L-arginine-nitric oxide pathway in end-stage renal disease

Indolent course of tubulointerstitial disease in a mouse model of subpressor, low-dose nitric oxide synthase inhibition

Deficiency of nitric oxide (NO) represents a consistent manifestation of endothelial dysfunction (ECD), and the accumulation of asymmetric dimethylarginine occurs early in renal disease. Here, we confirmed in vitro and in vivo the previous finding that a fragment of collagen XVIII, endostatin, was upregulated by chronic inhibition of NO production and sought to support a hypothesis that primary ECD contributes to nephrosclerosis in the absence of other profibrotic factors. To emulate more closely the indolent course of ECD, the study was expanded to an in vivo model with N(G)-monomethyl-L-arginine (L-NMMA; mimics effects of asymmetric dimethylarginine) administered to mice in the drinking water at subpressor doses of 0.3 and 0.8 mg/ml for 3-6 mo. This resulted in subtle but significant morphological alterations detected in kidneys of mice chronically treated with L-NMMA: 1) consistent perivascular expansion of interstitial matrix components at the inner stripe of the outer medulla and 2) collagen XVIII/endostatin abundance. Ultrastructural abnormalities were detected in L-NMMA-treated mice: 1) increased activity of the interstitial fibroblasts; 2) occasional detachment of endothelial cells from the basement membrane; 3) splitting of the vascular basement membrane; 4) focal fibrosis; and 5) accumulation of lipofuscin by interstitial fibroblasts. Preembedding labeling of microvasculature with anti-CD31 antibodies showed infiltrating leukocytes and agglomerating platelets attaching to the visibly intact or denuded capillaries. Collectively, the data indicate that the mouse model of subpressor chronic administration of L-NMMA is not a robust one (endothelial pathology visible only ultrastructurally), and yet it closely resembles the natural progression of endothelial dysfunction, microvascular abnormalities, and associated tubulointerstitial scarring.

Plasma amino acid profile and L-arginine uptake in red blood cells from malnourished uremic patients

BACKGROUND

Patients with end-stage chronic renal failure (CRF) (uremia) have a high prevalence of inflammation, malnutrition, and oxidative stress. All of these features seem to be associated with the increased cardiovascular mortality observed in these patients. Nitric oxide (NO) is involved in the pathogenesis of CRF. The present study investigates the effects of nutritional status on L-arginine transport (NO precursor), plasma amino acid profile, and concentration of tumor necrosis factor (TNF)-alpha in uremic patients on hemodialysis (HD).

METHODS

A total of 32 uremic patients on regular HD and 16 healthy controls were included in this study. Kinetic studies of L-arginine transport, mediated by cationic transport systems y(+) and y(+)L into red blood cells, plasma concentrations of amino acids (measured by high-performance liquid chromatography), and plasma TNF-alpha level (evaluated by enzyme-linked immunosorbent assay), were analyzed in malnourished and well-nourished patients (isolated by body mass index).

RESULTS

L-arginine influx by system y(+) in red blood cells (micromol/L cells(-1)h(-1)) was increased in both malnourished (377 +/- 41) and well-nourished (461 +/- 63) patients with CRF compared with controls (287 +/- 28). Plasma levels of all cationic amino acids (L-arginine, L-ornithine, and L-lysine) were low in uremic patients compared with controls. Among the uremic population, the reduction in plasma cationic amino acids levels was greater in malnourished patients. L-cysteine and L-glutamate, precursors of glutathione, were dramatically increased in plasma from uremic patients, independently of nutritional status. In addition, TNF-alpha concentration in plasma was enhanced in malnourished uremic patients (3.4 +/- 0.7 pg/mL) compared with controls (1.2 +/- 0.1 pg/mL) and well-nourished patients (1.9 +/- 0.1 pg/mL).

CONCLUSIONS

Our results suggest an increased catabolism of cationic amino acids, inflammatory markers, and oxidative stress in CRF, especially in malnourished patients. The reduced plasma concentration of plasma L-arginine is counterbalanced by enhanced rates of transport, resulting in an activation of NO synthesis in uremia.

CHARACTERIZATION OF CATIONIC AMINO ACID TRANSPORT SYSTEMS IN RAT ERYTHROCYTES: LACK OF EFFECT OF URAEMIA ON l-ARGININE INFLUX

1. Chronic renal failure (CRF) is associated with the abnormal regulation of nitric oxide (NO) synthesis at the systemic level. The transport of L-arginine, upregulated in blood cells from uraemic patients, modulates NO synthesis in this pathological condition. The model of partial nephrectomy in rats is widely accepted as a valid model of uraemia. Because there are no reports of L-arginine transport in blood cells from uraemic rats, the aim of the present study was to investigate L-arginine transport in red blood cells (RBCs) from these rats. 2. The kinetics of L-arginine transport in RBC and plasma and the amino acid profiles of RBC were investigated in control, sham-operated and subtotally nephrectomized rats. 3. L-Arginine transport was mediated via the cationic amino acid transport system y+ and a transport system with kinetics resembling the human system y+L. In control RBC, the apparent Ki for L-leucine inhibition of L-arginine transport via system y+L was 0.16 +/- 0.02 and 4.8 +/- 2 mmol/L in the presence of Li+ and Na+, respectively. 4. The Vmax values for L-arginine transport via system y+L and system y+ were similar in RBC from control sham-operated and uraemic rats. Moreover, L-arginine concentrations in plasma and RBC were not affected by uraemia. 5. The findings of the present study provide the first evidence that L-arginine transport in rat erythrocytes is mediated by two distinct cationic transport systems with characteristics of systems y+ and y+L, which accept neutral amino acids only in the presence of Li+. In contrast with previous studies in uraemic patients, plasma levels and maximal transport rates of L-arginine were not altered in this rat model of CRF.

Effects of renal diseases on the regulation and expression of renal and hepatic drug-metabolizing enzymes: a review

1. The activity of drug-metabolizing enzymes (DMEs) in extrahepatic organs is highest in the kidneys. Generally, the kidneys contain most, if not all, of the DMEs found in the liver. Surprisingly, some of these DMEs show higher activity in the kidneys than in the liver. 2. Most of the renal DMEs are localized in the cortex of the kidneys, especially in the proximal tubules. DMEs are also found in the distal tubules and collecting ducts. 3. Renal diseases such as acute and chronic renal failure and renal cell carcinoma alter the regulation of both hepatic and extrahepatic phase I and II DMEs. Changes in the expression of these DMEs seem to be tissue and species specific. 4. Generally, there is significant down-regulation of most of the phase I and a few of phase II DMEs at the protein, mRNA and activity levels. Unfortunately, the mechanisms leading to the alteration in DMEs in renal diseases remain unclear, although many theories have been made. 5. The presence of some circulating factors such as cytokines, nitric oxide, parathyroid hormones and increased intracellular calcium play a role in the regulation of DMEs in renal diseases.

Regulatory and functional interaction of vasoactive factors in the kidney and extracellular pH

A growing body of evidence suggests that vasoactive factors produced in the kidney such as nitric oxide, endothelins, angiotensin, and prostaglandins participate actively in the regulation of acid-base homeostasis under physiologic conditions. In addition, recent reports indicate that alterations in the systemic acid-base status may also influence the generation of vasoactive cytokines in the kidney, which in turn may mediate the renal effector processes that tend to restore normality under such conditions. Metabolic acidosis, which so frequently accompanies many forms of chronic renal failure (CRF), may contribute to down-regulation of intrarenal nitric oxide production that characterizes CRF. Reduced extracellular pH inhibits inducible nitric oxide production in mesangial cells by altering the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidation, an important posttranslational mechanism in the inducible nitric oxide synthase (iNOS) activation. The underlying defects resulting in the uncoupling of NADPH oxidation in acidemic microenvironment are discussed. Acidosis stimulates renal production of endothelins, which mediate proximal tubular acidification by enhancing sodium-hydrogen exchanger-3 (NHE-3) activity. Renal endothelins mediate enhanced urinary acid excretion following dietary acid ingestion, an effect that is effectively blocked by endothelin receptor blockers. Reduced extracellular pH stimulates endothelin secretion from renal microvascular endothelial cells, which may promote enhanced acid excretion from the distal tubule under conditions of acidosis. These phenomena as well as the role of angiotensin and renal prostaglandins in mediating renal acidification in normal and acidotic conditions are discussed in this review, which describe the regulatory interaction between extracellular pH and renal vasoactive factors.

Basic Science and Dialysis: Nitric Oxide and Hemodialysis

Nitric oxide (NO), previously thought of as a noxious gas, is now recognized as an important mediator of vascular responsiveness. Soon after its discovery, it was realized that the actions of NO are similar to the previously described endothelium-derived relaxing factor (EDRF). It is synthesized in the vascular endothelium utilizing the enzyme nitric oxide synthase (NOS) and diffuses in the adjacent vascular media, where it has a vasodilatory action. Opposing actions of NO and vasoconstrictor agents (such as endothelin-1, angiotensin IotaIota, and others) maintain the vascular tone of the renal arteries. The same balance at the level of the macula densa maintains glomerular filtration rate (GFR) during varying levels of salt excretion. Lack of NO can result in disruption of this fine balance, with resultant vasoconstriction and disease progression, hypertension, and accelerated atherosclerosis. In addition, hypertension may result from positive salt balance that occurs when macula densa NOS is inhibited. While most investigators report low levels of NO in uremic subjects, the levels in hemodialysis (HD) patients have not been characterized adequately. This is primarily because HD patients are exposed to both stimulatory and inhibitory factors for NO synthesis. Retention of inhibitors of NOS tends to decrease NO levels, whereas production of NO will be increased by cytokines generated during blood-dialyzer interaction. There is less disagreement, however, over the finding of elevated levels in those with dialyzer reactions and dialysis-induced hypotension. Recent developments in the isolation of inducible and constitutive forms of NOS makes understanding of its pathophysiologic effects more complete. Newer treatment directed at inhibiting only the inducible forms of NOS (sparing the constitutive forms) may soon be found useful for the treatment and prevention of hypotension and dialyzer reactions in HD patients.