Acute metabolic acidosis decreases muscle protein synthesis but not albumin synthesis in humans

Ammonium chloride-induced hypothermia is attenuated by transient receptor potential channel vanilloid-1, but augmented by ankyrin-1 in rodents

AIMS

Systemic administration of ammonium chloride (NH4Cl), an acidifying agent used in human patients and experimental conditions, causes hypothermia in mice, however, the mechanisms of the thermoregulatory response to NH4Cl and whether it develops in other species remained unknown.

MAIN METHODS

We studied body temperature (Tb) changes in rats and mice induced by intraperitoneal administration of NH4Cl after blockade of transient receptor potential vanilloid-1 (TRPV1) or ankyrin-1 (TRPA1) channels.

KEY FINDINGS

In rats, NH4Cl decreased Tb by 0.4-0.8°C (p < 0.05). The NH4Cl-induced hypothermia also developed in Trpv1 knockout (Trpv1-/-) and wild-type (Trpv1+/+) mice, however, the Tb drop was exaggerated in Trpv1-/- mice compared to Trpv1+/+ controls with maximal decreases of 4.0 vs. 2.1°C, respectively (p < 0.05). Pharmacological blockade of TRPV1 channels with AMG 517 augmented the hypothermic response to NH4Cl in genetically unmodified mice and rats (p < 0.05 for both). In contrast, when NH4Cl was infused to mice genetically lacking the TRPA1 channel, the hypothermic response was significantly attenuated compared to wild-type controls with maximal mean Tb difference of 1.0°C between the genotypes (p = 0.008). Pretreatment of rats with a TRPA1 antagonist (A967079) also attenuated the NH4Cl-induced Tb drop with a maximal difference of 0.7°C between the pretreatment groups (p = 0.003).

SIGNIFICANCE

TRPV1 channels limit, whereas TRPA1 channels exaggerate the development of NH4Cl-induced hypothermia in rats and mice, but other mechanisms are also involved. Our results warrant for regular Tb control and careful consideration of NH4Cl treatment in patients with TRPA1 and TRPV1 channel dysfunctions.

Pathophysiology of Diet-Induced Acid Stress

Diets can influence the body's acid-base status because specific food components yield acids, bases, or neither when metabolized. Animal-sourced foods yield acids and plant-sourced food, particularly fruits and vegetables, generally yield bases when metabolized. Modern diets proportionately contain more animal-sourced than plant-sourced foods, are, thereby, generally net acid-producing, and so constitute an ongoing acid challenge. Acid accumulation severe enough to reduce serum bicarbonate concentration, i.e., manifesting as chronic metabolic acidosis, the most extreme end of the continuum of "acid stress", harms bones and muscles and appears to enhance the progression of chronic kidney disease (CKD). Progressive acid accumulation that does not achieve the threshold amount necessary to cause chronic metabolic acidosis also appears to have deleterious effects. Specifically, identifiable acid retention without reduced serum bicarbonate concentration, which, in this review, we will call "covert acidosis", appears to cause kidney injury and exacerbate CKD progression. Furthermore, the chronic engagement of mechanisms to mitigate the ongoing acid challenge of modern diets also appears to threaten health, including kidney health. This review describes the full continuum of "acid stress" to which modern diets contribute and the mechanisms by which acid stress challenges health. Ongoing research will develop clinically useful tools to identify stages of acid stress earlier than metabolic acidosis and determine if dietary acid reduction lowers or eliminates the threats to health that these diets appear to cause.

Nutrition and Hydration for High-Altitude Alpinism: A Narrative Review

This report aims to summarise the scientific knowledge around hydration, nutrition, and metabolism at high altitudes and to transfer it into the practical context of extreme altitude alpinism, which, as far as we know, has never been considered before in the literature. Maintaining energy balance during alpine expeditions is difficult for several reasons and requires a deep understanding of human physiology and the biological basis for altitude acclimation. However, in these harsh conditions it is difficult to reconcile our current scientific knowledge in sports nutrition or even for mountaineering to high-altitude alpinism: extreme hypoxia, cold, and the logistical difficulties intrinsic to these kinds of expeditions are not considered in the current literature. Requirements for the different stages of an expedition vary dramatically with increasing altitude, so recommendations must differentiate whether the alpinist is at base camp, at high-altitude camps, or attempting the summit. This paper highlights nutritional recommendations regarding prioritising carbohydrates as a source of energy and trying to maintain a protein balance with a practical contextualisation in the extreme altitude environment in the different stages of an alpine expedition. More research is needed regarding specific macro and micronutrient requirements as well as the adequacy of nutritional supplementations at high altitudes.

Clinical Consequences of Metabolic Acidosis-Muscle

Metabolic acidosis is common in people with chronic kidney disease and can contribute to functional decline, morbidity, and mortality. One avenue through which metabolic acidosis can result in these adverse clinical outcomes is by negatively impacting skeletal muscle; this can occur through several pathways. First, metabolic acidosis promotes protein degradation and impairs protein synthesis, which lead to muscle breakdown. Second, metabolic acidosis hinders mitochondrial function, which decreases oxidative phosphorylation and reduces energy production. Third, metabolic acidosis directly limits muscle contraction. The purpose of this review is to examine the specific mechanisms of each pathway through which metabolic acidosis affects muscle, the impact of metabolic acidosis on physical function, and the effect of treating metabolic acidosis on functional outcomes.

Renohepatic crosstalk: a review of the effects of acute kidney injury on the liver

Several theories regarding acute kidney injury (AKI)-related mortality have been entertained, although mounting evidence supports the paradigm that impaired kidney function directly and adversely affects the function of several remote organs. The kidneys and liver are fundamental to human metabolism and detoxification, and it is therefore hardly surprising that critical illness complicated by hepatorenal dysfunction portends a poor prognosis. Several diseases can simultaneously impact the proper functioning of the liver and kidneys, although this review will address the impact of AKI on liver function. While evidence for this relationship in humans remains sparse, we present supportive studies and then discuss the most likely mechanisms by which AKI can cause liver dysfunction. These include 'traditional' complications of AKI (uremia, volume overload and acute metabolic acidosis, among others) as well as systemic inflammation, hepatic leukocyte infiltration, cytokine-mediated liver injury and hepatic oxidative stress. We conclude by addressing the therapeutic implications of these findings to clinical medicine.

How to Overcome Anabolic Resistance in Dialysis-Treated Patients?

A current hypothesis is that dialysis-treated patients are "anabolic resistant" i. e., their muscle protein synthesis (MPS) response to anabolic stimuli is blunted, an effect which leads to muscle wasting and poor physical performance in aging and in several chronic diseases. The importance of maintaining muscle mass and MPS is often neglected in dialysis-treated patients; better than to describe mechanisms leading to energy-protein wasting, the aim of this narrative review is to suggest possible strategies to overcome anabolic resistance in this patient's category. Food intake, in particular dietary protein, and physical activity, are the two major anabolic stimuli. Unfortunately, dialysis patients are often aged and have a sedentary behavior, all conditions which per se may induce a state of "anabolic resistance." In addition, patients on dialysis are exposed to amino acid or protein deprivation during the dialysis sessions. Unfortunately, the optimal amount and formula of protein/amino acid composition in supplements to maximixe MPS is still unknown in dialysis patients. In young healthy subjects, 20 g whey protein maximally stimulate MPS. However, recent observations suggest that dialysis patients need greater amounts of proteins than healthy subjects to maximally stimulate MPS. Since unneccesary amounts of amino acids could stimulate ureagenesis, toxins and acid production, it is urgent to obtain information on the optimal dose of proteins or amino acids/ketoacids to maximize MPS in this patients' population. In the meantime, the issue of maintaining muscle mass and function in dialysis-treated CKD patients needs not to be overlooked by the kidney community.

Oral exposure of pigs to the mycotoxin deoxynivalenol does not modulate the hepatic albumin synthesis during a LPS-induced acute-phase reaction

The sensitivity of pigs to deoxynivalenol (DON) might be influenced by systemic inflammation (SI) which impacts liver. Besides following acute-phase proteins, our aim was to investigate both the hepatic fractional albumin (ALB) synthesis rate (FSR) and the ALB concentration as indicators of ALB metabolism in presence and absence of SI induced by LPS via pre- or post-hepatic venous route. Each infusion group was pre-conditioned either with a control diet (CON, 0.12 mg DON/kg diet) or with a DON-contaminated diet (DON, 4.59 mg DON/kg diet) for 4 wk. A depression of ALB FSR was observed 195 min after LPS challenge, independent of feeding group or LPS application route, which was not paralleled by a down-regulated ALB mRNA expression but by a reduced availability of free cysteine. The drop in ALB FSR only partly explained the plasma ALB concentrations which were more depressed in the DON-pre-exposed groups, suggesting that ALB levels are influenced by further mechanisms. The abundances of haptoglobin, C-reactive protein, serum amyloid A, pig major acute-phase protein, fibrinogen and LPS-binding protein mRNA were up-regulated upon LPS stimulation but not accompanied by increases in the plasma concentrations of these proteins, pointing at an imbalance between synthesis and consumption.

New Insights About Albumin and Liver Disease

Decompensated liver cirrhosis has a dismal prognosis, with an overall survival of 2-4 years, which is worse than for many oncological diseases. Albumin is an important tool in the management of patients with cirrhosis, since it decreases for less than half the risk for post-paracentesis cardiocirculatory dysfunction and mortality associated with spontaneous bacterial infection, as well as, it triplicates the response to terlipressin in patients with hepatorenal syndrome. Recently, research on albumin has been a hot topic, with important new insights such as the characterization of the pleiotropic effects of albumin (which surpass its oncotic properties) and the concept of effective albumin concentration. In fact, patients with liver cirrhosis present posttranslational modifications on albumin that compromises its function. Those modified albumin forms were proved to have prognostic value and its knowledge may change the paradigm of albumin treatment. In this review, we critically summarize the latest evidence on the potential benefits of albumin in patients with end-stage liver disease.

Pilot Study Examining the Influence of Potassium Bicarbonate Supplementation on Nitrogen Balance and Whole-Body Ammonia and Urea Turnover Following Short-Term Energy Restriction in Older Men

With aging there is a chronic low-grade metabolic-acidosis that may exacerbate negative protein balance during weight loss. The objective of this randomized pilot study was to assess the impact of 90 mmol∙day⁻¹ potassium bicarbonate (KHCO₃) versus a placebo (PLA) on 24-h urinary net acid excretion (NAE), nitrogen balance (NBAL), and whole-body ammonia and urea turnover following short-term diet-induced weight loss. Sixteen (KHCO₃; n = 8, PLA; n = 8) older (64 ± 4 years) overweight (BMI: 28.5 ± 2.1 kg∙day⁻¹) men completed a 35-day controlled feeding study, with a 7-day weight-maintenance phase followed by a 28-day 30% energy-restriction phase. KHCO₃ or PLA supplementation began during energy restriction. NAE, NBAL, and whole-body ammonia and urea turnover (¹⁵N-glycine) were measured at the end of the weight-maintenance and energy-restriction phases. Following energy restriction, NAE was −9.8 ± 27.8 mmol∙day⁻¹ in KHCO₃ and 43.9 ± 27.8 mmol∙day⁻¹ in PLA (p < 0.05). No significant group or time differences were observed in NBAL or ammonia and urea turnover. Ammonia synthesis and breakdown tended (p = 0.09) to be higher in KHCO₃ vs. PLA following energy restriction, and NAE was inversely associated (r = −0.522; p < 0.05) with urea synthesis in all subjects. This pilot study suggests some benefit may exist with KHCO₃ supplementation following energy restriction as lower NAE indicated higher urea synthesis.

Adverse Effects of the Metabolic Acidosis of Chronic Kidney Disease

The kidney has the principal role in the maintenance of acid-base balance, and therefore, a fall in renal net acid excretion and positive H⁺ balance often leading to reduced serum [HCO₃^-] are observed in the course of CKD. This metabolic acidosis can be associated with muscle wasting, development or exacerbation of bone disease, hypoalbuminemia, increased inflammation, progression of CKD, protein malnutrition, alterations in insulin, leptin, and growth hormone, and increased mortality. Importantly, some of the adverse effects can be observed even in the absence of overt hypobicarbonatemia. Administration of base decreases muscle wasting, improves bone disease, restores responsiveness to insulin, slows progression of CKD, and possibly reduces mortality. Base is recommended when serum [HCO₃^-] is <22 mEq/L, but the target serum [HCO₃^-] remains unclear. Evidence that increments of serum [HCO₃^-] >26 mEq/L might be associated with worsening of cardiovascular disease adds complexity to treatment decisions. Further study of the mechanisms through which positive H⁺ balance in CKD contributes to its various adverse effects and the pathways involved in mediating the benefits and complications of base therapy is warranted.

Short inter-set rest blunts resistance exercise-induced increases in myofibrillar protein synthesis and intracellular signalling in young males

What is the central question of this study? Does shorter rest between sets of resistance exercise promote a superior circulating hormonal and acute muscle anabolic response compared with longer rest periods? What is the main finding and its importance? We demonstrate that short rest (1 min) between sets of moderate-intensity, high-volume resistance exercise blunts the acute muscle anabolic response compared with a longer rest period (5 min), despite a superior circulating hormonal milieu. These data have important implications for the development of training regimens to maximize muscle hypertrophy. Manipulating the rest-recovery interval between sets of resistance exercise may influence training-induced muscle remodelling. The aim of this study was to determine the acute muscle anabolic response to resistance exercise performed with short or long inter-set rest intervals. In a study with a parallel-group design, 16 males completed four sets of bilateral leg-press and knee-extension exercise at 75% of one-repetition maximum to momentary muscular failure, followed by ingestion of 25 g of whey protein. Resistance exercise sets were interspersed by 1 min (n = 8) or 5 min of passive rest (n = 8). Muscle biopsies were obtained at rest, 0, 4, 24 and 28 h postexercise during a primed continuous infusion of l-[ring-(13) C6 ]phenylalanine to determine myofibrillar protein synthesis and intracellular signalling. We found that the rate of myofibrillar protein synthesis increased above resting values from 0 to 4 h postexercise with 1 (76%; P = 0.047) and 5 min inter-set rest (152%; P < 0.001) and was significantly greater in the 5 min inter-set rest group (P = 0.001). Myofibrillar protein synthesis rates at 24-28 h postexercise remained elevated above resting values (P < 0.05) and were indistinguishable between groups. Postexercise p70S6K(Thr389) and rpS6(Ser240/244) phosphorylation were reduced with 1 compared with 5 min inter-set rest, whereas phosphorylation of eEF2(Thr56) , TSC2(Thr1462) , AMPK(Thr172) and REDD1 protein were greater for 1 compared with 5 min inter-set rest. Serum testosterone was greater at 20-40 min postexercise and plasma lactate greater immediately postexercise for 1 versus 5 min inter-set rest. Resistance exercise with short (1 min) inter-set rest duration attenuated myofibrillar protein synthesis during the early postexercise recovery period compared with longer (5 min) rest duration, potentially through compromised activation of intracellular signalling.

Metabolic Acidosis of CKD: An Update

The kidney has the principal role in the maintenance of acid-base balance. Therefore, a decrease in renal ammonium excretion and a positive acid balance often leading to a reduction in serum bicarbonate concentration are observed in the course of chronic kidney disease (CKD). The decrease in serum bicarbonate concentration is usually absent until glomerular filtration rate decreases to <20 to 25mL/min/1.73 m(2), although it can develop with lesser degrees of decreased kidney function. Non-anion gap acidosis, high-anion gap acidosis, or both can be found at all stages of CKD. The acidosis can be associated with muscle wasting, bone disease, hypoalbuminemia, inflammation, progression of CKD, and increased mortality. Administration of base may decrease muscle wasting, improve bone disease, and slow the progression of CKD. Base is suggested when serum bicarbonate concentration is <22 mEq/L, but the target serum bicarbonate concentration is unclear. Evidence that increments in serum bicarbonate concentration > 24 mEq/L might be associated with worsening of cardiovascular disease adds complexity to treatment decisions. Further study of the mechanisms through which metabolic acidosis contributes to the progression of CKD, as well as the pathways involved in mediating the benefits and complications of base therapy, is warranted.

Acute acidosis attenuates leucine stimulated signal transduction and protein synthesis in rat skeletal muscle

BACKGROUND

Critical illnesses are often complicated by acute metabolic acidosis, which if persistent, adversely affects outcome. Among the harmful effects that it might cause are impaired utilization of nutrients, increased proteolysis and depressed protein synthesis, leading to muscle wasting. As the amino acid leucine stimulates protein synthesis by activating mTOR signaling, we explored whether in acidosis, impaired leucine-stimulated signaling might be a contributor to the depressed protein synthesis.

METHODS

Male pair-fed rats were gavaged with NH4Cl (acidosis) or NaCl (control) for 2 days and then gavaged once with leucine and sacrificed 45 min later. Extensor digitorum longus muscles were isolated, incubated with or without leucine and protein synthesis measured. The anterior tibial muscle signaling was analysed by Western immunobloting.

RESULTS

Despite pair-feeding, acidotic rats lost body and muscle weight vs. controls. Moreover, leucine-induced protein synthesis in isolated muscle from acidotic rats was impaired. In-vivo, 45 min after an oral leucine load, anterior tibial muscle mTOR and 4E-BP1 phosphorylation increased significantly and comparably in control and acidotic rats. In contrast, leucine-stimulated phosphorylation of S6K1, a regulator of translation initiation and protein synthesis, was attenuated to approximately 56% of the control value (p < 0.05).

CONCLUSION

This study reveals that an acute metabolic acidosis impairs leucine-stimulated protein synthesis and activation of signaling downstream of mTOR at the level of S6K1. We propose that this S6K1 abnormality may account in part, for the resistance to leucine-stimulated muscle protein synthesis, and may thereby contribute to the impaired nutrient utilization and ultimately the muscle wasting that develops in acidosis.

The order of concurrent endurance and resistance exercise modifies mTOR signaling and protein synthesis in rat skeletal muscle

Concurrent training, a combination of endurance (EE) and resistance exercise (RE) performed in succession, may compromise the muscle hypertrophic adaptations induced by RE alone. However, little is known about the molecular signaling interactions underlying the changes in skeletal muscle adaptation during concurrent training. Here, we used an animal model to investigate whether EE before or after RE affects the molecular signaling associated with muscle protein synthesis, specifically the interaction between RE-induced mammalian target of rapamycin complex 1 (mTORC1) signaling and EE-induced AMP-activated protein kinase (AMPK) signaling. Male Sprague-Dawley rats were divided into five groups: an EE group (treadmill, 25 m/min, 60 min), an RE group (maximum isometric contraction via percutaneous electrical stimulation for 3 × 10 s, 5 sets), an EE before RE group, an EE after RE group, and a nonexercise control group. Phosphorylation of p70S6K, a marker of mTORC1 activity, was significantly increased 3 h after RE in both the EE before RE and EE after RE groups, but the increase was smaller in latter. Furthermore, protein synthesis was greatly increased 6 h after RE in the EE before RE group. Increases in the phosphorylation of AMPK and Raptor were observed only in the EE after RE group. Akt and mTOR phosphorylation were increased in both groups, with no between-group differences. Our results suggest that the last bout of exercise dictates the molecular responses and that mTORC1 signaling induced by any prior bout of RE may be downregulated by a subsequent bout of EE.

Regulation of muscle protein synthesis and the effects of catabolic states

Protein synthesis and degradation are dynamically regulated processes that act in concert to control the accretion or loss of muscle mass. The present article focuses on the mechanisms involved in the impairment of protein synthesis that are associated with skeletal muscle atrophy. The vast majority of mechanisms known to regulate protein synthesis involve modulation of the initiation phase of mRNA translation, which comprises a series of reactions that result in the binding of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. The function of the proteins involved in both events has been shown to be repressed under atrophic conditions such as sepsis, cachexia, chronic kidney disease, sarcopenia, and disuse atrophy. The basis for the inhibition of protein synthesis under such conditions is likely to be multifactorial and includes insulin/insulin-like growth factor 1 resistance, pro-inflammatory cytokine expression, malnutrition, corticosteroids, and/or physical inactivity. The present article provides an overview of the existing literature regarding mechanisms and signaling pathways involved in the regulation of mRNA translation as they apply to skeletal muscle wasting, as well as the efficacy of potential clinical interventions such as nutrition and exercise in the maintenance of skeletal muscle protein synthesis under atrophic conditions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

High-precision mass spectrometric analysis using stable isotopes in studies of children

The use of stable isotopes combined with mass spectrometry (MS) provides insight into metabolic processes within the body. Herein, an overview on the relevance of stable isotope methodology in pediatric research is presented. Applications for the use of stable isotopes with MS cover carbohydrate, fat, and amino acid metabolism as well as body composition, energy expenditure, and the synthesis of specific peptides and proteins, such as glutathione and albumin. The main focus of these studies is on the interactions between nutrients and the endogenous metabolism within the body and how these factors affect the health of a growing infant. Considering that the early imprinting of metabolic processes hugely impacts metabolism (and thus functional outcome) later in life, research in this area is important and is advancing rapidly. The major fluxes on a metabolic level are the synthesis and breakdown rates. They can be quantified using kinetic tracer analysis and mathematical modeling. Organic MS and isotope ratio mass spectrometry (IRMS) are the two most mature techniques for the isotopic analysis of compounds. Introduction of the samples is usually done by coupling gas chromatography (GC) to either IRMS or MS because it is the most robust technique for specific isotopic analysis of volatile compounds. In addition, liquid chromatography (LC) is now being used more often as a tool for sample introduction of both volatile and non-volatile compounds into IRMS or MS for (13)C isotopic analyses at natural abundances and for (13)C-labeled enriched compounds. The availability of samples is often limited in pediatric patients. Therefore, sample size restriction is important when developing new methods. Also, the availability of stable isotope-labeled substrates is necessary for measurements of the kinetics and concentrations in metabolic studies, which can be a limiting factor. During the last decade, the availability of these substrates has increased. Furthermore, improvements in the accuracy, precision, and sensitivity of existing techniques (such as GC/IRMS) and the development of new techniques (such as LC/IRMS) have opened up new avenues for tackling these limitations.

Lower serum bicarbonate and a higher anion gap are associated with lower cardiorespiratory fitness in young adults

Lower levels of serum bicarbonate and a higher anion gap have been associated with insulin resistance and hypertension in the general population. Whether these associations extend to other cardiovascular disease risk factors is unknown. To clarify this, we examined the association of serum bicarbonate and anion gap with cardiorespiratory fitness in 2714 adults aged 20–49 years in the 1999–2004 National Health and Nutrition Examination Survey. The mean serum bicarbonate was 24.6 mEq/L and the mean anion gap was 10.26 mEq/L, with fitness determined by submaximal exercise testing. After multivariable adjustment, gender, length of fasting, soft drink consumption, systolic blood pressure, serum phosphate, and hemoglobin were independently associated with both the serum bicarbonate and the anion gap. Low fitness was most prevalent among those in the lowest quartile of serum bicarbonate or highest quartile of anion gap. After multivariable adjustment, a one standard deviation higher serum bicarbonate or anion gap was associated with an odds ratio for low fitness of 0.80 (95% CI 0.70–0.91) and 1.30 (95% CI 1.15–1.48), respectively. The association of bicarbonate with fitness may be mediated by differences in lean body mass. Thus, lower levels of serum bicarbonate and higher levels of anion gap are associated with lower cardiorespiratory fitness in adults aged 20–49 years in the general population.

Consequences and therapy of the metabolic acidosis of chronic kidney disease

Metabolic acidosis is common in patients with chronic kidney disease (CKD), particularly once the glomerular filtration rate (GFR) falls below 25 ml/min/1.73 m(2). It is usually mild to moderate in magnitude with the serum bicarbonate concentration ([HCO(3)(-)]) ranging from 12 to 23 mEq/l. Even so, it can have substantial adverse effects, including development or exacerbation of bone disease, growth retardation in children, increased muscle degradation with muscle wasting, reduced albumin synthesis with a predisposition to hypoalbuminemia, resistance to the effects of insulin with impaired glucose tolerance, acceleration of the progression of CKD, stimulation of inflammation, and augmentation of β(2)-microglobulin production. Also, its presence is associated with increased mortality. The administration of base to patients prior to or after initiation of dialysis leads to improvement in many of these adverse effects. The present recommendation by the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (NKF KDOQI) is to raise serum [HCO(3)(-)] to ≥ 22 mEq/l, whereas Caring for Australians with Renal Impairment (CARI) recommends raising serum [HCO(3)(-)] to >22 mEq/l. Base administration can potentially contribute to volume overload and exacerbation of hypertension as well as to metastatic calcium precipitation in tissues. However, sodium retention is less when given as sodium bicarbonate and sodium chloride intake is concomitantly restricted. Results from various studies suggest that enhanced metastatic calcification is unlikely with the pH values achieved during conservative base administration, but the clinician should be careful not to raise serum [HCO(3)(-)] to values outside the normal range.

Nutrient provision increases signalling and protein synthesis in human skeletal muscle after repeated sprints

The effect of nutrient availability on the acute molecular responses following repeated sprint exercise is unknown. The aim of this study was to determine skeletal muscle cellular and protein synthetic responses following repeated sprint exercise with nutrient provision. Eight healthy young male subjects undertook two sprint cycling sessions (10 × 6 s, 0.75 N m torque kg(-1), 54 s recovery) with either pre-exercise nutrient (24 g whey, 4.8 g leucine, 50 g maltodextrin) or non-caloric placebo ingestion. Muscle biopsies were taken from vastus lateralis at rest, and after 15 and 240 min post-exercise recovery to determine muscle cell signalling responses and protein synthesis by primed constant infusion of L: -[ring-(13)C(6)] phenylalanine. Peak and mean power outputs were similar between nutrient and placebo trials. Post-exercise myofibrillar protein synthetic rate was greater with nutrient ingestion compared with placebo (~48%, P < 0.05) but the rate of mitochondrial protein synthesis was similar between treatments. The increased myofibrillar protein synthesis following sprints with nutrient ingestion was associated with coordinated increases in Akt-mTOR-S6K-rpS6 phosphorylation 15 min post-exercise (~200-600%, P < 0.05), while there was no effect on these signalling molecules when exercise was undertaken in the fasted state. For the first time we report a beneficial effect of nutrient provision on anabolic signalling and muscle myofibrillar protein synthesis following repeated sprint exercise. Ingestion of protein/carbohydrate in close proximity to high-intensity sprint exercise provides an environment that increases cell signalling and protein synthesis.

Effect of consecutive repeated sprint and resistance exercise bouts on acute adaptive responses in human skeletal muscle

We examined acute molecular responses in skeletal muscle to repeated sprint and resistance exercise bouts. Six men [age, 24.7 ± 6.3 yr; body mass, 81.6 ± 7.3 kg; peak oxygen uptake, 47 ± 9.9 ml·kg−1·min−1; one repetition maximum (1-RM) leg extension 92.2 ± 12.5 kg; means ± SD] were randomly assigned to trials consisting of either resistance exercise (8 × 5 leg extension, 80% 1-RM) followed by repeated sprints (10 × 6 s, 0.75 N·m torque·kg−1) or vice-versa. Muscle biopsies from vastus lateralis were obtained at rest, 15 min after each exercise bout, and following 3-h recovery to determine early signaling and mRNA responses. There was divergent exercise order-dependent phosphorylation of p70 S6K (S6K). Specifically, initial resistance exercise increased S6K phosphorylation (∼75% P < 0.05), but there was no effect when resistance exercise was undertaken after sprints. Exercise decreased IGF-I mRNA following 3-h recovery (∼50%, P = 0.06) independent of order, while muscle RING finger mRNA was elevated with a moderate exercise order effect ( P < 0.01). When resistance exercise was followed by repeated sprints PGC-1α mRNA was increased (REX1-SPR2; P = 0.02) with a modest distinction between exercise orders. Repeated sprints may promote acute interference on resistance exercise responses by attenuating translation initiation signaling and exacerbating ubiquitin ligase expression. Indeed, repeated sprints appear to generate the overriding acute exercise-induced response when undertaking concurrent repeated sprint and resistance exercise. Accordingly, we suggest that sprint-activities are isolated from resistance training and that adequate recovery time is considered within periodized training plans that incorporate these divergent exercise modes.

Inhibition of SNAT2 by metabolic acidosis enhances proteolysis in skeletal muscle

Insulin resistance is a major cause of muscle wasting in patients with ESRD. Uremic metabolic acidosis impairs insulin signaling, which normally suppresses proteolysis. The low pH may inhibit the SNAT2 l-Glutamine (L-Gln) transporter, which controls protein synthesis via amino acid-dependent insulin signaling through mammalian target of rapamycin (mTOR). Whether SNAT2 also regulates signaling to pathways that control proteolysis is unknown. In this study, inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate or metabolic acidosis (pH 7.1) depleted intracellular L-Gln and stimulated proteolysis in cultured L6 myotubes. At pH 7.1, inhibition of the proteasome led to greater depletion of L-Gln, indicating that amino acids liberated by proteolysis sustain L-Gln levels when SNAT2 is inhibited by acidosis. Acidosis shifted the dose-response curve for suppression of proteolysis by insulin to the right, confirming that acid increases proteolysis by inducing insulin resistance. Blocking mTOR or phosphatidylinositol-3-kinase (PI3K) increased proteolysis, indicating that both signaling pathways are involved in its regulation. When both mTOR and PI3K were inhibited, methylaminoisobutyrate or acidosis did not stimulate proteolysis further. Moreover, partial silencing of SNAT2 expression in myotubes and myoblasts with small interfering RNA stimulated proteolysis and impaired insulin signaling through PI3K. In conclusion, SNAT2 not only regulates mTOR but also regulates proteolysis through PI3K and provides a link among acidosis, insulin resistance, and protein wasting in skeletal muscle cells.

Albumin synthesis in premature neonates is stimulated by parenterally administered amino acids during the first days of life

BACKGROUND

We recently showed that parenteral administration of amino acids to premature infants immediately after birth is safe and results in a positive nitrogen balance and increased whole-body protein synthesis. However, we did not determine organ-specific effects; albumin, produced by the liver, is an important protein, but its concentration is often low in premature neonates during the first few days after birth.

OBJECTIVE

The objective of the study was to test the hypothesis that the fractional and absolute albumin synthesis rates would increase with the administration of amino acids after birth, even at low nonprotein energy intake.

DESIGN

Premature infants (<1500 g birth weight), who were on ventilation, received from birth onward either glucose only (control group, n = 7) or glucose and 2.4 g amino acid kg(-1) d(-1) (intervention group, n = 8). On postnatal day 2, all infants received a primed continuous infusion of [1-(13)C]leucine, and mass spectrometry techniques were used to determine the incorporation of the leucine into albumin. Results are expressed as medians and 25th and 75th percentiles.

RESULTS

Albumin fractional synthesis rates in the intervention group were significantly higher than those in the control group [22.9% (17.6-28.0%)/d and 12.6% (11.0-19.4%)/d, respectively; P = 0.029]. Likewise, the albumin absolute synthesis rates in the intervention group were significantly higher than those in the control group [228 (187-289) mg kg(-1) d(-1) and 168 (118-203) mg kg(-1) d(-1), respectively; P = 0.030].

CONCLUSION

Amino acid administration increases albumin synthesis rates in premature neonates even at a low energy intake.

Acidosis-sensing glutamine pump SNAT2 determines amino acid levels and mammalian target of rapamycin signalling to protein synthesis in L6 muscle cells

Wasting of lean tissue as a consequence of metabolic acidosis is a serious problem in patients with chronic renal failure. A possible contributor is inhibition by low pH of the System A (SNAT2) transporter, which carries the amino acid L-glutamine (L-Gln) into muscle cells. The aim of this study was to determine the effect of selective SNAT2 inhibition on intracellular amino acid profiles and amino acid-dependent signaling through mammalian target of rapamycin in L6 skeletal muscle cells. Inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate, metabolic acidosis (pH 7.1), or silencing SNAT2 expression with small interfering RNA all depleted intracellular L-Gln. SNAT2 inhibition also indirectly depleted other amino acids whose intracellular concentrations are maintained by the L-Gln gradient across the plasma membrane, notably the anabolic amino acid L-leucine. Consequently, SNAT2 inhibition strongly impaired signaling through mammalian target of rapamycin to ribosomal protein S6 kinase, ribosomal protein S6, and 4E-BP1, leading to impairment of protein synthesis comparable with that induced by rapamycin. It is concluded that even though SNAT2 is only one of several L-Gln transporters in muscle, it may determine intracellular anabolic amino acid levels, regulating the amino acid signaling that affects protein mass, nucleotide/nucleic acid metabolism, and cell growth.

Resistance exercise increases AMPK activity and reduces 4E-BP1 phosphorylation and protein synthesis in human skeletal muscle

Resistance exercise is a potent stimulator of muscle protein synthesis and muscle cell growth, with the increase in protein synthesis being detected within 2-3 h post-exercise and remaining elevated for up to 48 h. However, during exercise, muscle protein synthesis is inhibited. An increase in AMP-activated protein kinase (AMPK) activity has recently been shown to decrease mammalian target of rapamycin (mTOR) signalling to key regulators of translation initiation. We hypothesized that the cellular mechanism for the inhibition of muscle protein synthesis during an acute bout of resistance exercise in humans would be associated with an activation of AMPK and an inhibition of downstream components of the mTOR pathway (4E-BP1 and S6K1). We studied 11 subjects (seven men, four women) before, during, and for 2 h following a bout of resistance exercise. Muscle biopsy specimens were collected at each time point from the vastus lateralis. We utilized immunoprecipitation and immunoblotting methods to measure muscle AMPKalpha2 activity, and mTOR-associated upstream and downstream signalling proteins, and stable isotope techniques to measure muscle fractional protein synthetic rate (FSR). AMPKalpha2 activity (pmol min(-1) (mg protein)(-1)) at baseline was 1.7 +/- 0.3, increased immediately post-exercise (3.0 +/- 0.6), and remained elevated at 1 h post-exercise (P < 0.05). Muscle FSR decreased during exercise and was significantly increased at 1 and 2 h post-exercise (P < 0.05). Phosphorylation of 4E-BP1 at Thr37/46 was significantly reduced immediately post-exercise (P < 0.05). We conclude that AMPK activation and a reduced phosphorylation of 4E-BP1 may contribute to the inhibition of muscle protein synthesis during resistance exercise. However, by 1-2 h post-exercise, muscle protein synthesis increased in association with an activation of protein kinase B, mTOR, S6K1 and eEF2.

Toxicity of methionine in humans

The literature has been searched to identify evidence relating to the possible toxicity of the amino acid methionine in human subjects. Nutritional and metabolic studies have employed amounts of methionine, including the d and dl isomers, both below and above the requirement and have not reported adverse effects in adults and children. Although methionine is known to exacerbate psychopathological symptoms in schizophrenic patients, there is no evidence of similar effects in healthy subjects. The role of methionine as a precursor of homocysteine is the most notable cause for concern. A "loading dose" of methionine (0.1 g/kg) has been given, and the resultant acute increase in plasma homocysteine has been used as an index of the susceptibility to cardiovascular disease. Although this procedure results in vascular dysfunction, this is acute and unlikely to result in permanent damage. However, a 10-fold larger dose, given mistakenly, resulted in death. Longer-term studies in adults have indicated no adverse consequences of moderate fluctuations in dietary methionine intake, but intakes higher than 5 times normal resulted in elevated homocysteine levels. These effects of methionine on homocysteine and vascular function are moderated by supplements of vitamins B-6, B-12, C, and folic acid. In infants, methionine intakes of 2-5 times normal resulted in impaired growth and extremely high plasma methionine levels, but no adverse long-term consequences were observed.

Skeletal Muscle Protein Synthesis after Active or Passive Ascent to High Altitude

INTRODUCTION

The effects of acute exposure to high altitude on muscle protein synthesis rates in human volunteers were examined after active and passive ascent.

METHODS

Measurements were made initially at low altitude (550 m) and again after ascent to high altitude (4,559 m). To be able to separate the contribution of physical exercise, one group was flown by helicopter (air group, N=8), whereas the other group climbed to high altitude (foot group, N=9). Fractional rates of muscle protein synthesis rates (FSR) were determined from the incorporation of isotope into protein after injection of [H5ring] phenylalanine.

RESULTS

In the air group, there was no change in FSR at high altitude, whereas in the foot group, there was a 35% increase in FSR (P<0.05 for interaction) measured 19-23 h after the end of climbing. At high altitude, the degree of hypoxia and alkalosis were not different between the groups. The plasma concentration of insulin-like growth factor-1, free thyroxin, free triiodothyronine, and thyroid-stimulating hormone were not different between the groups. Urinary 24-h cortisol excretion increased significantly in both groups after ascent, but the increase in the foot group was significantly higher compared with the air group.

CONCLUSION

Physical exercise appeared to be responsible for the observed increase in muscle FSR. The significantly higher increase of 24-h cortisol excretion in the foot group suggests that the increase in FSR occurred despite higher levels of glucocorticoids, which generally affect muscle protein turnover by inhibiting protein synthesis.

Muscle Protein Metabolism During Hemodialysis

Despite improvement in many aspects of the care of maintenance hemodialysis (HD) patients, protein-calorie malnutrition, which is characterized by an insidious loss of somatic protein, is common and is a major risk factor for increased morbidity and mortality. We present here an overview of the current knowledge on protein metabolism in uremic patients with the expectation of providing insights into the mechanisms involved in HD-associated catabolism and outlining the rationale underlying intradialytic nutrition. We concentrate on the discussion of muscle protein metabolism because muscle is the predominant site of protein storage, and its integrity is mandatory for the maintenance of a good quality of life.

Response of protein synthesis to hypercapnia in rats: independent effects of acidosis and hypothermia

Acute metabolic acidosis has been shown to inhibit muscle protein synthesis, although little is known on the effect of acidosis of respiratory origin. The aim of this study was to investigate the effect of acute respiratory acidosis on tissue protein synthesis. Rats (n = 8) were made acidotic by increasing the CO2 content of inspired air to 12% for 1 hour. Similar rats breathing normal air served as controls (n = 8). Muscle and liver protein synthesis rates were then measured with L-[ 2H5 ]phenylalanine (150 micromol per 100 g body weight, 40 mol%). The results show that protein synthesis is severely depressed in skeletal muscle (-44% in gastrocnemius, -39% in plantaris, and -24% in soleus muscles, P < .01) and liver (-20%, P < .001) in acidotic animals. However, because breathing CO2 -enriched air was found to lower body temperature by approximately 2 degrees C, in a second experiment (n = 10), the difference in body temperature between treated and control animals was minimized by gently wrapping rats breathing CO2 -enriched air in porous cloths. This second experiment confirmed that respiratory acidosis depresses protein synthesis in muscle (-22% in gastrocnemius, P < .001; -19% in plantaris, P < .01; and -4% in soleus, P = NS). However, no effect on liver protein synthesis could be detected, suggesting that liver protein synthesis may be sensitive to changes in body temperature but is not affected by acute respiratory acidosis for 1 hour. The results show that respiratory acidosis inhibits protein synthesis in skeletal muscle and indicates that acidosis, whether of metabolic or respiratory origin, may contribute to loss of muscle protein in patients with compromised renal or respiratory function.

Risks of chronic metabolic acidosis in patients with chronic kidney disease

Risks of chronic metabolic acidosis in patients with chronic kidney disease. Metabolic acidosis is associated with chronic renal failure (CRF). Often, maintenance dialysis therapies are not able to reverse this condition. The major systemic consequences of chronic metabolic acidosis are increased protein catabolism, decreased protein synthesis, and a negative protein balance that improves after bicarbonate supplementation. Metabolic acidosis also induces insulin resistance and a decrease in the elevated serum leptin levels associated with CRF. These three factors may promote protein catabolism in maintenance dialysis patients. Available data suggest that metabolic acidosis is both catabolic and anti-anabolic. Several clinical studies have shown that correction of metabolic acidosis in maintenance dialysis patients is associated with modest improvements in nutritional status. Preliminary evidence indicates that metabolic acidosis may play a role in beta2-microglobulin accumulation, as well as the hypertriglyceridemia seen in renal failure. Interventional studies for metabolic acidosis have yielded inconsistent results in CRF and maintenance hemodialysis patients. In chronic peritoneal dialysis patients, the mitigation of acidemia appears more consistently to improve nutritional status and reduce hospitalizations. Large-scale, prospective, randomized interventional studies are needed to ascertain the potential benefits of correcting acidemia in maintenance hemodialysis patients. To avoid adverse events, an aggressive management approach is necessary to correct metabolic acidosis. Clinicians should attempt to adhere to the National Kidney Foundation Kidney Disease Outcome Quality Initiative (K/DOQI) guidelines for maintenance dialysis patients. The guidelines recommend maintenance of serum bicarbonate levels at 22 mEq/L or greater.

Metabolic acidosis and malnutrition-inflammation complex syndrome in chronic renal failure

Metabolic acidosis, a common condition in patients with renal failure, may be linked to protein-energy malnutrition (PEM) and inflammation, together also known as malnutrition-inflammation complex syndrome (MICS). Methods of serum bicarbonate measurement may misrepresent the true bicarbonate level, since the total serum carbon dioxide measurement usually overestimates the serum bicarbonate concentration. Moreover, the air transportation of blood samples to distant laboratories may lead to erroneous readings. In patients with chronic kidney disease (CKD) or end-stage renal disease (ESRD), a significant number of endocrine, musculoskeletal, and metabolic abnormalities are believed to result from acidemia. Metabolic acidosis may be related to PEM and MICS due to an increased protein catabolism, decreased protein synthesis, endocrine abnormalities including insulin resistance, decreased serum leptin level, and inflammation among individuals with renal failure. Evidence suggests that the catabolic effects of metabolic acidosis may result from an increased activity of the adenosine triphosphate (ATP)-dependent ubiquitin-proteasome and branched-chain keto acid dehydrogenase. In contrast to the metabolic studies, many epidemiologic studies in maintenance dialysis patients have indicated a paradoxically inverse association between mildly decreased serum bicarbonate and improved markers of protein-energy nutritional state. Hence metabolic acidosis may be considered as yet another element of the reverse epidemiology in ESRD patients. Interventional studies have yielded inconsistent results in CKD and ESRD patients, although in peritoneal dialysis patients, mitigating acidemia appears to more consistently improve nutritional status and reduce hospitalizations. Large-scale, prospective randomized interventional studies are needed to ascertain the potential benefits of correcting acidemia in malnourished and/or inflamed CKD and maintenance hemodialysis patients. Until then, all attempts should be made to adhere to the National Kidney Foundation Kidney Disease and Dialysis Outcome Quality Initiative guidelines to maintain a serum bicarbonate level in ESRD patients of at least 22 mEq/L.

Acid loading during treatment with sevelamer hydrochloride: mechanisms and clinical implications

Short-term and long-term studies indicate that patients treated with sevelamer hydrochloride have lower serum bicarbonate levels than patients treated with calcium-containing phosphate binders. This observation has previously been attributed to withdrawal of a source of base with discontinuation of calcium carbonate or calcium acetate. However, understanding of the chemistry of sevelamer hydrochloride suggests at least three potential mechanisms whereby it might induce a dietary acid load. Moreover, preliminary results from an animal model demonstrate that treatment with sevelamer hydrochloride results in a fall in urine pH, as well as an increase in urinary ammonium and calcium excretion consistent with an increase in net acid excretion. Chronic metabolic acidosis in maintenance dialysis patients is associated with major systemic effects. It is independently associated with an increased risk of death in dialysis patients. Metabolic acidosis has both catabolic and antianabolic effects that may lead to a net negative nitrogen balance and total body protein balance. Metabolic acidosis also leads to physiochemical dissolution of bone and promotes cell-mediated bone resorption due to enhanced osteoclast activity and reduced osteoblast activity. It may also exacerbate secondary hyperparathyroidism and renal osteodystrophy. Given the long-term risks of chronic metabolic acidosis in maintenance dialysis patients, Kidney/Dialysis Outcome Quality Initiative (K/DOQI) guidelines have recently recommended maintaining predialysis serum levels of CO2 above 22 mmol/L in order to improve bone histology, and to ameliorate excess protein catabolism.

Control of muscle protein kinetics by acid-base balance

PURPOSE OF REVIEW

Abnormalities of acid-base balance accompany many pathological conditions. Acidosis is associated with several diseases such as chronic renal failure, diabetic ketosis, severe trauma and sepsis, and chronic obstructive respiratory disease, which are often associated with muscle wasting. There is evidence that acidosis can induce muscle protein catabolism and it could therefore be an important factor contributing to loss of muscle protein in these conditions. This review aims at outlining the effects of acid-base balance abnormalities on muscle protein metabolism, and will in particular summarize and evaluate the most recent studies on the impact of pH on control of muscle protein metabolism.

RECENT FINDINGS

Acidosis has been shown to promote muscle protein catabolism by stimulating protein degradation and amino acid oxidation. This effect is achieved through up-regulation of the ubiquitin-proteasome pathway - one of the major enzyme systems for protein degradation. Recent studies in animals and humans have also shown that acidosis inhibits muscle protein synthesis. Little is known about the mechanisms by which acidosis depresses protein synthesis, or of the impact of alkalosis on protein metabolism.

SUMMARY

Increasing evidence suggests that acidosis promotes muscle protein wasting by both increasing protein degradation and inhibiting protein synthesis. Correction of acidosis may therefore help to preserve muscle mass and improve the health of patients with pathological conditions associated with acidosis.

The albumin controversy

There are relatively few studies of albumin use in neonates and children, with most showing no consistent benefit compared with the use of crystalloid solutions. Certainly, albumin treatment is not indicated for treatment of hypoalbuminemia alone. Studies also show that albumin is not indicated in neonates for the initial treatment of hypotension, respiratory distress, or partial exchange transfusions. In adults, albumin is not considered to be the initial therapy for hypovolemia, burn injury, or nutritional supplementation. Based on the evidence, albumin should be used rarely in the neonatal ICU. Albumin may be indicated in the treatment of hypovolemia only after crystalloid infusion has failed. In patients with acute hemorrhagic shock, albumin may be used with crystalloids when blood products are not available immediately. Inpatients with acute or continuing losses of albumin and normal capillary permeability and lymphatic function, such as during persistent thoracostomy tube or surgical site drainage, albumin supplementation will prevent the development of hypoalbuminemia, and possibly edema formation. This has not been studied systematically, however. In patients with hypoalbuminemia and increased capillary permeability, albumin supplementation often leads to greater albumin leakage across the capillary membrane, contributing to edema formation without improvement in outcome. As the disease process improves and capillary permeability normalizes, albumin supplementation may accelerate recovery, but long-term benefits of albumin treatment usually cannot be demonstrated. These patients will recover whether or not albumin is administered.

Metabolic acidosis in maintenance dialysis patients: clinical considerations

Metabolic acidosis in maintenance dialysis patients: Clinical considerations. Metabolic acidosis is a common consequence of advanced chronic renal failure (CRF) and maintenance dialysis (MD) therapies are not infrequently unable to completely correct the base deficit. In MD patients, severe metabolic acidosis is associated with an increased relative risk for death. The chronic metabolic acidosis of the severity commonly encountered in patients with advanced CRF has two well-recognized major systemic consequences. First, metabolic acidosis induces net negative nitrogen and total body protein balance, which improves upon bicarbonate supplementation. The data suggest that metabolic acidosis is both catabolic and antianabolic. Emerging data also indicate that metabolic acidosis may be one of the triggers for chronic inflammation, which may in turn promote protein catabolism among MD patients. In contrast to these findings, metabolic acidosis may be associated with a decrease in hyperleptinemia associated with CRF. Several studies have shown that correction of metabolic acidosis among MD patients is associated with modest improvements in the nutritional status. Second, metabolic acidosis has several effects on bone, causing physicochemical dissolution of bone and cell-mediated bone resorption (inhibition of osteoblast and stimulation of osteoclast function). Metabolic acidosis is probably also associated with worsening of secondary hyperparathyroidism. Data on the effect of correction of metabolic acidosis on renal osteodystrophy, however, are limited. Preliminary evidence suggest that metabolic acidosis may play a role in beta2-microglobulin accumulation, as well as the hypertriglyceridemia seen in renal failure. Given the body of evidence pointing to the several systemic consequences of metabolic acidosis, a more aggressive approach to the correction of metabolic acidosis is proposed.

Acute metabolic acidosis inhibits muscle protein synthesis in rats

In this study, we investigated the effect of acute metabolic acidosis on tissue protein synthesis. Groups of rats were made acidotic with intragastric administration of NH(4)Cl (20 mmol/kg body wt every 12 h for 24 h) or given equimolar amounts of NaCl (controls). Protein synthesis in skeletal muscle and a variety of different tissues, including lymphocytes, was measured after 24 h by injection of l-[(2)H(5)]phenylalanine (150 micromol/100 g body wt, 40 moles percent). Results show that acute acidosis inhibits protein synthesis in skeletal muscle (-29% in gastrocnemius, -23% in plantaris, and -17% in soleus muscles, P < 0.01) but does not affect protein synthesis in heart, liver, gut, kidney, and spleen. Protein synthesis in lymphocytes is also reduced by acidosis (-8%, P < 0.05). In a separate experiment, protein synthesis was also measured in acidotic and control rats by a constant infusion of l-[(2)H(5)]phenylalanine (1 micromol.100 g body wt(-1).h(-1)). The results confirm the earlier findings showing an inhibition of protein synthesis in gastrocnemius (-28%, P < 0.01) and plantaris (-19%, P < 0.01) muscles but no effect on heart and liver by acidosis. Similar results were also observed using a different model of acute metabolic acidosis, in which rats were given a cation exchange resin in the H(+) (acidotic) or the Na(+) (controls) form. In conclusion, this study demonstrates that acute metabolic acidosis for 24 h depresses protein synthesis in skeletal muscle and lymphocytes but does not alter protein synthesis in visceral tissues. Inhibition of muscle protein synthesis might be another mechanism contributing to the loss of muscle tissue observed in acidosis.

Inflammatory signals associated with hemodialysis

BACKGROUND

Inflammation is highly prevalent in chronic hemodialysis patients. Because hemodialysis involves the contact of blood with "foreign" surfaces, and the documented activation of several humoral and cellular pathways during the procedure, the hemodialysis procedure has been suggested as a potential source of inflammation in this patient population. Earlier studies did not provide clear-cut evidence of the potential contribution of the hemodialysis procedure to inflammation, as assessed by markers of inflammation such as cytokine levels and acute-phase protein production.

METHODS

Nine patients were studied using primed-constant infusion of l-(l-13C) leucine 2 hours before, during, and 2 hours after a single hemodialysis session. We evaluated the effects of hemodialysis on induction of interleukin-6 (IL-6) production as well as the fractional synthetic rates (FSR) of albumin and fibrinogen, two well-known acute-phase proteins.

RESULTS

During hemodialysis, albumin FSR and fibrinogen FSR increased significantly compared to the measurements obtained during baseline period. During this period, albumin and fibrinogen FSR increased 64% and 34%, respectively, compared to baseline (P < 0.05). While the increase in IL-6 concentration was modest during hemodialysis (14%), the levels further increased at the end of the 2-hour post-hemodialysis period (68% higher compared to baseline, P < 0.05). Fibrinogen FSR also demonstrated a further increase during the post-dialysis period (17% higher compared to the intradialytic period and 58% higher compared to baseline), while albumin FSR stabilized during this period.

CONCLUSIONS

The results provide clear evidence of hemodialysis-induced inflammatory response. The process is most notable during the 2-hour post-hemodialysis period.