Renalase in Peritoneal Dialysis Patients Is Not Related to Blood Pressure, but to Dialysis Vintage

The prevalence of hypertension in peritoneal dialysis (PD) patients ranges from 29% to 88%, and blood pressure (BP) control is often very poor in this group (1–3). Multiple factors are involved in BP control, including water retention and salt intake, increased activity of vasoconstrictive systems, decreased activity of vasodilatory systems, increased intracellular calcium, increased arterial stiffness, oxidative stress, hyperparathyroidism, erythropoietin, inflammation, sleep apnea, renovascular disease, and overactivation of the sympathetic nervous system (1,4–6).

Norepinephrine clearance is reduced by 20% in mild renal failure and by up to 40% in hemodialysis patients. A recent study by Xu et al. (7) identified a new flavin–adenine dinucleotide–containing hormone named renalase that is secreted by the kidney and circulated in blood. Renalase was shown to degrade catecholamines, and it may play a role in the regulation of sympathetic tone and BP.

The aim of the present study was to assess, in a cohort of 26 PD patients, the serum concentration of renalase and the relationship of renalase to BP control, type of antihypertensive therapy, presence of residual renal function, and selected PD parameters [Kt/V and peritoneal equilibration test (PET) results].

METHODS

The 26 PD patients included in the study (mean age: 62 years; 53.85% men; mean duration of PD: 34.21 months) were recruited from the Peritoneal Dialysis Center in Bialystok, Poland. Healthy volunteers (n = 27) were also enrolled to provide normal renalase ranges. All participants were informed about the aims of the study and gave informed consent. The study was approved by the medical university’s ethics committee.

During routine ambulatory follow-up, blood pressure was measured using an automatic manometer with the patient in the sitting position. The arithmetical average of three measurements taken in different ambulatory examinations was used for the analysis. Well-controlled BP was assessed as any reading lower then 140/90 mmHg, according to the Kidney Disease Outcomes Quality Initiative guidelines (8). The Kt/V and PET variables were assessed using standard methods. Residual renal function was assessed based on a 24-hour urine collection exceeding 100 mL. Data on all hypotensive drugs and other medications were collected from prescription charts for the individual patients. Blood for the estimation of serum renalase—performed using a commercially available ELISA assay from Uscn Life Science, Wuhan, China—was taken once during a routine ambulatory visit (when BP and weight were also assessed). For analyses, the members of the study cohort were divided into groups by BP control, presence of residual renal function, Kt/V, and PET results (high and high average in one group, and low average and low in another). The Statistica program (version 9.0: StatSoft, Tulsa, OK, USA) was used for statistical analyses, with the Shapiro–Wilks test used to determine normal distributions, the Student t-test and Mann–Whitney U-test used for comparisons between groups, and the Spearman test used to determine correlations.

RESULTS

In the preliminary results, BP control was considered to be abnormal in 15 patients of the study cohort (57.69%) according to the Kidney Disease Outcomes Quality Initiative guidelines. The main antihypertensive medications used in the PD population were beta-blockers (23 patients, 88.46%), calcium channel blockers (19 patients, 73.07%), angiotensin converting-enzyme inhibitors (18 patients, 69.23%), and diuretics (14 patients, 53.84%). Residual renal function was present in 16 patients (61.54%). Median Kt/V in the group was 1.8. Mean serum renalase was significantly higher in the PD patients than in the control group (19.24 ± 4.50 μg/mL vs 3.86 ± 0.73 μg/mL, p < 0.001).

We found that the serum concentration of renalase was significantly higher in patients dialyzed for more than 6 months than in those dialyzed for fewer than 6 months (21.15 ± 4.58 μg/mL vs 16.63 ± 2.86 μg/mL, p = 0.008). No significant differences were observed in the serum concentration of renalase between the PD patients with good and poor BP control (19.53 ± 4 μg/mL and 18.84±5.1 μg/mL respectively), between those with a Kt/V above and below 1.7 (20.4 ± 3.92 μg/mL vs 18.97 ± 4.88 μg/mL respectively), and between those grouped by PET category (high and high average vs low and low average: 19.41 ± 4.47 μg/mL vs 20.48 ± 4.67 μg/mL). Nor did we observe a significant difference in the serum concentration of renalase between the PD patients with and without residual renal function (18.65 ± 3.96 μg/mL vs 20.36 ± 5.46 μg/mL, p = 0.366). No effect of sex on serum renalase level was observed (18.31 ± 4.38 μg/mL in men and 20.32 ± 4.57 μg/mL in women). A significant correlation was observed between serum renalase and duration of PD (r = 0.5464, p = 0.003), as was a significant inverse correlation between serum renalase and residual renal function (r = –0.4286, p = 0.02). No correlation was observed between serum renalase and BP level, age, sex, or Kt/V.

DISCUSSION

For the first time, serum renalase has been found to be significantly higher in PD patients than in healthy volunteers. Even more interestingly, serum renalase was significantly elevated in the group of patients dialyzed for more than 6 months. Renalase was not related to BP control, BP level, sex, dialysis adequacy, or residual renal function.

So far, no data on renalase in PD are available. In their publication, Xu et al. (7), using Western blotting with polyclonal antibodies, found that, compared with 4 healthy control subjects, 8 hemodialyzed patients showed diminished renalase expression. Western blotting is a semi-qualitative method. Our ELISA assay, using a monoclonal antibody specific to renalase, reached different results. Also, the present study assessed only the concentration of renalase, and the ELISA assay measures only the total level of renalase 1, regardless of whether the peptide is active or inactive. It might also be possible that the antibody in the ELISA assay binds to common fragments of different isoforms, meaning that the measured level could be very high but unrelated to activity. Xu et al. (7) assessed renalase activity, and therefore those results should not be compared with our antigen results because of the potential presence of renalase isoforms, as indicated. The manufacturer of the assay claims that only renalase 1 levels are measured. No data on possible cross-reactivity are available. Because renalase is secreted not only by the kidney, but also by cardiomyocytes, liver, and adipose tissue, it is possible that, in the case of end-stage renal failure, other organs and tissues may oversecrete renalase, leading to very high levels. When renalase expression is assessed, we cannot be sure that the protein is active. In addition, Xu et al. (7) suggested that an inhibitor of renalase was present in plasma, meaning that the level of the enzyme could be very high, but that its activity could be very low or negligible. On the other hand, Li et al. (9) found that renalase circulates as a pro-enzyme that lacks enzymatic activity under baseline conditions, and that it is rapidly activated by elevated plasma catecholamines. We therefore speculate that, in a state of catecholamine excess (that is, end-stage renal disease), activation of pro-enzyme renalase into active renalase is rapid and efficient in variety of tissues. That activation may lead to the increased renalase level seen in PD patients, as in the present study.

Boomsma and Tipton (10) question the method used by Xu et al. (7), considering that the (patho) physiologic concentration of catecholamines in vivo are lower than the concentrations used in the experiments. They even concluded that it is unlikely that renalase is a catecholamine-metabolizing enzyme; it may have important cardiovascular functions, but through another mechanism. The same doubts were recently presented by Medvedev and biochemist colleagues (11). On the other hand, Luft (12), according to studies quoted by him, noted that dopamine is associated with lower BP and cardiovascular risk, which is why diminishing dopaminergic tone (prompted by renalase, for example) would increase BP and cardiovascular risk.

CONCLUSIONS

Based on our findings, serum renalase appears possibly to be related to either or both of declining kidney function (as time on renal replacement therapy passes, residual renal function is lower) and dysfunction in other organs (because renalase is synthesized by other tissues such as the heart) and could be responsible for cardiovascular complications (13). However, the problem of the pathogenesis of hypertension in renal replacement therapy remains an open issue. Further studies are needed to prove or disprove the possible role of renalase in the pathogenesis of hypertension in patients with kidney disease.

DISCLOSURES

The authors have no financial conflicts of interest to disclose.