1 Departments of Medicine, 2 Cardiology, 3 Clinical Chemistry, 4 Centre of Clinical Epidemiology, National Hospital, University of Oslo, Oslo, Norway
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Abstract |
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Methods. One hundred and seventy-three consecutive patients were prospectively examined 10 weeks after transplantation. An oral glucose tolerance test was completed in 167 patients. Questionnaires, medical records, and the results of various blood tests were used to evaluate a number of known cardiovascular risk factors in all patients.
Results. Glucose intolerance was present in about one-half the recipients and was associated with age, a positive family history of ischaemic heart disease, acute rejection, higher levels of serum triglycerides, apolipoprotein B and 2-h insulin, and lower levels of serum HDL cholesterol. After adjustment for age and sex, lower HDL cholesterol (P=0.005), higher serum triglycerides (P<0.001), apolipoprotein B (P=0.039) and 2-h insulin (P<0.001) were still associated with post-transplant glucose intolerance.
Conclusions. Ten weeks after renal transplantation glucose intolerance is associated with a clustering of cardiovascular risk factors and metabolic abnormalities, consistent with a post-transplant metabolic cardiovascular syndrome.
Keywords: cardiovascular disease; cardiovascular metabolic syndrome; dyslipidaemia; post-transplant diabetes mellitus; post-transplant glucose intolerance; renal transplantation
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Introduction |
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Reaven [3] introduced the concept of a metabolic cardiovascular syndrome focusing on the clustering of risk factors such as reduced high-density lipoprotein cholesterol (HDL) concentrations, hypertriglyceridaemia, and hypertension in insulin-resistant individuals. Both glucose intolerance and hyperinsulinaemia have been reported to be independent risk factors for atherosclerotic disease in the general population [4,5]. Even fasting blood glucose values in the upper normal range (4.86.0 mmol/l) are associated with increased risk for cardiovascular death [6]. Recently glycosylated haemoglobin was reported to be an independent risk factor for coronary heart disease in type 2 diabetic patients [7].
Renal transplant recipients are insulin resistant when compared with age- and sex-matched controls [8]. Impaired non-oxidative glucose disposal explains the insulin resistance observed in post-transplant diabetes mellitus (PTDM) similar to that observed in patients with type 2 diabetes in the general population [8]. Recipients with post-transplant impaired glucose tolerance (IGT) are equally insulin resistant as patients with PTDM but have a better insulin secretion response [9].
Diabetes mellitus is an established and powerful predictor for death from ischaemic heart disease (IHD) in renal transplant recipients [2], whereas data on CVD in recipients with PTDM are sparse. To our knowledge no previous study has addressed the issue of CVD in recipients with post-transplant IGT or impaired fasting glucose (IFG).
Recently we reported a high incidence of PTDM (18%) and IGT (31%) 10 weeks after renal transplantation [10]. Daily prednisolone dose and age were independent predictors of both PTDM and IGT. Furthermore the multivariate analysis showed a positive family history of diabetes, CMV-infection, and HLA-B27 phenotype to be associated with PTDM, and the use of beta-blockers was associated with IGT.
The aim of the present study was to assess whether glucose intolerance after renal transplantation is associated with metabolic disturbances known to increase risk for CVD, as has been observed in the general population.
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Subjects and methods |
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However, our study objective made it most appropriate to compare two groups, namely glucose-intolerant recipients (PTDM, IGT, and IFG) vs patients with normal glucose tolerance.
The study population, recruitment, immunosuppressive protocol, and OGTT have been described in detail previously [10].
Risk factors
All medical records were reviewed with respect to the presence of pre-transplant CVD. Myocardial infarction was confirmed by a history of typical chest pain and significant electrocardiographic or enzymatic changes. Coronary revascularization was defined as coronary artery bypass grafting or percutaneous transluminal coronary angioplasty. Cerebral vascular events included cerebral infarctions, transient ischaemic episodes, or intracerebral haemorrhage. The prevalence of nephrosclerosis and adult polycystic kidney disease were specifically addressed with reference to the reported increased risk for CVD [1,13] in these patients.
Electrocardiograms were recorded on the day of transplantation in all patients. Left ventricular hypertrophy was diagnosed in accordance with the SokolowLyon criteria.
On the day of OGTT all patients completed a questionnaire addressing CVD in first-degree relatives, present or previous symptoms of angina pectoris, intermittent claudication, and smoking habits. Height and weight were measured and body mass index calculated (kg/m2). Patients with repeated blood pressure values above systolic 140 mmHg or diastolic 90 mmHg in the sitting or recumbent position, and those treated with antihypertensive medication, were classified as hypertensive.
Laboratory analyses
The patients fasted overnight and refrained from medication on the day of examination. Blood samples were drawn in the fasting state, and immediately after centrifugation used for determination of glucose, creatinine and lipids or frozen for later analysis of insulin (-40°C), homocysteine (-70°C) and apolipoproteins (-70°C). In addition, blood samples were drawn after 1 and 2 h for analysis of glucose and insulin. Creatinine clearance was calculated using the CockcroftGault formula.
A glucose dehydrogenase method (Cobas Mira, Roche, Switzerland) was used for the analysis of serum glucose. HbA1c was determined in whole blood (EDTA tubes) by a turbidimetric immunoassay, Hitachi 911 or 917 (Boehringer Mannheim, GmhB, Germany).
Serum insulin was determined by a commercial radioimmunoassay (Coat-A-Count®, Diagnostic Products Corporation, Los Angeles, CA). Area under curve (AUC) insulin was calculated from fasting, 1-h, and 2-h serum insulin concentrations using the trapezoid rule.
Total cholesterol, HDL cholesterol, and triglycerides were analysed by an enzymatic calorimetric test, Hitachi 911 or 917 (Boehringer Mannheim, GmhB, Germany). Low-density lipoprotein cholesterol (LDL) concentrations were estimated with the Friedewald equation, triglyceride values below 4.0 mmol/l were multiplied with a correctional factor of 0.45; otherwise a factor of 0.37 was used. Lipoprotein (a) in serum was determined by a nephelometric assay (Dade Behring Marburg GmbH, Marburg, Germany), as well as apolipoproteins A1 and B (Behring Diagnostics GmbH, Marburg, Germany).
Serum total homocysteine was measured by HPLC and fluorescence detection after reduction of SS bonds with tri-n-butylphosphine, precipitation of proteins by perchloric acid, and derivatization with ammonium-7-fluorobenzo-2-oxa-1, 3-diazole-4-sulphonate. The method was slightly modified after Vester and Rasmussen [14].
Whole-blood CsA concentrations were measured using a CsA-specific fluorescence polarization immunoassay (TDx analyser; Abbott Laboratories, Chicago; IL).
Statistical analyses
Two-sided unpaired t-tests or one way analysis of variance (ANOVA) were used as appropriate to compare mean values between groups. The incidence of acute rejections in the two groups was analysed by the MannWhitney test. Chi-square tests were used to analyse categorical data. Multiple linear or logistic regression was performed for adjustments of age and sex. Because of the skewed distribution of serum triglycerides, lipoprotein (a), homocysteine, and insulin, these variables were log transformed to meet the assumptions of the statistical analyses. The analysis was implemented using SPSS® 10.0.
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Results |
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The incidence of acute rejections was significantly higher in the glucose-intolerant group (P<0.01), whereas renal function did not differ significantly between the groups (Table 3).
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CVD in first-degree relatives
Thirty nine per cent of the glucose-intolerant recipients had a positive family history of myocardial infarction or angina pectoris compared to 23% in the normal glucose tolerance group (P=0.03) (Table 4).
Lipids and lipoproteins
The mean serum triglycerides and apolipoprotein B levels were significantly higher, and the HDL-cholesterol concentration was significantly lower in the glucose-intolerant group (Table 5), even after adjustments for age and sex (triglycerides; P<0.001, HDL; P=0.005, Apo B; P=0.039). Both groups had high mean serum LDL-cholesterol concentrations (>4 mmol/l) with no significant difference between the groups.
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Insulin
The serum insulin levels during the OGTTs were analysed separately in three groups of patients (PTDM, IGT/IFG and NGT). Mean fasting serum insulin concentration did not differ significantly between the groups (Table 6). Patients with PTDM had lower 1-h, 2-h, and AUC insulin levels than the IGT/IFG group (independent t-test; P<0.001, 0.008 and 0.001). On the other hand, the latter group had significantly higher 2-h insulin level than patients with NGT (P<0.001). When glucose-intolerant recipients were compared with euglycaemic patients, the former had higher 2-h insulin concentrations (P<0.001) also after adjustment for age and sex (P<0.001).
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Homocysteine
Mean serum total homocysteine concentration was 31.1 µmol/l in the glucose-intolerant group and 26.5 µmol/l in the NGT group (P=0.13). Reference values in the general population are between 5 and 19 µmol/l.
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Discussion |
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Pre-transplant manifest IHD tended to be more prevalent in glucose-intolerant renal transplant recipients than in patients with NGT (P=0.07). Previous reports have documented that pre- and post-transplant IHD are closely linked [1], and pre-transplant atherosclerotic vascular disease may be a strong predictor for post-transplant atherosclerotic disease [15].
Patients with glucose intolerance also had a higher prevalence of coronary heart disease in first-degree relatives than recipients with NGT. Atherosclerotic CVD in first-degree relatives increases the risk of premature coronary artery disease three- to sevenfold in the general population [16], whereas data on inheritance of CVD in transplant populations is not available.
Both age and male gender are important risk factors for CVD in renal transplant patients [1]. Glucose-intolerant recipients were older than the euglycaemic, whereas the groups did not differ significantly with respect to gender.
In the present study post-transplant glucose intolerance was associated with lower concentrations of HDL cholesterol and higher levels of triglycerides. Low HDL cholesterol concentration has been shown to be a strong independent predictor for the development of IHD in a US study [1] and for death from IHD in a Norwegian study of renal transplant recipients [2]. In addition, patients with elevated triglycerides had an increased incidence of IHD in the former [1], and serum triglyceride levels were higher in non-survivors in the latter study [2]. However, in both studies the association between hypertriglyceridaemia and CVD failed to reach statistical significance in the multivariate model. Further, the level of serum triglycerides tends to be inversely associated with serum HDL cholesterol concentration [1], which makes it difficult to evaluate their independent influence on cardiovascular risk.
The level of serum apolipoprotein B, which reflects the number of LDL particles, was higher in the glucose-intolerant group. High levels of triglycerides, low HDL cholesterol concentration, and insulin resistance are all factors associated with increased number of small, dense LDL-cholesterol particles [17]. These particles appear particularly atherogenic and have been reported to increase the risk of coronary heart disease [17]. In a recently published study, the presence of small LDL particles seemed to be a feature of uraemic dyslipidaemia, which persisted after renal transplantation [18]. Our findings of both higher levels of apolipoprotein B and triglycerides and lower levels of HDL cholesterol in glucose-intolerant recipients, may imply that these patients also have an increased number of small, dense LDL particles. Unfortunately, determination of LDL particle size was not performed in the present study.
A recently published large prospective epidemiological study concluded that hyperinsulinaemia during an OGTT defined as area under curve, 1-h, and 2-h serum insulin, predicted increased risk of coronary heart disease (CHD) in the general population [5]. Others have reported fasting insulin concentration to be an independent risk factor for CHD [17]. In the present study the 2-h insulin concentration during the OGTT was significantly higher in glucose-intolerant recipients. Our study population of renal-transplant recipients appeared to have higher insulin levels than reported in the general population [5].
The combination of OGTTs and insulin analyses used in the present study made it difficult to evaluate directly any differences in insulin resistance between the groups. However, in a recently published study [9], using the hyperinsulinaemic, euglycaemic glucose clamp technique, we reported that renal-transplant recipients with IGT and PTDM showed a similar and significant reduction in insulin-stimulated glucose disposal rate compared to recipients with NGT, whereas patients with IGT and NGT shared a similar insulin response. The differences in the mean serum insulin levels during an OGTT reported in the present study, indirectly support our previous findings indicating that glucose-intolerant recipients are more insulin resistant than euglycaemic patients.
Pre-transplant and current hypertension was highly prevalent but not different between the groups. Serum homocysteine levels were high consistent with a previous report [19] but with no difference between the groups.
Glucose intolerant recipients had a higher incidence of acute rejection episodes, which is reported to imply increased risk for IHD [1]. It is well known that increasing steroid dosage during rejection episodes worsens glucose tolerance [10], but the question whether steroids, glucose intolerance, or the rejection process itself cause CVD remains unanswered.
The results of the present study should be interpreted with some caution. Firstly, the study design was observational and cross-sectional. Secondly, the patients were assessed at 10 weeks after transplantation when treated with high doses of prednisolone and CsA, which are known to aggravate both metabolic disturbances [20] and hypertension [21]. Further, cessation or tapering off the immunosuppressive medication may improve insulin sensitivity, hypertension, and lipid profile [2022].
Elevated fasting blood glucose, even in the normal range, has been reported to increase risk of cardiovascular death in the general population [6]. One may speculate that pre-transplant blood glucose levels may predict cardiovascular risk, but this has to be assessed in future studies.
To conclude, glucose intolerance is associated with a clustering of traditional cardiovascular risk factors and metabolic abnormalities early after renal transplantation, consistent with a post-transplant metabolic cardiovascular syndrome. Prospective studies are, however, needed to investigate any causal relationship between post-transplant glucose intolerance and cardiovascular end-points.
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Acknowledgments |
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Notes |
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References |
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