Hemodialysis hypotension: interaction of inhibitors, iNOS, and the interdialytic period

Interaction of Human Serum Albumin with Uremic Toxins: The Need of New Strategies Aiming at Uremic Toxins Removal

Chronic kidney disease (CKD) is acknowledged worldwide to be a grave threat to public health, with the number of US end-stage kidney disease (ESKD) patients increasing steeply from 10,000 in 1973 to 703,243 in 2015. Protein-bound uremic toxins (PBUTs) are excreted by renal tubular secretion in healthy humans, but hardly removed by traditional haemodialysis (HD) in ESKD patients. The accumulation of these toxins is a major contributor to these sufferers’ morbidity and mortality. As a result, some improvements to dialytic removal have been proposed, each with their own upsides and drawbacks. Longer dialysis sessions and hemodiafiltration, though, have not performed especially well, while larger dialyzers, coupled with a higher dialysate flow, proved to have some efficiency in indoxyl sulfate (IS) clearance, but with reduced impact on patients’ quality of life. More efficient in removing PBUTs was fractionated plasma separation and adsorption, but the risk of occlusive thrombosis was worryingly high. A promising technique for the removal of PBUTs is binding competition, which holds great hopes for future HD. This short review starts by presenting the PBUTs chemistry with emphasis on the chemical interactions with the transport protein, human serum albumin (HSA). Recent membrane-based strategies targeting PBUTs removal are also presented, and their efficiency is discussed.

The Influence of Sevelamer Hydrochloride and Calcium Carbonate on Markers of Inflammation and Oxidative Stress in Hemodialysis at Six Months of Follow-Up

Patients with end-stage renal disease (ESRD) present alterations in mineral and bone metabolism. Hyperphosphatemia in ESRD is considered an independent risk factor for cardiovascular disease (CVD), increasing morbidity, and mortality. Sevelamer hydrochloride is a calcium-free, non-absorbable phosphate-chelating polymer. Calcium carbonate chelator is helpful in controlling serum phosphate levels. There is insufficient information on the influence of sevelamer hydrochloride and calcium carbonate on the behavior of oxidative stress (OS) markers and inflammation in patients on hemodialysis (HD). A randomized open clinical trial was carried out on patients to evaluate sevelamer hydrochloride and calcium carbonate influence at 6 months of study follow-up. Levels of oxidants (LPO, NO, and 8-isoprostanes), antioxidants (SOD and TAC), oxidative DNA damage (8-OHdG and hOGG1), pro-inflammatory cytokines (IL-6 and TNF-α), and inflammation markers (ferritin and C-reactive protein) were measured with colorimetric and ELISA methods. We found a significant increase in oxidants LPO and NO, and antioxidants SOD and TAC, and downregulation of IL-6 and TNF-α. Ferritin decrease at 6 months follow-up in the sevelamer hydrochloride group. Increase in C-reactive protein was found in the group of patients treated with calcium carbonate. In conclusion, we found an oxidative state imbalance with increase in LPO and NO oxidants. The activity of the antioxidant enzymes (SOD and TAC) was also found to increase, suggesting a compensatory effect in the face of increase in oxidants. The same phenomenon was observed with increase in the oxidative damage marker to DNA and the increase in the DNA repair enzyme, suggesting a compensatory effect. Pro-inflammatory cytokines were predominantly downregulated by TNF-α in the group that ingested sevelamer hydrochloride in the final determination at 6 months of follow-up. Serum ferritin levels decreased significantly at the end of follow-up in patients on HD in the sevelamer hydrochloride group. The management of hyperphosphatemia with sevelamer hydrochloride appears to have obvious anti-inflammatory and antioxidant benefits.

The effect of hemodialysis on ocular changes in patients with the end-stage renal disease

Background: Numerous metabolic parameters can be changed during hemodialysis in the end-stage renal disease (ESRD) caused by systemic diseases, such as diabetes mellitus, hypertension. Some ocular parameters also can be variable due to the changes after hemodialysis. This study evaluates the effects of ocular parameters, including best-corrected visual acuity (BCVA), intraocular pressure (IOP), central macular thickness (CMT), subfoveal choroidal thickness (SFCT), retinal arteriolar caliber (RAC), retinal venular calibre (RVC), in ESRD patients following hemodialysis.

Materials and methods: Two-hundred and two ESRD patients were recruited resulting in 404 eyes evaluations. All patients underwent hemodialysis in the Dialysis Unit of the Second Hospital of Tianjin Medical University. BCVA, CMT, IOP, SFCT, RAC and RVC were evaluated before and after hemodialysis. Systemic parameters were collected such as age, body weight, systolic blood pressure (SBP), diastolic blood pressure (DBP), duration of hemodialysis, body weight changes, high density lipoprotein cholesterol (HDLC), low density lipoprotein cholesterol (LDLC), very low density lipoprotein cholesterol (VLDLC), glycosylated hemoglobin (HbA1c).

Results: The causes of ESRD patients included chronic glomerulonephritis (n = 65), diabetes mellitus (n = 60), hypertensive nephrosclerosis (n = 37), and other causes (n = 40). In our study, BCVA (p = .817), CMT (p = .252) and IOP (p = .978) did not significantly change after hemodialysis. SFCT significantly decreased from 254.29 ± 69.36 μm to 235.54 ± 659.90 μm (p = .002) following hemodialysis. SFCT changes were significantly correlated with SBP (p = .042) and body weight changes (p = .044). The RAC and RVC were dilated significantly (p = .033, p = .007). RVC changes were correlated with baseline DBP (p = .003), HDLC (p = .009), LDLC (p = .004) and changes in DBP (p = .037) and body weight (p = .001).

Conclusion: Hemodialysis can affect various ocular parameters including SFCT, RAC and RVC, which changed significantly following hemodialysis. Whereas BCVA, IOP and CMT did not change after hemodialysis in ESRD patients. The systemic compensatory mechanisms of the changes in SBP, DBP, body weight following hemodialysis need further study.

The renal transport protein OATP4C1 mediates uptake of the uremic toxin asymmetric dimethylarginine (ADMA) and efflux of cardioprotective L-homoarginine

Elevated plasma concentrations of the uremic toxin asymmetrical dimethylarginine (ADMA) and low plasma concentrations of L-homoarginine are independently associated with cardiovascular events and total mortality. Enzymes degrading ADMA [dimethylaminohydrolase 1 (DDAH1)] and synthesizing L-homoarginine [L-arginine:glycine amidinotransferase (AGAT)] are expressed in human proximal tubule cells. So far, it is not known which transport protein in the basolateral membrane of proximal tubule cells is mediating the uptake of ADMA into the cells for subsequent degradation or the export of intracellularly synthesized L-homoarginine. One study suggested that the uptake transporter OATP4C1 (gene symbol SLCO4C1) may be involved in the transport of ADMA and other uremic toxins. OATP4C1 is a member of the SLCO/SLC21 family of solute carriers, localized in the basolateral membrane of human proximal tubule cells. By using stably-transfected HEK cells overexpressing human OATP4C1, we demonstrate that ADMA and L-homoarginine are substrates of OATP4C1 with K_m values of 232.1 μM and 49.9 μM, respectively. ADMA and the structurally related uremic toxin SDMA (100 μM) inhibited OATP4C1-mediated L-homoarginine uptake (P < 0.01), whereas other tested uremic toxins such as urea and p-cresyl sulfate have no effect on OATP4C1-mediated transport. Preloading experiments (300 μM for 60 min) with subsequent efflux studies revealed that OATP4C1 also facilitates efflux e.g. of L-homoarginine. Both ADMA and L-homoarginine are substrates of human OATP4C1. Because proximal tubule cells are one site of ADMA metabolism and L-homoarginine synthesis, we postulate a protective role of OATP4C1 by mediating uptake of ADMA from and export of L-homoarginine into the systemic circulation.

Age-related changes in ADMA-DDAH-NO pathway in rat liver subjected to partial ischemia followed by global reperfusion

BACKGROUND

Liver function is affected during ischemia/reperfusion (IR). We evaluated the effect of the aging process on selected parameters determining the NO level in rat liver subjected to IR.

METHODS

The animals were divided into the C-2 and the IR-2 group of young rats (2-4 months old) and the C-12 and the IR-12 group of older rats (12-14 months old). Livers belonging to the IR-2 and the IR-12 group were subjected to partial ischemia (60 min) and reperfusion (4 h). Blood samples were obtained after surgeries to estimate the activity of aminotransferases, as well as just before ischemia and during reperfusion (15, 120, and 240 min) to estimate concentration of arginine (Arg) and its derivatives: asymmetric and symmetric dimethylarginine (ADMA, SDMA). After IR, dimethylarginine dimethylaminohydrolase (DDAH) activity and protein concentration of inducible nitric oxide synthase (iNOS) were measured in liver homogenates.

RESULTS

In the IR-2 group ADMA level increased the most between 15 and 120 min of reperfusion and was the highest of all the groups (0.72±0.2 μmol/l). In the IR-12 group ADMA level decreased significantly and was lower compared to all the other groups at 15 min (0.42±0.2 μmol/l) and to IR-2 at 120 (0.52±0.1 μmol/l) and 240 min (0.38±0.1 μmol/l) of reperfusion. Only the IR-2 group SDMA level increased significantly between 15 (0.75±0.9 μmol/l) and 240 min (1.0±1.2 μmol/l) of reperfusion. At the beginning of the surgery the Arg level was significantly higher in young rats (C-2: 102.1±35.7 μmol/l; IR-2: 114.63±28.9 μmol/l) than in older ones (C-12: 41.88±44.7 μmol/l; IR-12: 28.64±30.6 μmol/l). In the C-2 group the Arg level (77.41±37.5 μmol/l) and Arg/ADMA (A/A) ratio (138.03±62.8 μmol/l) were significantly higher compared to the ischemic groups at 15 min and to all the other groups at 120 (Arg: 47.17±31.7 μmol/l; A/A: 88.28±66.2 μmol/l) and 240 min (Arg: 43.87±21.9 μmol/l; A/A: 118.02±106.3 μmol/l). In the IR-2 group Arg level (11.4±12.0 μmol/l) and A/A ratio (16.11±16.2 μmol/l) decreased significantly at 15 min and during the next phase of reperfusion the levels of those parameters were low, comparably to those in IR-12. As a result of IR, a decrease in DDAH activity and an increase in iNOS protein concentration were observed only in the young rats.

CONCLUSIONS

We found that in the non-ischemic groups the Arg level may be affected by the aging process. Under IR conditions, important changes in DDAH-ADMA-NO pathway were observed only in young livers.

Microvascular dilatation after haemodialysis is determined by the volume of fluid removed and fall in mean arterial pressure

BACKGROUND/AIMS

The effects of haemodialysis on the microcirculation are poorly understood. This study examined the changes in small vessel calibre.

METHODS

24 patients (including 12 males, median age 62.5 years, range 30-87) underwent digital retinal photography immediately before and after routine haemodialysis. Arteriolar and venular calibres were measured from the images by a trained grader using a highly reproducible, computer-assisted method.

RESULTS

Patients had an average 2.0 ± 0.3 litres of fluid removed with dialysis, and their mean arterial blood pressure fell by 6.8 mm Hg (CI 13.8-0.2, p = 0.06). Retinal arteriole calibre did not change (mean difference 2.3 µm, CI -1.1 to 5.7, p = 0.17) but the venules dilated (mean difference 12.7 µm, CI 7.3-18.3, p < 0.001). Calibre returned to baseline by 2 h. Venules dilated less in diabetics than non-diabetics (mean difference -6.2 µm, CI -9.6 to -2.9, p < 0.01). Retinal venular dilatation correlated positively with the volume of fluid removed per kilogramme body weight (5.9, CI 0.2-11.5, p = 0.04), and negatively with the fall in mean arterial pressure (-0.36, CI -0.72 to -0.01, p < 0.05) after adjusting for age, gender, diabetes and dyslipidaemia.

CONCLUSION

Haemodialysis is associated with systemic venular dilatation.

Haemodialysis acutely reduces the plasma levels of ADMA without reversing impaired NO-dependent vasodilation

End-stage renal disease patients have endothelial dysfunction and high plasma levels of ADMA (asymmetric ω-NG,NG-dimethylarginine), an endogenous inhibitor of NOS (NO synthase). The actual link between these abnormalities is controversial. Therefore, in the present study, we investigated whether HD (haemodialysis) has an acute impact on NO-dependent vasodilation and plasma ADMA in these patients. A total of 24 patients undergoing maintenance HD (HD group) and 24 age- and gender-matched healthy controls (Control group) were enrolled. The increase in forearm SkBF (skin blood flow) caused by local heating to 41 °C (SkBF41), known to depend on endothelial NO production, was determined with laser Doppler imaging. SkBF41 was expressed as a percentage of the vasodilatory reserve obtained from the maximal SkBF induced by local heating to 43 °C (independent of NO). In HD patients, SkBF41 was assessed on two successive HD sessions, once immediately before and once immediately after HD. Plasma ADMA was assayed simultaneously with MS/MS (tandem MS). In the Control group, SkBF41 was determined twice, on two different days, and plasma ADMA was assayed once. In HD patients, SkBF41 was identical before (82.2±13.1%) and after (82.7±12.4%) HD, but was lower than in controls (day 1, 89.6±6.1; day 2, 89.2±6.9%; P<0.01 compared with the HD group). In contrast, plasma ADMA was higher before (0.98±0.17 μmol/l) than after (0.58±0.10 μmol/l; P<0.01) HD. ADMA levels after HD did not differ from those obtained in controls (0.56±0.11 μmol/l). These findings show that HD patients have impaired NO-dependent vasodilation in forearm skin, an abnormality not acutely reversed by HD and not explained by ADMA accumulation.

Asymmetrical dimethylarginine in renal disease: limits of variation or variation limits? A systematic review

Asymmetrical dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, is increasingly recognized as a putative biomarker in cardiovascular and renal disease. Elevated plasma levels of ADMA are the consequence of increased synthesis, reduced renal clearance or reduced enzymatic degradation. Based upon the metabolic fate the highest plasma concentrations of ADMA have been reported in patients with renal failure in whom this molecule accumulates. However, the range of published ADMA levels in patients with chronic renal failure as well as in patients with end-stage renal failure undergoing maintenance hemodialysis, peritoneal dialysis or kidney transplant recipients is widely scattered and overlaps with the levels reported in healthy individuals. This wide distribution can in part be explained by different bioanalytical techniques and the lack of standardization of such assays. This review summarizes available literature on ADMA in patients with kidney disease and stresses the urgent need for a consensus regarding reference values for different analytical methods in order to appreciate the prognostic significance of elevated ADMA levels. At present, one cannot advocate this molecule for risk assessment or individual patient prognosis in the clinical work-up of patients with renal impairment.

Inconsistency of reported uremic toxin concentrations

Discrepancies in reported uremic toxin concentrations were evaluated for 78 retention solutes. For this analysis, 378 publications were screened. Up to eight publications per toxin were retained. The highest and the lowest reported concentrations, as well as the median reported concentration were registered. The ratio between the highest and the lowest (H/L) concentrations and, for some solutes, also the ratio between the highest and the median (H/M) concentrations were calculated. The compounds were arbitrarily subdivided into three groups based on their H/L ratio: group A, H/L < 3 (n = 33); group B, 3 < H/L < 8.5 (n = 20); and group C, H/L > 8.5 (n = 25). Solutes of groups A and B showed a low to intermediate scatter, suggesting a homogeneity of reported data. Group C showed a more substantial scatter. For at least 10 compounds of group C, extremely divergent concentrations were registered (H/M > 5.5) using scatter plot analysis. For all solutes of groups A and B, the highest reported concentration could be used as a reference. For some solutes of group C and for the compounds showing a divergent scatter analysis, however, more refined directives should be followed.

ACTIVATION OF l-ARGININE TRANSPORT IN UNDIALYSED CHRONIC RENAL FAILURE AND CONTINUOUS AMBULATORY PERITONEAL DIALYSIS PATIENTS

1. Treatment with haemodialysis and continuous ambulatory peritoneal dialysis (CAPD) presents different pathophysiological profiles and it has been suggested that clinical outcome in chronic renal failure may depend on the mode of dialysis. The transport of L-arginine, a precursor of nitric oxide, into blood cells is increased in uraemic patients on haemodialysis. The present study was designed to investigate L-arginine transport into red blood cells (RBC) in uraemic patients not yet on dialysis and on CAPD therapy. 2. Eleven uraemic patients not yet on dialysis and 17 on CAPD were included in the study. L-Arginine transport into RBC and plasma and RBC amino acid profiles were analysed in these sets of patients. 3. L-Arginine transport via system y(+), but not y(+)L, into RBC, was significantly increased in undialysed uraemic patients (459 +/- 40 micromol/L per cell per h) and CAPD patients (539 +/- 61 micromol/L per cell per h) compared with controls (251 +/- 39 micromol/L per cell per h). High-pressure liquid chromatography measurements demonstrated low levels of plasma L-arginine in uraemic patients both on CAPD (54 +/- 3 micromol/L) and not yet on dialysis (80 +/- 6 micromol/L) compared with control subjects (146 +/- 14 micromol/L). 4. Our findings provide the first evidence that uraemic patients not yet on dialysis and on CAPD present with an activation of L-arginine transport via system y(+) into RBC associated with reduced plasma levels of L-arginine.

Relationship of asymmetric dimethylarginine to haemodialysis hypotension

Hypotension is one of the major complications in patients undergoing haemodialysis (HD), that is well evident in patients defined as "hypotension-prone." The mechanisms underlying the hypotensive episodes are not known. We carried out a clinical study on hypotension-prone HD patients to test the existence of a dysregulation in the nitric oxide (NO) generating pathway. Since asymmetric dimethylarginine (ADMA) is an endogenous compound which regulates NO synthesis, we measured its variation in plasma of stable-HD and hypotension-prone patients before, during, and at the end of HD. Before HD, the hypotension-prone patients have higher ADMA levels than stable-HD patients. The HD procedure significantly removes ADMA from plasma of stable-HD patients, while in the hypotension-prone ADMA levels are unchanged at the end of the HD. Moreover, in the hypotension-prone patients, during the hypotensive episode, a dramatic drop of ADMA levels is observed, followed by a rapid increase at the end of the HD. The symmetric dimethylarginine (SDMA), which has no effect on NO synthesis, is also high in plasma of both groups of HD patients compared to normal subjects, and in both groups its levels at the end of HD are significantly reduced. The hypotension-prone patients have basal TNF-alpha levels lower than the stable-HD groups, that significantly increase during the hypotensive episode. On the basis of these findings, we suggest that the hypotensive syndrome could be related to a dysregulation between ADMA metabolism and clearance due both to cytokines release and to an extremely fast ADMA clearance during HD, leading to an increase in NO blood levels.

Preoperative supplementation with a carbohydrate mixture decreases organ dysfunction-associated risk factors

BACKGROUND & AIMS

Recently, both asymmetrical dimethylarginine and IL-6 have been suggested to be associated with the induction and severity of single and multiple organ dysfunction. The aims of the present study were to elucidate if these factors were increased in an ischemia reperfusion (IR) model and whether pre-operative carbohydrate supplementation can reduce the risk factors along with the IR injury.

METHODS

One group of male Wistar rats was fasted for 16 h (water ad libitum) prior to clamping the superior mesenteric artery (IR fasted n=14). A second group had ad libitum access to a carbohydrate solution prior to clamping (IR fasted CHO group n=11). Sham-fasted animals, which only received laparotomy and no clamping, served as controls (n=4).

RESULTS

Plasma urea and ALAT activity were both increased in the IR fasted animals when compared to the sham rats (P=0.007 and P<0.02, respectively). Furthermore, it was shown that IR fasted rats had increased ADMA and IL-6 concentration in plasma when compared to sham animals (P<0.02). Moreover, the GSH level in lung was significantly decreased in the IR fasted animals (P=0.014). IR CHO supplemented showed no significant increase of ALAT activity and decrease of lung GSH. Furthermore, significantly lower plasma urea, ADMA and IL-6 concentration was seen in the IR CHO supplemented group when compared to the IR fasted rats (P=0.028, P<0.01 and P<0.02, respectively). The liver glycogen concentration in IR fasted rats was 48% of that IR rats supplemented the carbohydrate mixture.

CONCLUSION

The present rat intestinal ischemia reperfusion model not only induces organ injury indicated by the classical parameters such as plasma urea and ALAT activity, but also increased plasma IL-6 and ADMA and decreased lung GSH concentration in IR fasted rats. Pre-operative supplementation with the carbohydrate mixture significantly lowered the plasma urea, IL-6 and ADMA concentrations and maintained lung GSH concentration. This indicates that pre-operative carbohydrate supplementation reduces post-operative organ injury.

Effect of haemodialysis on retinal circulation in patients with end stage renal disease

AIMS

To investigate the effect of haemodialysis on retinal circulation in patients with end stage renal disease (ESRD).

METHOD

Seventeen consecutive patients with ESRD were recruited into the study. The authors simultaneously measured changes in vessel diameter and blood velocity and calculated the retinal blood flow (RBF) in the retinal veins in patients with ESRD before and after haemodialysis using a laser Doppler velocimetry system. In addition, the relations between the changes in systemic and retinal circulatory parameters were examined.

RESULTS

There was a group averaged increase in vessel diameter (p = 0.003) after haemodialysis. However, the blood velocity and RBF values obtained after haemodialysis were not significantly different from those before haemodialysis (p = 0.66 and p = 0.63, respectively). The changes in vessel diameter were negatively (r = -0.549, p = 0.02) correlated with the change in MABP, but the changes in blood velocity and RBF were positively correlated with the change in MABP (r = 0.683, p<0.002 and r = 0.589, p<0.01, respectively). The change in RBF was also inversely correlated with the increase in haematocrit (r = -0.693, p<0.002) and the amount of fluid removed (r = -0.597, p<0.01).

CONCLUSION

The results indicate that haemodialysis and the associated changes in systemic circulatory parameters may affect the retinal circulation in patients with ESRD.

Low water-soluble uremic toxins

The uremic syndrome is the result of the retention of solutes, which under normal conditions are cleared by the healthy kidneys. Uremic retention products are arbitrarily subdivided according to their molecular weight. Low-molecular-weight molecules are characterized by a molecular weight below 500 D. The purpose of the present publication is to review the main water soluble, nonprotein bound uremic retention solutes, together with their main toxic effects. We will consecutively discuss creatinine, glomerulopressin, the guanidines, the methylamines, myo-inositol, oxalate, phenylacetyl-glutamine, phosphate, the polyamines, pseudouridine, the purines, the trihalomethanes, and urea per se.

Nitric Oxide Synthetic Capacity in Relation to Dialysate Temperature

BACKGROUND

During hemodialysis, vascular reactivity is impaired, which can be corrected by lowering dialysate temperature. It has also been shown that nitric oxide (NO) is related to intradialytic hypotension. As NO synthesis may be temperature-dependent, this study addressed the influence of dialysate temperature on the NO synthetic capacity of plasma.

METHODS

NO synthetic capacity was studied during hemodialysis with a dialysate temperature of 37.5 degrees C (dialysis-37.5 degrees C) and programmed extracorporeal blood cooling (cool dialysis; Blood Temperature Monitor; Fresenius C) in 12 stable patients. NO synthetic capacity was assessed ex vivo by [3H]L-citrulline formation from [3H]L-arginine in cultured endothelial cells after incubation with plasma samples obtained during the respective sessions.

RESULTS

Core temperature decreased (-0.32 +/- 0.10 degrees C) and energy transfer rate was significantly lower (-27.5 +/- 2.8 W; p < 0.05) during cool dialysis compared to dialysis-37.5 degrees C (0.19 +/- 0.06 degrees C and -0.8 +/- 1.2 W respectively; p < 0.05). Systolic blood pressure decreased during dialysis-37.5 degrees C (-19 +/- 4 mm Hg; p < 0.05), but not during cool dialysis (-6 +/- 5 mm Hg). NO synthetic capacity increased during dialysis-37.5 degrees C (55.5 +/- 9.3 to 73.5 +/- 10.2 pmol/10(5) cells; p < 0.05), in contrast to cool dialysis (67.3 +/- 11.1 to 66.2 +/- 10.8 pmol/10(5) cells).

CONCLUSION

The stimulatory effect of uremic plasma on endothelial NO synthesis was augmented during dialysis-37.5 degrees C but not during cool dialysis.

Handling of asymmetrical dimethylarginine and symmetrical dimethylarginine by the rat kidney under basal conditions and during endotoxaemia

BACKGROUND

Asymmetrical dimethylarginine (ADMA) is capable of inhibiting nitric oxide synthase enzymes, whereas symmetrical dimethylarginine (SDMA) competes with arginine transport. The potential role of inflammation in the metabolism of ADMA has been elucidated in an in vitro model using tumour necrosis factor-alpha, resulting in a decreased activity of the ADMA-degrading enzyme dimethylarginine dimethylaminohydrolase (DDAH). The kidney probably plays a crucial role in the metabolism of ADMA by both urinary excretion and degradation by DDAH. We aimed to further elucidate the role of the kidney in a rat model under basal conditions and during endotoxaemia.

METHODS

Twenty-five male Wistar rats weighing 275-300 g were used for this study. The combination of arteriovenous concentration differences and kidney blood flow allowed calculation of net organ fluxes. Blood flow was measured using radiolabelled microspheres according to the reference sample method. Concentrations of ADMA, SDMA and arginine were measured by high-performance liquid chromatography.

RESULTS

The kidney showed net uptake of both ADMA and SDMA and fractional extraction rates were 35% and 31%, respectively. Endotoxaemia resulted in a lower systemic ADMA concentration (P = 0.01), which was not explained by an increased net renal uptake. Systemic SDMA concentrations increased during endotoxaemia (P = 0.007), which was accompanied by increased creatinine concentrations.

CONCLUSIONS

The rat kidney plays a crucial role in the regulation of concentrations of dimethylarginines, as both ADMA and SDMA were eliminated from the systemic circulation in substantial amounts. Furthermore, evidence for the role of endotoxaemia in the metabolism of dimethylarginines was obtained as plasma levels of ADMA were significantly lower in endotoxaemic rats.

Asymmetric dimethylarginine, C-reactive protein, and carotid intima-media thickness in end-stage renal disease

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase that has been linked to endothelial dysfunction and atherosclerosis in the general population. ADMA is also elevated in end-stage renal disease and may contribute to the high cardiovascular risk in patients with chronic renal failure. A prospective cohort study was performed to investigate the relationship between plasma ADMA, C-reactive protein (CRP), and intima-media thickness (IMT) in 90 patients undergoing hemodialysis. In the baseline study, plasma ADMA was directly related to IMT both on univariate analysis (r = 0.32, P = 0.002) and on multiple regression analysis (beta = 0.23, P = 0.01). In the follow-up study (15 mo) IMT changes were significantly related to ADMA (r = 0.51, P = 0.02) and serum CRP (r = 0.53, P = 0.01) in patients with initially normal IMT. In these patients, ADMA and CRP were strongly interrelated (r = 0.64, P = 0.002), and on multiple regression analysis the interaction between ADMA and CRP emerged as the sole independent predictor of the progression of intimal lesions. Independently of other risk factors, plasma ADMA in patients on hemodialysis is significantly related to IMT. Furthermore, in patients with initially normal IMT, ADMA and CRP are interacting factors in the progression of carotid intimal lesions. These data support the hypothesis that accumulation of this endogenous inhibitor of NO synthase is an important risk factor for cardiovascular disease in chronic renal failure and suggest a possible link between ADMA and inflammation.

Elevated serum endogenous inhibitor of nitric oxide synthase and endothelial dysfunction in aged rats

1. The purpose of the present study was to determine the relationship between endothelial dysfunction and the endogenous inhibitor of nitric oxide synthase NG,NG'-asymmetric dimethylarginine (ADMA) in aged rats. 2. Studies were performed in male adult Sprague-Dawley rats (6 months old; n = 8) and in aged rats (20 months old; n = 8). Serum levels of ADMA and L-arginine were measured by high-performance liquid chromatography and responses of endothelium-intact aortic rings to acetylcholine (ACh) were tested. Nitric oxide synthase activity in kidney tissue and serum concentrations of nitrite, a stable end-product of nitric oxide, were assayed and serum contents of malondialdehyde, derived from lipid peroxidation and serum lipid and creatinine level were determined. 3. Serum levels of ADMA increased significantly in aged rats compared with adult rats (P < 0.01), whereas serum levels of L-arginine were similar in both groups (P = NS). Accordingly, the ratio of L-arginine/ADMA in old rats was lower than that in young rats (P < 0.01). Endothelium-dependent relaxation responses to ACh in aortic rings from aged rats were impaired and these impaired responses were improved by pre-incubation of aortic rings with L-arginine. 4. Nitric oxide synthase activity in the kidney, together with serum concentration of nitrite, was significantly decreased and serum contents of malondialdehyde, cholesterol and triglycerides were increased in old compared with young rats. However, the serum creatinine level was not significantly different between adult and aged rats. 5. Endogenous ADMA may be a contributor to age-related endothelial dysfunction and increases in endogenous ADMA may be linked to lipid peroxidation in aged rats.