Abnormalities of Glucose Metabolism in Patients With Early Renal Failure

A total of 321 untreated essential hypertensive patients (aged 53 ± 12 years; 184 men, 137 women), who were referred to the outpatient clinic of our department, were included in a cross-sectional study. High blood pressure (systolic blood pressure ≥160 mmHg and/or diastolic blood pressure ≥95 mmHg) was measured at least twice on two different occasions and subsequently confirmed on at least two more visits during the next 4 weeks. Blood pressure was measured by a mercury sphygmomanometer (14) after each subject had been supine for 15 min. The average of three readings obtained in 5 min was recorded. The patients seen at our clinic include individuals with all grades of hypertension living in Northeast Italy and are representative of hypertensive patients in this geographic area (15). Exclusion criteria were age <30 or >75 years; BMI >30 kg/m²; severe hypertension, as defined by diastolic blood pressure of ≥115 mmHg; pregnancy; diabetes or family history of diabetes; renal failure with creatinine clearance <30 ml · min⁻¹ · 1.73 m⁻² body surface area; and presence of other diseases or treatments that might interfere with glucose metabolism. Renovascular and endocrine causes of hypertension were excluded on the basis of exhaustive laboratory testing (16). Renal function was assessed by measurement of 24-h creatinine clearance, urine protein excretion, and microalbuminuria. The cardiovascular status was assessed in all patients by a complete history, physical examination, electrocardiogram, echocardiography, and ultrasound examination of abdominal aorta, carotid, iliac, and femoral arteries. Additional laboratory tests, including treadmill exercise stress testing, myocardial perfusion scan, and coronary arteriography were performed following specific indications (17). The retrospective diagnosis of myocardial infarction was confirmed by documented history, electrocardiographic changes, and greater than twofold elevation of the serum creatine kinase with positive MB-fraction. The neurological diagnosis of transient ischemic attack, prolonged reversible neurological deficits, and atherothrombotic stroke was confirmed by documented history, clinical signs, and computerized cerebral axial tomography. The assessment of cardiovascular status was done without prior knowledge of patients’ glucose metabolism parameters.

Ninety-two normotensive individuals (aged 52 ± 18 years; 48 men, 44 women) served as control subjects. These subjects were selected from the general population of the same geographic area as the hypertensive patients by frequency matching after specification of inclusion criteria to avoid age and sex as potential confounding variables.

Impairment of renal function in hypertensive patients was considered to be present when the 24-h creatinine clearance was <90 ml · min⁻¹ · 1.73 m⁻² body surface area. This limit was selected a priori, as performed in earlier studies (6). History, review of medical records, urinalysis, repeated urine cultures, and renal ultrasound examination were used to establish the etiology of renal failure. Other tests, such as specific blood biochemistries, pyelography, renal scintigraphy, renal arteriography, computerized tomography, and renal biopsy, were performed when needed. Patients with decreased creatinine clearance were subdivided into three groups according to their renal function: 1) patients with creatinine clearance between 70 and 89 ml · min⁻¹ · 1.73 m⁻²; 2) patients with creatinine clearance between 50 and 69 ml · min⁻¹ · 1.73 m⁻²; and 3) patients with creatinine clearance between 30 and 49 ml · min⁻¹ · 1.73 m⁻². All patients had stable renal function for at least 6 months before the study, and none of them were treated with erithropoietin or vitamin D. A total of 212 (66%) of the hypertensive patients included in the study were treated with antihypertensive drugs (monotherapy 44%, multiple drug therapy 56%, calcium antagonists 46%, ACE inhibitors 39%, diuretics 36%, β-blockers 29%, angiotensin II receptor antagonists 13%, and α-blockers 6%) and were withdrawn from treatment a minimum of 1 week before measurement of blood parameters. At the time of the study, patients were allowed to maintain their usual unrestricted diet. The study was approved by the Ethical Committee of the University of Udine. A sample of venous blood was obtained without venous stasis after fasting for 12–14 h and after the patients were in the sitting position for 10 min for analysis of glucose, insulin, C-peptide, lipids, Hb, pH, bicarbonate, potassium, magnesium, glucagon, and growth hormone. The fasting glucose-to-insulin ratio was calculated as an index of insulin sensitivity, as previously reported (16,18,19).

Glucose tolerance was evaluated with the use of a 180-min OGTT as previously described (18). Briefly, a solution of 75 g glucose in 200 ml water was given orally, and blood for determination of plasma glucose, insulin, and C-peptide was drawn at baseline and after 30, 60, 90, 120, and 180 min. The area under the curve (AUC) for blood glucose and plasma insulin concentration during the glucose tolerance test (>180 min) was calculated by the trapezoidal rule (18).

Insulin sensitivity was further evaluated in a subgroup of 104 patients by a hyperinsulinemic-euglycemic clamp that was performed according to De Fronzo et al. (20) as previously described (21). In brief, a priming insulin (Humulin R; Eli Lilly, Sesto Fiorentino, Italy) dose of 100 mU/kg body wt was administered intravenously over a period of 10 min, and then a sustained infusion of insulin (dissolved in 0.9% NaCl), at a rate of 2 mU · kg body wt⁻¹ · min⁻¹, was started to maintain serum insulin concentrations at ∼700 pmol/l. Concomitantly, an intravenous infusion of a 20% glucose solution was started to stabilize blood glucose values at 5.0 mmol/l. To this purpose, plasma glucose was determined every 10 min during the clamp. Sensitivity to insulin was assessed as the glucose metabolic clearance rate (MCR) (milliliters per kilogram per minute) during 60 min of the clamp.

Plasma glucose was assayed using the glucose oxidase method with an oxygen electrode. Total cholesterol and triglycerides were assayed enzymatically by an automated method (International Laboratory, Milan, Italy). HDL cholesterol was assayed enzymatically after magnesium chloride-dextran sulfate precipitation of apolipoprotein-B-containing lipoproteins. The concentration of LDL cholesterol was calculated with the formula of Friedewald et al. (22). Plasma insulin, C-peptide, glucagon, growth hormone, and microalbuminuria were measured by radioimmunoassay as previously described (6,16,18,23), and intact parathyroid hormone (i-PTH) by IRMA (immunoradiometric test with monoclonal antibody).

All values are expressed as means ± SD. The Student’s t test was used for comparisons between two groups, and ANOVA (two-way) was used for comparisons of more than two groups. The Pearson χ² test was used to compare frequency distributions. The relationship between different parameters was examined by linear regression analysis, and the correlation was expressed by the correlation coefficient.

Clinical, lipid, and glucose metabolism parameters of hypertensive and control subjects are presented in Table 1. In addition to greater triglyceride and lower HDL cholesterol levels, hypertensive patients had greater fasting plasma insulin and C-peptide levels, greater AUC for blood glucose and plasma insulin, and lower fasting glucose-to-insulin ratio than control subjects. Creatinine clearance <90 ml · min⁻¹ · 1.73 m⁻² was found in 116 of 321 hypertensive patients. Decreased creatinine clearance was caused by hypertensive nephrosclerosis in all patients, a diagnosis that was made after exclusion of other renal diseases. As shown in Table 2, age, systolic blood pressure, and duration of hypertension were significantly and inversely related to creatinine clearance. Sex distribution, anthropometric indexes, current smoking status, and plasma lipids, with the exception of triglycerides, were comparable among the four creatinine clearance groups. Among patients with early renal failure, those with creatinine clearance <50 ml · min⁻¹ · 1.73 m⁻² had significantly greater fasting plasma insulin and C-peptide levels, greater AUC for blood glucose and plasma insulin, and lower fasting glucose-to-insulin ratio than patients with normal renal function (Table 3 and Fig. 1). These differences remained significant after correction of values for age. All measures of glucose metabolism were comparable among patients groups with creatinine clearance of ≥50 ml · min⁻¹ · 1.73 m⁻². During the hyperinsulinemic clamp, comparable steady-state plasma glucose and plasma insulin levels were maintained in the four creatinine clearance groups. Consistent with the results of the OGTT, lower glucose MCR was observed in patients with creatinine clearance <50 ml · min⁻¹ · 1.73 m⁻², indicating reduced peripheral sensitivity to insulin in this group (Table 3). No differences in glucose MCR were found among patient groups with creatinine clearance of ≥50 ml · min⁻¹ · 1.73 m⁻². Patients with creatinine clearance <50 ml · min⁻¹ · 1.73 m⁻² had significantly lower levels of Hb and serum bicarbonate and greater levels of serum potassium than patients with creatinine clearance of ≥50 ml · min⁻¹ · 1.73 m⁻², whereas no significant differences were found in plasma magnesium, calcium, phosphorus, parathyroid hormone, glucagon, and growth hormone levels (Table 4).

Comparison of hypertensive patients with and without microalbuminuria showed significantly greater systolic blood pressure and duration of hypertension in patients with microalbuminuria (Table 5); no significant difference was found in glucose metabolism parameters with the exception of C-peptide levels. Analysis of correlations showed that creatinine clearance was significantly and inversely correlated with age (r = −0.276, P < 0.001), plasma C-peptide (r = −0.183, P < 0.01), systolic blood pressure (r = −0.177, P < 0.01), and microalbuminuria (r = −0.177, P < 0.01). No correlations were found between the other parameters of glucose metabolism and creatinine clearance or microalbuminuria.

Clinical and instrumental evidence of one or more events attributed to atherosclerosis was found in 10.7% of patients with creatinine clearance of ≥90 ml · min⁻¹ · 1.73 m⁻² and in 14.5, 31.0, and 38.9% of patients with creatinine clearance between 70 and 89, 50 and 69, and 30 and 49 ml · min⁻¹ · 1.73 m⁻², respectively (P < 0.001) (Table 6). Most of these events (97%) occurred in the 5 years before the study. Significant difference in prevalence of atherosclerotic complications was also observed between patients with normal and impaired renal function when coronary artery, cerebrovascular, and peripheral vascular disease were considered separately. Among patients with decreased creatinine clearance, those with evidence of atherosclerotic complications were older, had significantly greater levels of systolic blood pressure, total and LDL cholesterol, cigarette consumption, and microalbuminuria and significantly lower HDL cholesterol than patients without such evidence (Table 7). All parameters of glucose metabolism were comparable in renal failure patients, with and without evidence of atherosclerotic complications.

The present study was performed in a large group of patients with hypertensive nephrosclerosis to investigate at which stage of renal failure abnormalities of glucose metabolism develop. The results demonstrate that fasting hyperinsulinemia, abnormal glucose and insulin response to an OGTT, and decreased sensitivity to insulin were not detectable at the earliest stages of renal failure, whereas they are present when creatinine clearance is <50 ml · min⁻¹ · 1.73 m⁻². These abnormalities do not appear to be related to glomerular damage as evaluated by measurement of microalbuminuria. Although patients with early renal failure have increased prevalence of atherosclerotic complications, these are not associated with significant changes in the parameters of glucose metabolism.

Many studies have shown that the sensitivity to the action of insulin with respect to carbohydrate metabolism is markedly impaired in patients with end-stage renal failure who are undergoing conservative or dialytic treatment (10–13). Because hyperglycemia and hyperinsulinemia may contribute to development of atherosclerosis (8,9) and accumulation of glycosylation end products in various tissues and because they have adverse effects on protein metabolism, it is important to define the stage in which renal failure abnormalities of glucose metabolism develop. Only a few studies have addressed this question in patients with mild impairment of renal function. Eidemak et al. (24) studied 29 patients with renal failure of mixed etiology and a median glomerular filtration rate (GFR) of 25 ml · min⁻¹ · 1.73 m⁻² and observed normal blood glucose response to the OGTT, hyperinsulinemia during both fasting and the OGTT, and decreased sensitivity to insulin as compared with 15 healthy control subjects. The major limitation of this study was that it included patients with obesity and patients treated with β-blockers, thiazides, or ACE inhibitors, drugs that are known to affect glucose metabolism and insulin sensitivity (25–27). Insulin resistance and hyperinsulinemia were also observed by Stenvinkel et al. (28) early in the course of renal insufficiency in patients with nephrotic syndrome (GFR 64 ± 6 ml/min) and in renal transplant recipients (GFR 44 ± 6 ml/min). However, in these categories of patients, factors other than renal insufficiency per se, such as dyslipidemia and immunosuppressive drugs, are likely to contribute to the derangement of glucose metabolism. More recently, Fliser et al. (29) evaluated 50 renal patients with IgA glomerulonephritis or adult polycystic kidney disease in different stages of renal failure. These authors observed hyperinsulinemia and insulin resistance of the same degree throughout the range of renal function considered (∼20–130 ml/min), including renal patients with GFR in the normal range, suggesting that abnormal glucose metabolism may be part of the phenotype of these two specific pathological conditions, independent of renal function. This possibility was supported by another study by Vareesangthip et al. (30), who observed insulin resistance and hyperinsulinemia in 15 patients with adult polycystic kidney disease and GFR in the normal range. The present study examined the largest sample of renal patients and was devoid of potential confounders, such as different etiology of the renal failure, inclusion of patients with obesity or family history of diabetes, and treatments with drugs that might affect glucose metabolism. Moreover, in addition to calculation of the fasting glucose-to-insulin ratio (16,19), in order to better define the sensitivity to insulin of peripheral tissues, we performed a hyperinsulinemic-euglycemic clamp in a subgroup of patients. Our study demonstrates that abnormal plasma glucose response to an oral glucose load, hyperinsulinemia, and insulin resistance develop when the GFR is <50 ml · min⁻¹ · 1.73 m⁻² but are not present at earlier stages of renal failure.

Different mechanisms may contribute to the abnormal glucose metabolism in chronic renal failure, including decreased sensitivity to insulin, inadequate insulin secretion, and increased hepatic gluconeogenesis (13,31). In addition to some conditions intrinsically related to renal failure (such as anemia [32] and metabolic acidosis [33]), accumulation of some toxic substance(s), including free fatty acids (31), hormones with antagonistic actions to insulin (31), “middle molecules” (34), pseudouridine (35), nitrogenous substances derived from protein metabolism (36), and acute phase reactants (37), may contribute to the impaired insulin-mediated glucose metabolism occurring after a certain degree of renal function loss. Some of these potential contributors were evaluated in our study. In patients with creatinine clearance <50 ml · min⁻1 · 1.73 m⁻² and altered glucose metabolism, we found significant differences in Hb, bicarbonate, and potassium concentrations but not in plasma magnesium, free fatty acid, glucagon, and growth hormone concentrations as compared with patients with GFR ≥50 ml · min⁻¹ · 1.73 m⁻². Moreover, because insulin resistance and hyperinsulinemia of uremia may be related to secondary hyperparathyroidism (38), we evaluated plasma calcium, phosphorus, and i-PTH levels and found a trend, though not significant, for increased i-PTH in patients with decreased creatinine clearance, suggesting a possible contributor to the impairment of glucose metabolism in these patients.

Although increased cardiovascular morbidity and mortality have been repeatedly reported in patients with end-stage renal failure (1,2,39,40), the incidence of cardiovascular events in patients with early impairment of renal function is still debated. In a prospective cohort study over a 10-year period, Jungers et al. (7) observed a threefold increase in the incidence of cardiovascular events in patients with moderate renal failure as compared with the general population. In addition to other cardiovascular risk factors (3–6), hyperinsulinemia may contribute to increased cardiovascular morbidity in chronic renal failure patients (13). The present study is the first to evaluate the relationship between the prevalence of cardiovascular disease and parameters of glucose metabolism in patients with early impairment of renal function. The results showed that abnormalities of carbohydrate metabolism were not associated with cardiovascular events, suggesting that hyperinsulinemia and altered response to oral glucose load were unlikely contributors to cardiovascular morbidity in these patients. This conclusion requires the support of longitudinal studies.

Some possible limitations of the present study should be discussed. First, the use of a clinic sample might limit the generalizability of the conclusions to the general population because of a bias in the referral of patients to the source of care. This might also explain the high incidence of renal failure in hypertensive patients in our patient sample. Second, because the underlying renal disease in patients with decreased creatinine clearance was arteriolar nephrosclerosis, results should be extrapolated to other forms of renal disease with caution.

In conclusion, in patients with hypertensive nephrosclerosis and early impairment of GFR, abnormalities of carbohydrate metabolism and hyperinsulinemia become evident only when creatinine clearance is <50 ml · min⁻¹ · 1.73 m⁻². These abnormalities are not related to microalbuminuria. Although patients with early renal failure have increased prevalence of cardiovascular events, these events are not associated with abnormal glucose metabolism. Abnormalities of glucose metabolism do not appear to be significant contributors to cardiovascular damage in early renal disease, but this conclusion needs further evaluation in longitudinal studies.

This research was supported by CNR (Consiglio Nazionale delle Ricerche) Grants 92.01096.CT04, 94.084231.CT 04, and 94.02438.CT04; a Fulbright Senior Fellowship; and a Ferrero Foundation grant to L.A.S.