.2–12.9 µg/L). In a complete reevaluation at 6
months follow-up, results were unchanged from the initial evaluation.
For the 18 patients with sufficient residual volume for analysis, the
mean Enzymun cTnT was 0.18 (0.01–2.18) µg/L, and in 3 (17%) of
these patients cTnT concentrations were above-normal in both cTnT
assays (Fig. 1
). Enzymun cTnT concentrations in these 3 patients were
increased in 8 of 49 (16.3%) samples. There was no significant
correlation between ELISA cTnT and Enzymun cTnT (r =
0.185; P = 0.21). Only in patient 13 (all three
specimens) were cTnI concentrations increased over those of the 18
patients studied for comparison, but the Enzymun cTnT concentration was
normal for all three samples. Overall, the Enzymun cTnT assay exhibited
an approximately threefold increase in potential false-positive results
compared with cTnI (16.3% vs 5.6%, respectively) in these 18
patients.
The mean BUN for all 67 samples was 640 mg/L (340–970 mg/L) and the mean creatinine was 110 mg/L (50–190 mg/L). The highest BUN and creatinine for a given patient was often observed on Monday or Tuesday, the day after the longest hiatus between dialysis assays. There was no significant correlation between the higher BUN or creatinine and the increase of any serum cardiac markers.
skeletal muscle biopsy group
Clinical data for the five patients who underwent skeletal muscle
biopsy are presented in Table 1
. All patients were receiving chronic hemodialysis or had
recently begun hemodialysis treatment. Four of the five patients had
hypertension, two had diabetes, and one (Fig. 2
, lane 4) had a history of coronary artery disease with
percutaneous coronary angioplasty of the right coronary artery. No
patient in this group had myocardial ischemia at the time of
biochemical sampling. For these 5 patients, 1 of 5 had an increased
CK-MB mass, 2 of 5 had an increased ELISA cTnT concentration, 0 of 5
had an increased Enzymun cTnT concentration, and 0 of 5 had an
increased cTnI concentration. As shown in Fig. 2A
, Western blot
analysis expressed a single isoform of cTnT (Mr
41 kDa) for 4 of 5 skeletal muscles (lanes 4, 5, 7, and 9) comigrating
with both cTnT expressed in the human heart (lane 3) and the cTnT
standard (lane 6). Bands of lower Mr (36–38
kDa) are skeletal isoforms of TnT, made visible by the slight
cross-reactivity of the JS-2 antibody at high concentrations of
protein. cTnT was not expressed in the apparently healthy skeletal
muscle control (lane 2). In identical Western blot gel for cTnI (Fig. 2B
), cTnI expression was not detectable in the 5 renal patients'
skeletal muscle or in apparently healthy skeletal muscle. Further, the
11E12 anti-cTnI antibody showed no cross-reactivity for either skeletal
troponin I or skeletal troponin T (lane 1).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Our findings are consistent with those of Jaffe et al.[1], Chan et al. [2], and Lal et al.
(3), who have demonstrated false elevations of CK-MB in
chronic dialysis patients without acute ischemic heart disease. Chronic
dialysis is associated with abnormal protein metabolism and muscle
wasting (35). This may be the source of elevated CK-MB in
these patients (1). Our observations also support those of
earlier investigators who demonstrated frequent elevation of cTnT
measured by the first-generation ELISA in chronic dialysis patients
(17)(18)(19)(20). Although the ELISA may give falsely elevated
cTnT values caused by nonspecific binding, another potential source of
the elevated cTnT may be the cross-reactivity with skeletal troponin T
in the first-generation (ELISA) assays. We found fewer false elevations
of cTnT with the second-generation (Enzymun) assay. This improved
specificity of the Enzymun assay has also been reported by Braun et al.
(36). Our 6-month follow-up telephone interviews revealed
no heart disease in this study group, most likely because the
population studied was a highly selected, younger patient group,
selected because of the absence of ischemic heart disease. One patient
exhibited unexplained increases of serum cTnI concentrations. Although
this cTnI increase was possibly an analytical false-positive result,
increased serum cTnI concentrations in this patient (Fig. 1
, patient
13), as measured by the Stratus II analyzer, were also confirmed by an
alternative immunoassay system (Behring Opus) that uses different
anti-cTnI antibodies (37). Patient 13 had no evidence of
LV hypertrophy, valvular heart disease, or pericardial disease. His LV
systolic and diastolic functions were normal.
In the second part of the study, we have demonstrated expression of cTnT in skeletal muscle in four of five renal failure patients biopsied. This finding was consistent with reexpression of cTnT isoforms identified in failing hearts (38). The mechanism for expression of cTnT in skeletal muscle from renal failure patients is likely associated with the peripheral myopathy associated with renal disease (35). Expression of cTnT in diseased or regenerating skeletal muscle appears to represent reexpression of a fetal gene, given that fetal skeletal muscle expresses cTnT (12). Previous studies have demonstrated a reexpression of cTnT isoforms in regenerating rat skeletal muscle (12) and in skeletal muscle from patients with Duchenne muscular dystrophy and polymyositis (13). In light of these studies, our findings implicate skeletal muscle as a possible source of abnormally elevated serum cTnT in this population. It would be informative to perform the Western blot experiments with the M7 monoclonal antibody, which was reported by Katus et al. (23) to have absolute specificity for cTnT, if the antibody should become available for general experimentation. The absence of cTnI in skeletal muscle from the patients appears to reflect its superior tissue specificity, as described by Bodor et al. (11).
This study involved a relatively small number of patients, but the findings are compelling. Preenrollment echocardiograms for two patients showed mildly decreased LV function. These patients had echo evidence of hypertensive heart disease. Both patients' echocardiograms were unchanged at the time of biochemical sampling and both had unremarkable measurements for all biochemical markers, with the exception of a single elevated cTnT concentration (ELISA). This study was limited to patients without acute myocardial injury. The clinical performance of these markers in dialysis patients with acute myocardial injury is unknown. An alternative anti-cTnT antibody was used for Western blot analysis rather than the anti-cTnT antibodies used in the ELISA and Enzymun immunoassays.
In conclusion, patients maintained on chronic dialysis without evidence of acute myocardial injury often have chronically increased CK-MB mass and cTnT concentrations. The second-generation cTnT (Enzymun) assay has better specificity than the first-generation (ELISA) assay for dialysis patients without acute ischemic heart disease. Only a single patient, who had no clinical evidence of myocardial injury, exhibited an increased serum cTnI. cTnT, but not cTnI, was expressed in the skeletal muscle of chronic renal failure patients. Thus, cTnI appears to be a superior marker of cardiac injury in the chronic dialysis population.
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Acknowledgments |
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Footnotes |
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References |
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-Tropomyosin, cardiac troponin T mutations cause familial hypertrophic cardiomyopathy: a disease of the sarcomere. Cell 1994;77:701-712.
[ISI][Medline]
[Order article via Infotrieve]
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