Muscle ammonia and glutamine exchange during chronic liver insufficiency in the rat

Nutraceuticals for the treatment of sarcopenia in chronic liver disease

BACKGROUND AND AIMS

Sarcopenia, defined as loss of muscle mass, strength and function, is associated with adverse clinical outcomes in patients with cirrhosis. Despite improved understanding of the multifaceted pathogenesis, there are few established therapies to treat or prevent muscle loss in this population. This narrative review examines the available literature investigating the role of nutraceuticals for the prevention or treatment of muscle wasting in chronic liver disease.

METHODS

A comprehensive search or Medline and PubMED databases was conducted. Reference lists were screened to identify additional articles.

RESULTS

A number of nutritional supplements and vitamins target the specific metabolic derangements that contribute to sarcopenia in cirrhosis including altered amino acid metabolism, hyperammonaemia and inflammation. Branched chain amino acid (BCAA) supplementation has proposed anabolic effects through dual pathways of enhanced ammonia clearance and stimulation of muscle protein synthesis. l-carnitine also has multimodal effects on muscle and shows promise as a therapy for muscle loss through anti-inflammatory, antioxidant and ammonia lowering properties. Other nutraceuticals including l-ornithine l-aspartate, omega-3 polyunsaturated fatty acids and zinc and vitamin D supplementation, may similarly have positive effects on muscle homeostasis, however further evidence to support their use in cirrhotic populations is required.

CONCLUSION

Nutraceuticals offer a promising and likely safe adjunct to standard care for sarcopenia in cirrhosis. While there is most evidence to support the use of BCAA and l-carnitine supplementation, further well-designed clinical trials are needed to elucidate their efficacy as a therapy for muscle loss in this population.

Serum zinc concentration and quality of life in chronic liver diseases

Health related quality of life (HRQOL) in chronic liver disease (CLD) patients has been attracting much attention these days because it is closely associated with clinical outcomes in CLD patients. HRQOL has become established as an important concept and target for research and practice in the fields of medicine. A critique of HRQOL research is the lack of conceptual clarity and a common definition of HRQOL. Using a clear definition of HRQOL may increase the conceptual understanding. In this study, we aimed to elucidate the association between serum zinc (Zn) level and HRQOL as assessed by the Beck Depression Inventory-2nd edition (BDI-II), Pittsburgh Sleep Quality Index Japanese version (PSQI-J) and the 36-Item Short Form Health Survey (SF-36) in CLD patients (n = 322, median age = 65 years, 121 liver cirrhosis (LC) patients (37.6%)). The median serum Zn level for all cases was 73.2 μg/dl. The median BDI-II score and PSQI-J score were 6 and 5, respectively. Patients with higher BDI-II score tended to have lower serum Zn level compared with those with lower BDI-II score. Similar tendencies were observed in patients with higher PSQI-J score. In the SF-36, physical functioning, role physical and physical component summary score significantly correlated with serum Zn level regardless of age, liver disease etiology and the LC status. While mental health and mental component summary score did not significantly correlate with serum Zn level regardless of age, liver disease etiology and the LC status. In conclusion, serum Zn level can be a useful marker for decreased HRQOL in patients with CLDs, especially for physical components.

Differential ammonia metabolism and toxicity between avian and mammalian species, and effect of ammonia on skeletal muscle: A comparative review

Comparative aspects of ammonia toxicity, specific to liver and skeletal muscle and skeletal muscle metabolism between avian and mammalian species are discussed in the context of models for liver disease and subsequent skeletal muscle wasting. The purpose of this review is to present species differences in ammonia metabolism and to specifically highlight observed differences in skeletal muscle response to excess ammonia in avian species. Ammonia, which is produced during protein catabolism and is an essential component of nucleic acid and protein biosynthesis, is detoxified mainly in the liver. While the liver is consistent as the main organ responsible for ammonia detoxification, there are evolutionary differences in ammonia metabolism and nitrogen excretory products between avian and mammalian species. In patients with liver disease and all mammalian models, inadequate ammonia detoxification and successive increased circulating ammonia concentration, termed hyperammonemia, leads to severe skeletal muscle atrophy, increased apoptosis and reduced protein synthesis, altogether having deleterious effects on muscle size and strength. Previously, an avian embryonic model, designed to determine the effects of increased circulating ammonia on muscle development, revealed that ammonia elicits a positive myogenic response. Specifically, induced hyperammonemia in avian embryos resulted in a reduction in myostatin, a well-known inhibitor of muscle growth, expression, whereas myostatin expression is significantly increased in mammalian models of hyperammonemia. These interesting findings imply that species differences in ammonia metabolism allow avians to utilize ammonia for growth. Understanding the intrinsic physiological mechanisms that allow for ammonia to be utilized for growth has potential to reveal novel approaches to muscle growth in avian species and will provide new targets for preventing muscle degeneration in mammalian species.

The role of antioxidants and zinc in minimal hepatic encephalopathy: a randomized trial

BACKGROUND

Minimal hepatic encephalopathy (MHE) has a far-reaching impact on quality and function ability in daily life and may progress to overt hepatic encephalopathy. There is a synergistic effect between systemic oxidative stress and ammonia that is implicated in the pathogenesis of hepatic encephalopathy. The aim of this study is to investigate the effectiveness of oral supplementation of antioxidants and zinc gluconate on MHE versus lactulose.

METHODS

Our study included 58 patients with cirrhosis diagnosed as having MHE by neuropsychometric tests, including number connection test part A (NCT-A), digit symbol test (DST) and block design tests (BDTs). Patients were randomized to receive 175 mg zinc gluconate, 50,000 IU vitamin A, 500 mg vitamin C and 100 mg vitamin E once daily plus lactulose, dose 30-60 ml/day for 3 months [group A (n = 31)] or initiated and maintained on lactulose dose 30-60 ml/day for 3 months [group B (n = 27)]. Neuropsychometric tests and laboratory investigations were repeated after 3 months of therapy.

RESULTS

Compared with the baseline neuropsychometric tests, a significant improvement was reported in patients with MHE after 3 months of antioxidant and zinc therapy (group A) versus patients with lactulose therapy (group B) (NCT-A, p <0.001; DST, p = 0.006; BDT, p < 0.001). Antioxidant and zinc supplementation significantly decreased arterial ammonia level, alanine aminotransferase (ALT), aspartate aminotransferase (AST) (p < 0.001) and improved Child-Pugh score in MHE after 3 months of therapy (p= 0.024).

CONCLUSION

Antioxidant and zinc supplementation can improve MHE in patients with liver cirrhosis.

Ammonia elicits a different myogenic response in avian and murine myotubes

Increased myostatin expression, resulting in muscle loss, has been associated with hyperammonemia in mammalian models of cirrhosis. However, there is evidence that hyperammonemia in avian embryos results in a reduction of myostatin expression, suggesting a proliferative myogenic environment. The present in vitro study examines species differences in myotube and liver cell response to ammonia using avian and murine-derived cells. Primary myoblasts and liver cells were isolated from embryonic day 15 and 17 chick embryos to be compared with mouse myoblasts (C2C12) and liver (AML12) cells. Cells were exposed to varying concentrations of ammonium acetate (AA; 2.5, 5, or 10 mM) to determine the effects of ammonia on the cells. Relative expression of myostatin mRNA, determined by quantitative real-time PCR, was significantly increased in AA (10 mM) treated C2C12 myotubes compared to both ages of chick embryonic myotube cultures after 48 h (P < 0.02). Western blot analysis of myostatin protein confirmed an increase in myostatin expression in AA-treated C2C12 myotubes compared to the sodium acetate (SA) controls, while myostatin expression was decreased in the chick embryonic myotube cultures when treated with AA. Myotube diameter was significantly decreased in AA-treated C2C12 myotubes compared to controls, while avian myotube diameter increased with AA treatment (P < 0.001). There were no significant differences between avian and murine liver cell viability, assessed using 2', 7'- bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein, acetoxymethyl ester, when treated with AA. However, after 24 h, AA-treated avian myotubes showed a significant increase in cell viability compared to the C2C12 myotubes (P < 0.05). Overall, it appears that there is a positive myogenic response to hyperammonemia in avian myotubes compared to murine myotubes, which supports a proliferative myogenic environment.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Zinc and liver disease

Zinc is an essential trace element required for normal cell growth, development, and differentiation. It is involved in DNA synthesis, RNA transcription, and cell division and activation. It is a critical component in many zinc protein/enzymes, including critical zinc transcription factors. Zinc deficiency/altered metabolism is observed in many types of liver disease, including alcoholic liver disease (ALD) and viral liver disease. Some of the mechanisms for zinc deficiency/altered metabolism include decreased dietary intake, increased urinary excretion, activation of certain zinc transporters, and induction of hepatic metallothionein. Zinc deficiency may manifest itself in many ways in liver disease, including skin lesions, poor wound healing/liver regeneration, altered mental status, or altered immune function. Zinc supplementation has been documented to block/attenuate experimental ALD through multiple processes, including stabilization of gut-barrier function, decreasing endotoxemia, decreasing proinflammatory cytokine production, decreasing oxidative stress, and attenuating apoptotic hepatocyte death. Clinical trials in human liver disease are limited in size and quality, but it is clear that zinc supplementation reverses clinical signs of zinc deficiency in patients with liver disease. Some studies suggest improvement in liver function in both ALD and hepatitis C following zinc supplementation, and 1 study suggested improved fibrosis markers in hepatitis C patients. The dose of zinc used for treatment of liver disease is usually 50 mg of elemental zinc taken with a meal to decrease the potential side effect of nausea.

Clinical trial: oral zinc in hepatic encephalopathy

BACKGROUND

Hepatic encephalopathy has a negative effect on patient health-related quality of life (HRQOL). Zinc supplementation has been effective with regard to altered nitrogen metabolism.

AIM

To investigate the effectiveness of oral zinc supplementation on hepatic encephalopathy and HRQOL.

METHODS

Seventy-nine cirrhotic patients with hepatic encephalopathy were randomized to receive 225 mg of polaprezinc in addition to standard therapies of a protein-restricted diet including branched-chain amino acid and lactulose, or to continue only standard therapies for 6 months. The change of HRQOL by Short Form-36, hepatic encephalopathy grade, laboratory parameters, and neuropsychological (NP) tests were compared at baseline and at 6 months. We also evaluated via multivariate analysis whether zinc supplementation and clinical variables correlated with the changes in physical component scale (PCS) and mental component scale (MCS) between the two visits.

RESULTS

Zinc supplementation significantly improved the PCS (P = 0.04), but not the MCS (P = 0.95). Zinc supplementation significantly decreased hepatic encephalopathy grade and blood ammonia levels (P = 0.03 and P = 0.01), and improved Child-Pugh score and NP tests compared with standard therapy (P = 0.04 and P = 0.02). In multivariate analysis, zinc supplementation was significantly associated with improvement in PCS (P = 0.03), whereas it was not significantly associated with change in MCS (P = 0.98).

CONCLUSION

Zinc supplementation is effective in hepatic encephalopathy and consequently improves patients HRQOL.

Keratinocytes as depository of ammonium-inducible glutamine synthetase: age- and anatomy-dependent distribution in human and rat skin

In inner organs, glutamine contributes to proliferation, detoxification and establishment of a mechanical barrier, i.e., functions essential for skin, as well. However, the age-dependent and regional peculiarities of distribution of glutamine synthetase (GS), an enzyme responsible for generation of glutamine, and factors regulating its enzymatic activity in mammalian skin remain undisclosed. To explore this, GS localization was investigated using immunohistochemistry and double-labeling of young and adult human and rat skin sections as well as skin cells in culture. In human and rat skin GS was almost completely co-localized with astrocyte-specific proteins (e.g. GFAP). While GS staining was pronounced in all layers of the epidermis of young human skin, staining was reduced and more differentiated among different layers with age. In stratum basale and in stratum spinosum GS was co-localized with the adherens junction component beta-catenin. Inhibition of, glycogen synthase kinase 3beta in cultured keratinocytes and HaCaT cells, however, did not support a direct role of beta-catenin in regulation of GS. Enzymatic and reverse transcriptase polymerase chain reaction studies revealed an unusual mode of regulation of this enzyme in keratinocytes, i.e., GS activity, but not expression, was enhanced about 8-10 fold when the cells were exposed to ammonium ions. Prominent posttranscriptional up-regulation of GS activity in keratinocytes by ammonium ions in conjunction with widespread distribution of GS immunoreactivity throughout the epidermis allows considering the skin as a large reservoir of latent GS. Such a depository of glutamine-generating enzyme seems essential for continuous renewal of epidermal permeability barrier and during pathological processes accompanied by hyperammonemia.

Interorgan ammonia and amino acid metabolism in metabolically stable patients with cirrhosis and a TIPSS

Ammonia is central to the pathogenesis of hepatic encephalopathy. This study was designed to determine the quantitative dynamics of ammonia metabolism in patients with cirrhosis and previous treatment with a transjugular intrahepatic portosystemic stent shunt (TIPSS). We studied 24 patients with cirrhosis who underwent TIPSS portography. Blood was sampled and blood flows were measured across portal drained viscera, leg, kidney, and liver, and arteriovenous differences across the spleen and the inferior and superior mesenteric veins. The highest amount of ammonia was produced by the portal drained viscera. The kidneys also produced ammonia in amounts that equaled total hepatosplanchnic area production. Skeletal muscle removed more ammonia than the cirrhotic liver. The amount of nitrogen that was taken up by muscle in the form of ammonia was less than the glutamine that was released. The portal drained viscera consumed glutamine and produced ammonia, alanine, and citrulline. Urea was released in the splenic and superior mesenteric vein, contributing to whole-body ureagenesis in these cirrhotic patients. In conclusion, hyperammonemia in metabolically stable, overnight-fasted patients with cirrhosis of the liver and a TIPSS results from portosystemic shunting and renal ammonia production. Skeletal muscle removes more ammonia from the circulation than the cirrhotic liver. Muscle releases excessive amounts of the nontoxic nitrogen carrier glutamine, which can lead to ammonia production in the portal drained viscera (PDV) and kidneys. Urinary ammonia excretion and urea synthesis appear to be the only way to remove ammonia from the body.

Interorgan ammonia metabolism in liver failure

In the post-absorptive state, ammonia is produced in equal amounts in the small and large bowel. Small intestinal synthesis of ammonia is related to amino acid breakdown, whereas large bowel ammonia production is caused by bacterial breakdown of amino acids and urea. The contribution of the gut to the hyperammonemic state observed during liver failure is mainly due to portacaval shunting and not the result of changes in the metabolism of ammonia in the gut. Patients with liver disease have reduced urea synthesis capacity and reduced peri-venous glutamine synthesis capacity, resulting in reduced capacity to detoxify ammonia in the liver. The kidneys produce ammonia but adapt to liver failure in experimental portacaval shunting by reducing ammonia release into the systemic circulation. The kidneys have the ability to switch from net ammonia production to net ammonia excretion, which is beneficial for the hyperammonemic patient. Data in experimental animals suggest that the kidneys could have a major role in post-feeding and post-haemorrhagic hyperammonemia.During hyperammonemia, muscle takes up ammonia and plays a major role in (temporarily) detoxifying ammonia to glutamine. Net uptake of ammonia by the brain occurs in patients and experimental animals with acute and chronic liver failure. Concomitant release of glutamine has been demonstrated in experimental animals, together with large increases of the cerebral cortex ammonia and glutamine concentrations. In this review we will discuss interorgan trafficking of ammonia during acute and chronic liver failure. Interorgan glutamine metabolism is also briefly discussed, since glutamine synthesis from glutamate and ammonia is an important alternative pathway of ammonia detoxification. The main ammonia producing organs are the intestines and the kidneys, whereas the major ammonia consuming organs are the liver and the muscle.

Glutamine: the pivot of our nitrogen economy?

Glutamine serves as a shuttle of useful nontoxic nitrogen, supplying nitrogen from glutamine-producing (eg, muscle) to glutamine-consuming tissues. True production rates of glutamine are difficult to measure, but probably are less than 60 to 100 g/d for a 70-kg man. During catabolic stress increased amounts of glutamine are released from muscle, consisting of protein derived glutamine, newly synthesized glutamine, and glutamine losses from the intramuscular free pool. The large and rapid losses of free muscle glutamine are difficult to restore, presumably as a result of disturbances in the Na+ electrochemical gradient across the cell membrane. Whereas increased amounts of glutamine are released from muscle, glutamine consumption by the immune system (liver, spleen) also is enhanced. Thus, during catabolic stress changes occur in the flow of glutamine between organs. These changes are not necessarily reflected by alterations in the whole-body appearance rate of glutamine. In contrast with the gut, where glutamine is taken up in a concentration dependent manner, the immune system actively takes up glutamine despite decreased plasma concentrations. Supplementation with glutamine influences uptake by both the gut and the immune system, as evidenced by increased mucosal glutamine concentrations and gut glutathione production. There is evidence suggesting that this improves gut barrier function. Although the benefit of glutamine supplementation is most evident from experimental studies, clinical studies on the effect of glutamine do exist and suggest that glutamine supplementation has beneficial effects with regard to patient outcome.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

The hepato-renal syndrome: renal amino acid transport in bile duct ligated rats (DL)--influence of treatment with triiodothyronine or dexamethasone on renal amino acid handling in amino acid loaded rats

The influence of triiodothyronine or dexamethasone on renal amino acid handling was investigated in anaesthetized, bile duct-ligated (DL) adult female rats. 3 days after DL, glomerular filtration rate (GFR) was unchanged whereas urine flow was decreased. Plasma concentrations of 5 out of 16 amino acids were significantly enhanced after DL. On the other hand, the fractional excretion (FE) of 11 out of 16 amino acids was significantly reduced as a sign of improved reabsorption capacity. Bolus injections of leucine (20 mg/100 g b.wt.), glutamine (45 mg/100 g b.wt.), or taurine (45 mg/100 g b.wt.) were followed by a temporary increase in the FE of the administered amino acids as well of the endogenous amino acids which were not administered. This phenomenon was more pronounced in DL than in control rats. Under load conditions, dexamethasone (60 microg/100 g b.wt.) or triiodothyronine (20 microg/100 g b.wt.) treatment for 3 days, i.p. once daily, was followed by a stimulation of renal amino acid reabsorption in DL rats. The increase in fractional amino acid excretion after amino acid load was significantly lower than in untreated rats. This effect was also more pronounced in DL rats.

Ammonia and glutamine metabolism during liver insufficiency: the role of kidney and brain in interorgan nitrogen exchange

BACKGROUND

During liver failure, urea synthesis capacity is impaired. In this situation the most important alternative pathway for ammonia detoxification is the formation of glutamine from ammonia and glutamate. Information is lacking about the quantitative and qualitative role of kidney and brain in ammonia detoxification during liver failure.

METHODS

This review is based on own experiments considered against literature data.

RESULTS AND CONCLUSIONS

Brain detoxifies ammonia during liver failure by ammonia uptake from the blood, glutamine synthesis and subsequent glutamine release into the blood. Although quantitatively unimportant, this may be qualitatively important, because it may influence metabolic and/or neurotransmitter glutamate concentrations. The kidney plays an important role in adaptation to hyperammonaemia by reversing the ratio of ammonia excreted in the urine versus ammonia released into the blood from 0.5 to 2. Thus, the kidney changes into an organ that netto removes ammonia from the body as opposed to the normal situation in which it adds ammonia to the body pools.