Association of dialysate calcium concentration with fetuin a level and carotid intima-media thickness in peritoneal dialysis patients

Abstract

Serum fetuin A has been shown to be associated with the risk of vascular calcification and atherosclerosis, and it can predict the onset of cardiovascular mortality in dialysis patients. The carotid intima-media thickness (cIMT) is an accessible and reliable method to identify the subclinical atherosclerosis. The aim of this study was to investigate the relationships between dialysate calcium concentrations and fetuin A or cIMT in patients undergoing peritoneal dialysis (PD). Forty patients, newly diagnosed end-stage renal disease (ESRD) and undergoing peritoneal dialysis, were enrolled in the study, with a calcium content of the peritoneal dialysis (PD) solution of 1.25 mmol/L in 20 patients (low-Ca group) and 1.75 mmol/L in 20 patients (standard-Ca group). The patients were followed up for 12 months after the PD conducted. Serum fetuin A was determined using a human fetuin A enzyme-linked immunosorbent assay kit and cIMT was detected using ultrasonic wave. We observed no difference between two groups with regard to the baseline data of fetuin A, cIMT, calcium, phosphorus, calcium-phosphorus product, high sensitivity CRP (hsCRP), parathyroid hormone (PTH), or lipid parameters. After 12 months follow-up, fetuin A (263.92 ± 16.1 vs. 282.76 ± 21.0, p = 0.017) and calcium-phosphorus product (39.85 ± 7.76 vs. 47.50 ± 6.65, p = 0.009) were obviously lower in the low-Ca group than standard-Ca group, the other serum parameters were not different between these two groups. Compared with baseline data, serum fetuin A concentration significantly reduced in low-Ca group ( p < 0.05). The number of patients with increased cIMT and newly occurring cardiovascular events in the low-Ca group were significantly reduced than standard-Ca group ( p < 0.05). In conclusion, our data suggest that low calcium dialysate treatment is associated with the decreased serum fetuin A concentration and serum calcium-phosphorus product, and it is associated with the reduced number of PD patients with increased cIMT or with newly occurring cardiovascular events. However, more studies with lager sample size should be performed in the future.

• Calcium
• carotid intima-media thickness
• fetuin A
• peritoneal dialysis

Introduction

End-stage renal disease (ESRD) is an ever-increasing problem worldwide, and it is a major health problem to be associated with very high morbidity and mortality.1–4 It is undoubtedly the peritoneal dialysis (PD) that serves the patients with ESRD as the most common method for renal replacement therapy worldwide.5 Cardiovascular disease (CVD) is the most significant cause of death in patients with ESRD undergoing PD treatment.6 Carotid intima-media thickness (cIMT) can be reliably determined in vivo by carotideum ultrasound, and it is an accessible and reliable method to identify the subclinical atherosclerosis.7,8 As a sign of atherosclerosis, increased cIMT has been widely accepted as a strong predictor of CVD and mortality in the ESRD patients.9

Fetuin A, a circulating plasma glycoprotein synthesized in the liver, is known as a potent circulating inhibitor in the ectopic calcification process.10,11 There were some reports showing that fetuin A was associated with the disorders of calcification metabolism and the risk of CVD. Wang et al.12 observed that there was an association between serum fetuin A and valvular calcification, atherosclerosis, and fetuin A was linked to mortality and cardiovascular events in PD patients via its close relationship with the atherosclerosis, calcification (MIAC) syndrome. Laughlin et al.13 reported that low level of fetuin A could predict the greater risk for CVD mortality in older adults without diabetes. Cagli et al.14 observed that serum fetuin A was significantly decreased in patients with rheumatic mitral valve disease, and was an independent predictor of calcium concentration in the mitral valve tissue.

The most common dialysate solutions utilized in PD patients are 1.25 mmol/L calcium PD solution and 1.75 mmol/L calcium PD solution. The relationship between calcium PD solution, fetuin A, and cIMT remains unclear. Therefore, in this study, we investigated whether the calcium concentration of PD solution was associated with the increased cIMT and the expression of fetuin A.

Patients and methods

This study was performed in a single center (Department of Nephrology, the Second Affiliated Hospital of GuangZhou Medical University, GuangZhou, China). Between December 2008 and December 2011, 40 PD patients were enrolled into the study, and were newly treated with steady continuous ambulatory peritoneal dialysis (CAPD) for at least three months, but no more than six months. Exclusion criteria included: inflammatory disease, malignancy, chronic liver disease, chronic obstructive airway disease, chronic rheumatic heart disease, congenital heart disease, and age <18 years or >70 years. The causes of ESRD included chronic glomerulonephritis in 11 patients, diabetic nephropathy in 13 patients, hypertensive nephrosclerosis in 10 patients, obstructive uropathy in 2 patients, tubulointerstitial nephritis in 2 patients, and unknown etiology in 2 patients. Patients were divided into two groups according to their admission order; 20 patients received a solution of 1.25 mmol/L calcium (low-Ca group) PD solution, and the other 20 patients received a 1.75 mmol/L calcium (standard-Ca group) PD solution. Twenty healthy individuals were used as the control group in order to define basal serum fetuin A level. The study was approved by the Ethics Committee of our Medical Faculty and written informed consents were obtained from all patients and healthy individuals before study entry.

Fasting venous blood was collected for the measurement of serum fetuin A, high sensitivity CRP (hs-CRP), serum albumin, calcium, phosphorus, parathyroid hormone (PTH), cholesterol (CHOL), triglycercide (TG), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). Serum fetuin A was determined using a human fetuin A enzyme-linked immunosorbent assay (ELISA) kit (Shanghai Lengton Bioscience Co., Shanghai, China). All specimens were measured in triplicate.

Ultrasonographic studies of the common carotid arteries were performed by a single experienced sonographer who was blinded to all clinical details. cIMT was measured during end diastole as the distance from the leading edge of the second echogenic line of the far walls of the distal segment of the common carotid artery, the carotid bifurcation, and the initial tract of the internal carotid artery on both sides. Measurements were performed 0.5, 1, and 2 cm below the bifurcation (three measurements were taken on each side), and the average measurement was denoted as IMT. The IMT measurements were always performed in plaque-free arterial segments.

Total weekly urea clearance (Kt/V) and creatinine clearance (CCr) were measured every four weeks in order to ensure adequacy of dialysis in PD patients. We prescribed calcium carbonate, calcitriol, and sevelamer hydrochloride to maintain the target range of iPTH and normal calcium, phosphorus, and calcium-phosphorus product concentrations. In patients who developed peritonitis, exit site infections, other infective complications or volume overload, all of the above assessments were deferred for at least one month following complete resolution of the complication. Cardiovascular events were defined as myocardial infarction, heart failure, arrhythmia, transient ischemic attacks, strokes, other thrombotic events, or peripheral vascular disease.

Statistical analyses

Data were expressed as mean ± SD (standard deviation). Baseline characteristics of PD patients between two groups were compared using analysis of variance for continuous data and chi-square analyses for categorical data. p < 0.05 was considered statistically significant.

Results

Baseline characteristics of patients in two groups

Baseline characteristics of the low-Ca and standard-Ca groups are listed at Table 1. The average age was 52.87 ± 12.0 years in the low-Ca group and 57.00 ± 13.0 years in the standard-Ca group, male/female ratio was 12/8 and 13/7 in the low- and standard-Ca group, respectively. The incidence of diabetes, cardiovascular events, cIMT, calcium, phosphorus, and calcium-phosphorus product were not different between two groups. Fetuin A concentrations were 276.3 ± 16.5 μg/mL in low-Ca group and 276.0 ± 18.0 μg/mL in standard-Ca group, and no difference was observed between those two groups (p = 0.976, Table 1).

Table 1. Baseline characteristics of patients in two groups.

	Low-Ca group (n = 20)	Standard-Ca group (n = 20)	p
Age years	52.87 ± 12.0	57.00 ± 13.0	0.285
Male man/woman	12/8	13/7	0.744
Diabetes	5	8	0.311
Cardiovascular events	7	5	0.705
cIMT	6	8	0.464
BMI (kg/m^2)	23.90 ± 1.73	24.36 ± 2.85	0.592
Fetuin A (μg/ml)	276.3 ± 16.5	276.0 ± 18.0	0.976
iPTH (pg/L)	31.70 ± 17.57	35.15 ± 18.67	0.628
ALB (g/L)	34.06 ± 5.26	33.29 ± 2.94	0.626
HsCRP (mg/L)	4.75 ± 4.62	8.53 ± 18.2	0.418
Ca (mg/dl)	7.78 ± 1.20	8.08 ± 1.01	0.458
P (mg/dl)	6.50 ± 1.32	6.08 ± 1.88	0.481
Ca-P (mg^2/dl^2)	48.08 ± 12.33	49.88 ± 9.37	0.701
TC (mmol/L)	4.43 ± 1.46	4.69 ± 1.18	0.565
TG (mmol/L)	1.77 ± 0.61	1.69 ± 0.82	0.715
HDL (mmol/L)	0.75 ± 0.26	0.88 ± 0.40	0.220
LDL (mmol/L)	3.56 ± 1.24	3.63 ± 1.07	0.869

Notes: Low-Ca: 1.25 mmol/L Ca; Standard-Ca: 1.75 mmol/L.

Serum fetuin A level and increased cIMT in two groups after 12 months follow-up

After 12 months follow-up, serum fetuin A concentration in the low-Ca group was increased when compared to that in the standard-Ca group (p < 0.05; Table 2). Interestingly, serum fetuin A concentration in the low-Ca group after 12 months treatment was decreased when compared with that in the baseline (p < 0.05; Tables 1 and 2), and the serum fetuin A concentration in the standard-Ca group after 12 months treatment was increased than that in the baseline (Tables 1 and 2). Furthermore, the number of PD patients with increased cIMT or with newly occurring cardiovascular events in the low-Ca group was significantly lower than that in the standard-Ca group after 12 months follow-up (each p < 0.05; Table 2). The calcium-phosphorus product in the low-Ca group was much lower than the standard-Ca group (p < 0.05; Table 2). We found no significant differences in calcium, phosphorus, iPTH, and the other serum parameters in our study between the two groups at 12 months follow-up (Table 2).

Table 2. Clinical characteristics of patients between groups at 12 months follow-up.

	Low-Ca group (n = 20)	Standard-Ca group (n = 20)	p
Age years	53.87 ± 12.0	58.00 ± 13.0	0.285
Male man/woman	12/8	13/7	0.744
Diabetes	5	8	0.311
Newly occurring  cardiovascular events	5	12	0.025
Increased cIMT	2	9	0.021
BMI (kg/m^2)	24.66 ± 2.71	24.86 ± 2.43	0.826
Fetuin A (μg/ml)	263.92 ± 16.1*	282.76 ± 21.0	0.017
iPTH (pg/L)	30.88 ± 15.89	24.06 ± 15.21*	0.256
ALB (g/L)	37.37 ± 3.49	35.36 ± 5.00	0.122
HsCRP (mg/L)	7.56 ± 13.80	5.04 ± 5.22	0.342
Ca (mg/dl)	8.84 ± 1.19	8.98 ± 0.89	0.712
P (mg/dl)	5.42 ± 0.88	5.74 ± 1.07	0.168
Ca-P (mg^2/dl^2)	39.85 ± 7.76	47.50 ± 6.65	0.009
TC (mmol/L)	4.74 ± 1.57	4.88 ± 1.77	0.838
TG (mmol/L)	1.56 ± 0.50	1.83 ± 0.61	0.199
HDL (mmol/L)	0.93 ± 0.24	0.93 ± 0.32	0.989
LDL (mmol/L)	3.46 ± 1.04	3.64 ± 1.35	0.738

Notes: Low-Ca: 1.25 mmol/L Ca; Standard-Ca: 1.75 mmol/L; cIMT = carotid intima-media thickness; BMI = body mass index; *p < 0.05, between baseline and month 12 in low-Ca group.

Discussion

Fetuin A has been identified as a potent circulating inhibitor of calcification, which can inhibit extraskeletal calcification.10,15 Vascular calcification is a major challenge to the long-term health of patients receiving PD therapy.16 There were some studies reporting that fetuin A was associated with cardiac valvular calcification, cardiovascular events, and cardiovascular mortality in dialysis patients.15,17,18 There was no report on the association of dialysate calcium concentration with fetuin A level in PD patients. However, Kim et al.19 performed a prospective study, and 20 patients on maintenance hemodialysis with dialysate calcium concentrations of 1.75 mmol/L were enrolled, and the dialysate calcium level was lowered to 1.50 mmol/L at the start of the study. They found that lowering dialysate calcium concentration significantly improved arterial stiffness parameters, which might have been associated with upregulation of serum fetuin A. In our study, we found that low calcium dialysate (1.25 mmol/L) treatment is associated with the decreased serum fetuin A in PD patients. Our result for fetuin A was not consistent with that in Kim et al.19 However, the characteristics of these two studies were different (patients’ types: PD patients vs. hemodialysis patients; low calcium dialysate: 1.25 mmol/L vs. 1.50 mmol/L). In different dialysis patients, whether the association of the calcium concentrations with fetuin A level was different, it was not clear. Some interesting reviews indicated that, in different tissues, cells, and diseases model, the role of some factors are controversial, such as LMX1B, PHB, PAX2, apoE, etc.20–23 We speculated that fetuin A was a protective factor for patients with dialysis, and fetuin A might be upregulated in the early phase of dialysate as a protective reflection, and it would be downregulated in later. Low calcium dialysate would get a better curative effect when compared with high calcium dialysate, accompanying with the downregulated fetuin A. However, more studies should be conducted in the future to elucidate this association.

Increased cIMT has been widely accepted as a reliable method to identify the subclinical atherosclerosis.7,8 There was no any report investigating the association between cIMT and low calcium dialysate treatment. In our study, we observed that the number of PD patients with increased cIMT in the low-Ca group was significantly reduced than standard-Ca group. It indicated that low-Ca group treatment could reduce the increased cIMT in PD patients. Low calcium dialysate might be an alternative treatment for the PD patients. However, more studies should be performed in further.

Newly occurring cardiovascular events is an important indicator to evaluate the effect of PD patients. There was no any report investigating the association of newly occurring cardiovascular events and low calcium dialysate treatment. In our study, we observed that the number of PD patients with newly occurring cardiovascular events in the low-Ca group were significantly reduced than standard-Ca group. It indicated that low-Ca group treatment could reduce the risk of newly occurring cardiovascular events in PD patients. Low calcium dialysate might be an alternative treatment for the PD patients. However, further studies should be conducted.

The increased calcium-phosphorus product is a determinant of the risk of cardiovascular lesion and events. Kahnooj et al.24 included 72 consecutive patients with the final diagnosis of ESRD candidate for chronic hemodialysis, and reported that a positive relationship between the calcium-phosphorus product value and the severity of aortic insufficiency was expected, and achieving an appropriate control of the calcium-phosphorus product level might decrease aortic valve calcification and improve the survival of patients on chronic hemodialysis. There was no any investigation reporting the association of calcium-phosphorus product and low calcium dialysate treatment in PD patients. In our study, we observed that the calcium-phosphorus product value in the low-Ca group was significantly reduced than standard-Ca group. It indicated that low-Ca group treatment could reduce the calcium-phosphorus product in PD patients. Low calcium dialysate might be a beneficial treatment for the PD patients. However, further studies should be conducted to explore this association.

In summary, our data suggested that low calcium dialysate treatment was associated with the decreased serum fetuin A concentration and serum calcium-phosphorus product, and it can also reduce the number of PD patients with increased cIMT or with newly occurring cardiovascular events. However, more studies with lager sample size should be performed in the future. More studies with lager sample size should be performed in the future.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

This work was supported by Guangdong provincial science and technology projects (gov number: 2007B031503009).