Urinary excretion and efflux from the leg of 3-methylhistidine before and after major surgical operation

The application of stable-isotope tracers to study human musculoskeletal protein turnover: a tale of bag filling and bag enlargement

The nutritional regulation of protein and amino acid balance in human skeletal muscle carried out by the authors with Mike Rennie is reviewed in the context of a simple physiological model for the regulation of the maintenance and growth of skeletal muscle, the "Bag Theory". Beginning in London in the late 1970s the work has involved the use of stable isotopes to probe muscle protein synthesis and breakdown with two basic experimental models, primed-dose continuous tracer infusions combined with muscle biopsies and arterio-venous (A-V) studies across a limb, most often the leg, allowing both protein synthesis and breakdown as well as net balance to be measured. In this way, over a 30 year period, the way in which amino acids and insulin mediate the anabolic effect of a meal has been elaborated in great detail confirming the original concepts of bag filling within the muscle endomysial "bag", which is limited by the "bag" size unless bag enlargement occurs requiring new collagen synthesis. Finally we briefly review some new developments involving ² H₂ O labelling of muscle proteins.

Tight Glycemic Control With Insulin Does Not Affect Skeletal Muscle Degradation During the Early Postoperative Period Following Pediatric Cardiac Surgery

OBJECTIVE

Critical illness is associated with significant catabolism, and persistent protein loss correlates with increased morbidity and mortality. Insulin is a potent anticatabolic hormone; high-dose insulin decreases skeletal muscle protein breakdown in critically ill pediatric surgical patients. However, insulin's effect on protein catabolism when given at clinically utilized doses has not been studied. The objective was to evaluate the effect of postoperative tight glycemic control and clinically dosed insulin on skeletal muscle degradation in children after cardiac surgery with cardiopulmonary bypass.

DESIGN

Secondary analysis of a two-center, prospective randomized trial comparing tight glycemic control with standard care. Randomization was stratified by study center.

PATIENTS

Children 0-36 months who were admitted to the ICU after cardiac surgery requiring cardiopulmonary bypass.

INTERVENTIONS

In the tight glycemic control arm, insulin was titrated to maintain blood glucose between 80 and 110 mg/dL. Patients in the control arm received standard care. Skeletal muscle breakdown was quantified by a ratio of urinary 3-methylhistidine to urinary creatinine.

MEASUREMENTS AND MAIN RESULTS

A total of 561 patients were included: 281 in the tight glycemic control arm and 280 receiving standard care. There was no difference in 3-methylhistidine to creatinine between groups (tight glycemic control, 249 ± 127 vs standard care, 253 ± 112, mean ± SD in μmol/g; p = 0.72). In analyses restricted to the patients in tight glycemic control arm, higher 3-methylhistidine to creatinine correlated with younger age, as well as lower weight, weight-for-age z score, length, and body surface area (p < 0.005 for each) and lower postoperative day 3 serum creatinine (r = -0.17; p = 0.02). Sex, prealbumin, and albumin were not associated with 3-methylhistidine to creatinine. During urine collection, 245 patients (87%) received insulin. However, any insulin exposure did not impact 3-methylhistidine to creatinine (t test, p = 0.45), and there was no dose-dependent effect of insulin on 3-methylhistidine to creatinine (r = -0.03; p = 0.60).

CONCLUSION

Although high-dose insulin has an anabolic effect in experimental conditions, at doses necessary to achieve normoglycemia, insulin appears to have no discernible impact on skeletal muscle degradation in critically ill pediatric cardiac surgical patients.

Assessing the metabolic effects of prednisolone in healthy volunteers using urine metabolic profiling

Background

Glucocorticoids, such as prednisolone, are widely used anti-inflammatory drugs, but therapy is hampered by a broad range of metabolic side effects including skeletal muscle wasting and insulin resistance. Therefore, development of improved synthetic glucocorticoids that display similar efficacy as prednisolone but reduced side effects is an active research area. For efficient development of such new drugs, in vivo biomarkers, which can predict glucocorticoid metabolic side effects in an early stage, are needed. In this study, we aim to provide the first description of the metabolic perturbations induced by acute and therapeutic treatments with prednisolone in humans using urine metabolomics, and to derive potential biomarkers for prednisolone-induced metabolic effects.

Methods

A randomized, double blind, placebo-controlled trial consisting of two protocols was conducted in healthy men. In protocol 1, volunteers received placebo (n = 11) or prednisolone (7.5 mg (n = 11), 15 mg (n = 13) or 30 mg (n = 12)) orally once daily for 15 days. In protocol 2, volunteers (n = 6) received placebo at day 0 and 75 mg prednisolone at day 1. We collected 24 h urine and serum samples at baseline (day 0), after a single dose (day 1) and after prolonged treatment (day 15) and obtained mass-spectrometry-based urine and serum metabolic profiles.

Results

At day 1, high-dose prednisolone treatment increased levels of 13 and 10 proteinogenic amino acids in urine and serum respectively, as well as levels of 3-methylhistidine, providing evidence for an early manifestation of glucocorticoid-induced muscle wasting. Prednisolone treatment also strongly increased urinary carnitine derivatives at day 1 but not at day 15, which might reflect adaptive mechanisms under prolonged treatment. Finally, urinary levels of proteinogenic amino acids at day 1 and of N-methylnicotinamide at day 15 significantly correlated with the homeostatic model assessment of insulin resistance and might represent biomarkers for prednisolone-induced insulin resistance.

Conclusion

This study provides evidence that urinary metabolomics represents a noninvasive way of monitoring the effect of glucocorticoids on muscle protein catabolism after a single dose and can derive new biomarkers of glucocorticoid-induced insulin resistance. It might, therefore, help the development of improved synthetic glucocorticoids.

Trial Registration

ClinicalTrials.gov NCT00971724

Effect of intraoperative acetated Ringer's solution with 1% glucose on glucose and protein metabolism

PURPOSE

To investigate the effects of the intraoperative administration of Ringer's solution with 1% glucose on the metabolism of glucose, lipid and muscle protein during surgery.

METHODS

Thirty-one adult patients, American Society of Anesthesiologists physical status I or II, undergoing elective otorhinolaryngeal, head and neck surgeries were randomly assigned to one of two patient groups: those receiving acetated Ringer's solution with 1% glucose (Group G) or those receiving acetated Ringer's solution without glucose (Group R) throughout the surgical procedure. Plasma glucose was measured at anesthetic induction (T0), artery 1 h (T1), 2 h (T2), 3 h after anesthetic induction (T3) and at the end of surgery (T4). Plasma ketone bodies, insulin and 3-methylhistidine were measured at T0 and T4.

RESULTS

The intravenous infusion for patients in Group G and R was 6.1 + or - 0.8 and 6.3 + or - 1.7 ml/kg/h, respectively, with Group G patients receiving a dose of 4.1 g/h glucose. Plasma glucose levels were significantly higher in Group G than in Group R patients at T1, T2, T3 and T4; however, plasma glucose remained <150 mg/dl in both groups. The plasma concentration of ketone bodies was significantly higher (P < 0.05) in Group R than in Group G patients at T4. Changes in plasma 3-methylhistidine concentration was significantly lower in Group G than in Group R patients. These results indicate that acetated Ringer's solution with 1% glucose decreased protein catabolism without hyperglycemia among the Group G patients.

CONCLUSION

The infusion of a small dose of glucose (1%) during minor otorhinolaryngeal, head and neck surgeries may suppress protein catabolism without hyperglycemia and hypoglycemia.

Potential antiproteolytic effects of L-leucine: observations of in vitro and in vivo studies

The purpose of present review is to describe the effect of leucine supplementation on skeletal muscle proteolysis suppression in both in vivo and in vitro studies. Most studies, using in vitro methodology, incubated skeletal muscles with leucine with different doses and the results suggests that there is a dose-dependent effect. The same responses can be observed in in vivo studies. Importantly, the leucine effects on skeletal muscle protein synthesis are not always connected to the inhibition of skeletal muscle proteolysis. As a matter of fact, high doses of leucine incubation can promote suppression of muscle proteolysis without additional effects on protein synthesis, and low leucine doses improve skeletal muscle protein ynthesis but have no effect on skeletal muscle proteolysis. These research findings may have an important clinical relevancy, because muscle loss in atrophic states would be reversed by specific leucine supplementation doses. Additionally, it has been clearly demonstrated that leucine administration suppresses skeletal muscle proteolysis in various catabolic states. Thus, if protein metabolism changes during different atrophic conditions, it is not surprising that the leucine dose-effect relationship must also change, according to atrophy or pathological state and catabolism magnitude. In conclusion, leucine has a potential role on attenuate skeletal muscle proteolysis. Future studies will help to sharpen the leucine efficacy on skeletal muscle protein degradation during several atrophic states.

Skeletal muscle proteolysis in response to short-term unloading in humans

Skeletal muscle atrophy is evident after muscle disuse, unloading, or spaceflight and results from decreased protein content as a consequence of decreased protein synthesis, increased protein breakdown or both. At this time, there are essentially no human data describing proteolysis in skeletal muscle undergoing atrophy on Earth or in space, primarily due to lack of valid and accurate methodology. This particular study aimed at assessing the effects of short-term unloading on the muscle contractile proteolysis rate. Eight men were subjected to 72-h unilateral lower limb suspension (ULLS) and intramuscular interstitial levels of the naturally occurring proteolytic tracer 3-methylhistidine (3MH) were measured by means of microdialysis before and on completion of this intervention. The 3MH concentration following 72-h ULLS (2.01 +/- 0.22 nmol/ml) was 44% higher (P < 0.05) than before ULLS (1.56 +/- 0.20 nmol/ml). The present experimental model and the employed method determining 3MH in microdialysates present a promising tool for monitoring skeletal muscle proteolysis or metabolism of specific muscles during conditions resulting in atrophy caused by, e.g., disuse and real or simulated microgravity. This study provides evidence that the atrophic processes are evoked rapidly and within 72 h of unloading and suggests that countermeasures should be employed in the early stages of space missions to offset or prevent muscle loss during the period when the rate of muscle atrophy is the highest.

Initiation factors for translation of proteins in the rectus abdominis muscle from patients on overnight standard parenteral nutrition before surgery

Previous studies have provided conflicting conclusions concerning the efficacy of improving protein balance in patients by standard intravenous nutrition [TPN (total parenteral nutrition)], which is either explained by suboptimal nutritional regimens or insensitive clinical methods. The aim of the present study was therefore to evaluate the effects on the initiation of translation of skeletal muscle proteins by standard overnight TPN. A total of 12 patients who underwent standard surgery were included. TPN was provided as an all-in-one treatment by constant infusion [0.16 gN·kg−1 of body weight·day−1 (30 kcal·kg−1 of body weight·day−1)]. Saline-infused patients served as controls. Rectus abdominis muscle biopsies were taken at the time of the operation. The phosphorylation state of the proteins for initiation of translation was quantified. Plasma glucose, and serum insulin, glycerol, triacylglycerols (triglycerides) and NEFAs (non-esterified fatty acids; ‘free fatty acids’) were not significantly altered during TPN infusion, whereas total plasma amino acids increased, as shown by increases in methionine, phenylalanine, threonine, alanine, arginine, aspartic acid, glycine and histidine (P<0.05). Overnight TPN increased the formation of active eIF4G–eIF4E (where eIF is eukaryotic-initiation factor) complexes (P<0.05), whereas the inhibitory complex 4E-BP1 (eIF4E-binding protein)–eIF4E was moderately decreased (P<0.06). TPN increased the amount of the most phosphorylated form of 4E-BP1 (P<0.05), and increased the amount (P<0.04) and phosphorylation (P<0.01) of p70S6K (70 kDa ribosomal protein S6 kinase). In conclusion, an overnight pre-operative constant infusion of standard TPN altered initiation factor complexes, indicating activation of the initiation of protein translation in rectus abdominis muscle in the presence of increased plasma amino acid levels, but without a concomitant increase in energy substrates and insulin. In contrast with our results from previous studies, the methodology used in the present study appears to be more sensitive in reflecting directional changes in human muscle protein synthesis compared with traditional methods, particularly based on measurements of amino acid flux.

Myofibrillar proteolysis in response to voluntary or electrically stimulated muscle contractions in humans

Knowledge about the effects of exercise on myofibrillar protein breakdown in human subjects is limited. Our purpose was to measure the changes in the degradation of myofibrillar proteins in response to different ways of eliciting muscle contractions using the local interstitial 3-methyl-histidine (3-MH) concentration as a marker for myofibrillar protein breakdown. Untrained males (n=8, 22-27 years, range) performed 210 maximal isokinetic eccentric contractions with each leg on an isokinetic dynamometer. One leg performed voluntary (VOL) and the other leg performed electrically induced contractions (ES). Microdialysis probes were placed in m. vastus lateralis in both the legs immediately after, and 1 and 3 days post-exercise. Interstitial 3-MH was higher in ES vs VOL immediately after exercise (P<0.05). One and 3 days post-exercise no difference between the two exercise types was observed. Only after ES did the histochemical stainings show significant disruption of cytoskeletal proteins. Furthermore, intracellular disruption and destroyed Z-lines were markedly more pronounced in ES vs VOL. In conclusion, the local level of interstitial 3-MH in the skeletal muscle was significantly enhanced after ES compared with VOL immediately after exercise, while the level of 3-MH did not change in the post-exercise period after VOL. These results indicate that the local myofibrillar breakdown is accelerated after ES associated with severe myofiber damage.

Methods for measuring tissue protein breakdown rate in vivo

PURPOSE OF REVIEW

To describe the latest innovations in measuring protein breakdown in vivo, particularly in muscle.

RECENT FINDINGS

The traditional method of using 3-methylhistidine excretion to measure muscle protein breakdown has been updated to include arteriovenous or microdialysis measurements, which address the concern that there are alternative sources of 3-methylhistidine in the body other than muscle. Several variations of a precursor-product method to measure fractional breakdown rate of tissues have been developed that are analogous to fractional synthesis rate of tissues. These methods are more generally applicable than the 3-methylhistidine methods and are less invasive than arteriovenous methods. The various precursor-product methods are distinguished by whether they require an isotopic steady state or multiple tracers and by how many biopsies are required.

SUMMARY

The new precursor-product methods have enabled assessment in clinical trials of protein breakdown for proteins other than myofibrillar proteins and in circumstances in which arteriovenous sampling is not feasible.

Impact of surgical trauma on human skeletal muscle protein synthesis

Muscle protein catabolism is a considerable clinical problem following surgery. However, the impact of surgical trauma on muscle protein synthesis is not well characterized. In this pilot study, we therefore investigated whether the severity of surgical trauma is related to a decrease in muscle protein synthesis rate in humans. Metabolically healthy patients (n=28) were included in the study. Eight of the patients were day-care patients undergoing minor breast surgery (defined as minor surgery). The other 20 patients were subjected to major abdominal surgery and were therefore scheduled to stay overnight in the recovery room during the first postoperative night (defined as major surgery). Protein FSRs (fractional synthesis rates) in skeletal muscle were determined during a measurement period of 90 min before surgery and immediately after termination of surgery. FSR in skeletal muscle of the minor surgery patients was 1.72±0.25%/24 h before surgery and 1.67±0.29%/24 h after surgery (P=0.68). In the major surgery group, FSR was 1.62±0.30%/24 h before surgery and 1.57±0.40%/24 h (P=0.59) immediately following surgery. The observations made in this pilot study could not confirm a size-related decrease in muscle protein synthesis immediately following minor and major surgery. This finding is discussed in relation to confounders, postoperative course and to muscle protein degradation. The shortage of knowledge in this field is emphasized.

Influence of age and resistance exercise on human skeletal muscle proteolysis: a microdialysis approach

We combined the interstitial sampling method of microdialysis with the natural tracer qualities (i.e. non-recyclability) of the amino acid 3-methylhistidine (3MH) to uniquely study in vivo degradation of the two most abundant skeletal muscle proteins, myosin and actin. Interstitial 3MH concentration was measured before and for 24 h following a single bout of resistance exercise in eight young (27 +/- 2 years) and eight old (75 +/- 4 years) men. The exercise bout consisted of four exercises (3 sets of 8 repetitions at 80% one-repetition maximum (1RM) per exercise) emphasizing the quadriceps. Interstitial 3MH concentration was calculated using the internal reference method from microdialysate samples that were obtained from two microdialysis probes placed in the vastus lateralis. Resting interstitial 3MH concentration was 44% higher (P < 0.05) in the old (6.16 +/- 0.56 nmol ml(-1)) as compared with the young (4.28 +/- 0.27 nmol ml(-1)). Interstitial 3MH was not different (P > 0.05) from preexercise at any time point within the 24 h following exercise in both the young and the old. Leg arteriovenous exchange measurements in a separate group of young subjects also showed no increase in 3MH release during the 4 h following a resistance exercise bout compared with a non-exercised control leg (control leg: -28 +/- 6, exercise leg: -28 +/- 11 nmol min(-1)). These results suggest that myosin and actin proteolysis are not increased in the first 24 h following a standard bout of resistance exercise, and this response is not altered with ageing. The higher interstitial 3MH concentration in the old suggests an increased proteolysis of the two main contractile proteins in the rested and fasted state, which is consistent with a decrease in muscle mass with ageing. Microdialysis is an appropriate methodology for use in ageing individuals and is compatible with high-intensity resistance exercise.

Metabolic effects of intraportal nutrition in humans

OBJECTIVE

We investigated the metabolic effects of intravenous nutrition through a portal (PN) or systemic (SN) peripheral vein.

METHODS

Twenty patients were randomized to receive PN or SN nutrition after colorectal surgery. The daily regimen included 900 kcal and 100 g of amino acid (AA). Visceral proteins and hepatic enzymes were measured on days 0, 1, 3, 5, and 7, and plasma arterovenous differences and limb flux of AA were measured on days 0, 3, and 7; urinary nitrogen and 3-CH3-histidine were analyzed daily.

RESULTS

Serum albumin on day 7 was still depressed (P = 0.01) in SN and fully restored in PN patients. Prealbumin levels increased significantly (P = 0.05) in the PN group only. Plasma levels of glutamine and asparagine were higher in PN than in SN patients, and this difference was statistically significant (P = 0.05). SN patients had significantly more negative limb-muscle balance of valine and tyrosine, whereas PN patients had a higher muscle release of citrulline and taurine.

CONCLUSIONS

In conclusion, short-term PN is safe and has some metabolic benefits: it accelerates recovery from postoperative hypoalbuminemia and hypopnealbuminemia and is associated with a higher plasma level of glutamine and an AA plasma pattern that is closer to normal. PN blunts the catabolic response of the muscle, decreasing loss of proteins and release of some AA involved in hepatic gluconeogenesis.

The effect of glutamine on protein balance and amino acid flux across arm and leg tissues in healthy volunteers

BACKGROUND

Glutamine is important in nitrogen transportation and the physiological control of acid-base regulation. In addition, it has been assumed that glutamine regulates protein balance in skeletal muscles based on findings in both experimental and clinical studies. However, little information on glutamine and its effect on protein dynamics in normal individuals is available. Therefore, the aim of this study was to evaluate whether glutamine improves protein balance and uptake of various indispensable amino acids across peripheral tissue in healthy individuals.

MATERIAL AND METHODS

Standard primed constant infusions of L-[ring-2H5]phenylalanine and [ring 3,3-2H2]tyrosine (2 micromol kg(-1) h(-1)) were performed after overnight fast in five healthy male volunteers before and during infusions of a standard and a glutamine/tyrosine enriched amino acid solution. Flux measurements of amino acids (AA) including 3-methylhistidine, glucose, lactate and free fatty acids (FFA) were performed across arm and leg tissues.

RESULTS

Infusion of the standard AA solution (0.2 g N kg(-1) day(-1)) increased the net uptake of individual amino acids, but provision of the enriched solution (0.4 g N kg(-1) day(-1)) with increased amounts of glutamine and tyrosine seemed to compete unfavourably with the net uptake of other key amino acids as methionine and phenylalanine, which are indispensable in muscles for protein synthesis. Increased flux of amino acids across peripheral tissues did not influence on flux of glucose, free fatty acid and lactate.

CONCLUSIONS

Glutamine provision did neither stimulate protein synthesis nor attenuate breakdown of either globular or myofibrillar proteins in skeletal muscles of healthy volunteers.

Postprandial resynthesis of myofibrillar proteins is translationally rather than transcriptionally regulated in human skeletal muscle

Feeding stimulates protein synthesis in skeletal muscles, although the regulatory mechanisms are incompletely understood. The aim of this study was to determine whether this could be detected at the gene transcription level for postprandial stimulation of the synthesis of muscle proteins. Healthy male volunteers were investigated after an overnight fast. Open muscle biopsies were performed in the starved state and 3 h after meal intake, consisting of 0.15 gN/kg, 12 kcal/kg. Blood samples were drawn every 15 to 30 min for 5 h. Myosin mRNA and insulin growth factor-I (IGF-I) mRNA were measured by solution hybridization assay in homogenized muscle specimens. After food intake, plasma glucose concentrations increased from 5.0 +/- 0.1 to 7.3 +/- 0.3 (P < or = 0.001), and insulin concentration rose from 3.8 +/- 0.5 mU/L before to 75.3 +/- 11.4 15 min after the meal (P < or = 0.001). Plasma concentration of free fatty acids declined after food intake (P < or = 0.001). Plasma concentrations of amino acids increased from basal values (2864 +/- 128 microM) to 4419 +/- 262 microM (P < or = 0.05) 90 min after meal ingestion. Myosin mRNA concentration in the biopsied muscle tissue was higher during starvation and was reduced by 20% after food intake: 10.8 +/- 1.3 amol mRNA/microgram DNA in the starved state and 8.5 +/- 1.3 amol mRNA/microgram DNA after food intake (P < or = 0.05). Feeding did not alter IGF-I mRNA concentrations in muscle: 0.51 +/- 0.05 and 0.55 +/- 0.06 amol/microgram DNA in the starved and fed state, respectively (P < or = 0.48). Improved protein balance by stimulation of protein synthesis has been related to increased plasma amino acids. Interestingly, in the short term, this was not related to increases in gene transcription of either myofibrillar proteins (myosin) or muscle IGF-I. Thus, postprandial stimulation of protein synthesis appears not to be regulated by increased gene transcription but by increased translation using the increased concentrations of amino acids. In contrast, as far as the 2X myosin mRNA level is concerned, this is enhanced during starvation, which facilitates rapid recovery once the availability of substrate is resumed.

Perioperative growth hormone treatment and functional outcome after major abdominal surgery: a randomized, double-blind, controlled study

OBJECTIVE

To evaluate short- and long-term effects of perioperative human growth hormone (hGH) treatment on physical performance and fatigue in younger patients undergoing a major abdominal operation in a normal postoperative regimen with oral nutrition.

SUMMARY BACKGROUND DATA

Muscle wasting and functional impairment follow major abdominal surgery.

METHODS

Twenty-four patients with ulcerative colitis undergoing ileoanal J-pouch surgery were randomized to hGH (12 IU/day) or placebo treatment from 2 days before to 7 days after surgery. Measurements were performed 2 days before and 10, 30, and 90 days after surgery.

RESULTS

The total muscle strength of four limb muscle groups was reduced by 7.6% in the hGH group and by 17.1% in the placebo group at postoperative day 10 compared with baseline values. There was also a significant difference between treatment groups in total muscle strength at day 30, and at the 90-day follow-up total muscle strength was equal to baseline values in the hGH group, but still significantly 5.9% below in the placebo group. The work capacity decreased by approximately 20% at day 10 after surgery, with no significant difference between treatment groups. Both groups were equally fatigued at day 10 after surgery, but at day 30 and 90 the hGH patients were less fatigued than the placebo patients. During the treatment period, patients receiving hGH had reduced loss of limb lean tissue mass, and 3 months after surgery the hGH patients had regained more lean tissue mass than placebo patients.

CONCLUSIONS

Perioperative hGH treatment of younger patients undergoing major abdominal surgery preserved limb lean tissue mass, increased postoperative muscular strength, and reduced long-term postoperative fatigue.

Tissue protein synthesis rates in critically ill patients

OBJECTIVES

The aims of this study were to simultaneously determine the in vivo rates of protein synthesis in skeletal muscle, peripheral blood lymphocytes, and serum albumin in critically ill patients; to establish whether a relationship between the responses of these tissues could be observed; and to demonstrate if a protein synthesis pattern characteristic of critical illness exists.

DESIGN

Descriptive study.

SETTING

Intensive care unit of a 1000-bed university hospital.

PATIENTS

Fifteen patients treated in the intensive care unit.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Rates of tissue protein synthesis were determined in vivo once during the course of critical illness, using the flooding method with L-(2H5)phenylalanine. Protein synthesis in muscle was 1.49 +/- 0.16%/day; in circulating lymphocytes (i.e., mononuclear cells), protein synthesis was 11.10 +/- 1.82%/day. Albumin synthesis was 12.81 +/- 1.23%/day when expressed as the fractional rate, and was 184 +/- 19 mg/kg/day when expressed as the absolute rate.

CONCLUSIONS

The individual tissues responded differently to trauma, and showed a wide range of values. The responses were not significantly correlated with each other and no pattern of tissue protein synthesis characteristic of critical illness was observed. However, both muscle protein and albumin synthesis rates correlated with metabolic status and clinical indices of the severity of illness.

Responsiveness of muscle protein synthesis to growth hormone administration in HIV-infected individuals declines with severity of disease

This study was undertaken to determine if human recombinant growth hormone (hrGH, 6 mg/d for 2 wk) would stimulate muscle protein synthesis in AIDS wasting. Healthy controls were compared with patients who were HIV+, had AIDS without weight loss, and had AIDS with > 10% weight loss. Before hrGH, rates of skeletal muscle protein synthesis, measured with l-[2H5]phenylalanine, were the same in controls and in all stages of disease. Rates of myofibrillar protein degradation, however, assessed from urinary excretion of 3-methyl histidine, were higher in AIDS and AIDS wasting than in HIV+ or healthy individuals. The group with weight loss had significantly higher TNFalpha levels but not higher HIV viral loads. Muscle function, as determined by isokinetic knee extension and shoulder flexion, was significantly higher in controls than all infected individuals. After GH, rates of protein synthesis were stimulated 27% in controls, with a smaller increase (11%) in HIV+, and a significant depression (42%) in AIDS with weight loss, despite fourfold elevation in insulin-like growth factor-I in all groups. There was a significant correlation of hrGH-induced changes in muscle protein synthesis with severity of disease (P = 0.002). The results indicate increased basal muscle protein degradation and decreased responsiveness of muscle protein synthesis to GH in the later stages of disease.

Hormonal regulation of human muscle protein metabolism

A continuous turnover of protein (synthesis and breakdown) maintains the functional integrity and quality of skeletal muscle. Hormones are important regulators of this remodeling process. Anabolic hormones stimulate human muscle growth mainly by increasing protein synthesis (growth hormone, insulin-like growth factors, and testosterone) or by decreasing protein breakdown (insulin). Unlike in growing animals, insulin's main anabolic effect on muscle protein in adult humans is an inhibition of protein breakdown. Protein synthesis is stimulated only in the presence of a high amino acid supply. A combination of the stress hormones (glucagon, glucocorticoids, and catecholamines) cause muscle catabolism, but the effects of the individual hormones on human muscle and their mechanisms of action remain to be clearly defined. Although thyroid hormone is essential during growth, both an excess and a deficiency cause muscle wasting by yet unknown mechanisms. A greater understanding of the regulation of human muscle protein metabolism is essential to elucidate mechanisms of muscle wasting.

The measurement of tissue protein turnover

Tissue protein turnover can be assessed by a number of semi-, quantitative and qualitative methods. There are a number of static indices of the state of turnover of protein, for example amount of RNA per DNA or protein, the state of aggregation of ribosomes (i.e. the polyribosome index), the abundance of mRNA for particular proteins, and the enzymatic activity of proteins such as proteases, ribonuclease, etc. In addition, the concentration of particular amino acids such as glutamine or non-re-utilizable amino acids, formed post-translationally, such as 3-methylhistidine or hydroxyproline, are able to provide snapshot indices. However, since turnover is a dynamic process it should, ideally, be probed using methods such as the incorporation of tracer amino acids into protein or the dilution of tracer amino acids in the free pool by protein breakdown. The combination of tracer and tissue or limb balance methods is especially powerful since all the dynamic processes can potentially be quantified. The use of stable isotopes to label metabolic tracers has dramatically increased the feasibility of carrying out measurements of protein synthesis and breakdown and there has been a substantial growth in the application of the methods to a wide variety of tissues sampled by biopsy or at operation. Summaries of a number of currently feasible methods are provided, together with commentary on the relative efficacy of the methods and of the instrumental techniques required. There is also a discussion of suitable tracer labels and amino acids, plus a summary of the most reliable current values for protein turnover in a variety of tissues. The review also contains descriptions of potential methods which have not yet been applied in human beings but which are feasible, given the current recent increases in the accuracy and sensitivity of instrumentation for measurement of stable isotope labelling.

Increased contribution by myofibrillar protein to whole-body protein breakdown according to severity of surgical stress

A study was conducted to clarify the contribution by myofibrillar protein to whole-body protein breakdown in surgically stressed patients. Thirteen patients who underwent esophagectomy (group E) and 22 who underwent gastric or colorectal operation (group GC) were studied. Patients were all male and younger than 65 y old. Whole-body protein breakdown was determined using constant infusion of 15N-glycine. Urinary excretion of total catecholamines and 3-methylhistidine (3-MH) were measured. Amino acid composition of femoral arterial and venous blood was also analyzed. All the patients were fed exclusively by total parenteral nutrition providing 1.5 g protein and 40 kcal.kg-1.d-1 throughout the study. Whole-body protein breakdown increased significantly in group E (P < 0.01) and group GC (P < 0.05) on the 3rd postoperative day. The increase was significantly greater in group E than group GC (P < 0.01). Urinary excretion of 3-MH also increased significantly in group E (P < 0.01) and in group GC (P < 0.01) on the 3rd postoperative day. The increase was also greater in group E than group GC (P < 0.01). The ratio of urinary 3-MH excretion to whole-body breakdown protein (mumol/g), which is a indicator for the contribution of myofibrillar protein to the whole-body protein breakdown, increased significantly from 0.84 +/- 0.30 of preoperative value to 1.79 +/- 0.38 in group E (mean +/- SD; P < 0.01) and 1.42 +/- 0.18 in group GC (P < 0.05) on the 3rd postoperative day. This ratio was significantly higher in group E (P < 0.05). Furthermore, the ratio of myofibrillar to whole-body protein breakdown correlated significantly with urinary excretion of total catecholamines (r = 0.546; P < 0.01). Therefore, the contribution of myofibrillar protein to whole-body protein breakdown increased proportionately with the severity of surgical stress. On the other hand, femoral-arteriovenous differences of BCAA, Ala, Gln, Tyr, and Phe correlated significantly with the urinary excretion of 3-MH. These data suggest that skeletal muscle protein degradation is proportional to the breakdown of total myofibrillar proteins and both correlate with the severity of stress. From these data, it may be suggested that the contribution of skeletal muscle to whole-body protein catabolism is increased postoperatively, and that the increase is correlated with the severity of surgical stress.

Protein metabolism in critical illness

In summary, protein metabolism of critically ill patients is a field open to new investigations that will help us to understand better the mechanism behind 'autocannibalism', which is still today associated with mortality. Although the underlying disease is the major determinant of mortality, nutritional depletion will add morbidity, an addition that grows over time in the ICU. With conventional treatment the velocity of the catabolic process can at best be slowed down and the patient be bought time for other types of treatment to work. New forms of specific nutrition and adjuvant therapies may give us tools to prevent muscle depletion, without endangering the supply of essential substrates to the tissues in the splanchnic area. Muscle is at present a limiting organ for the ICU patient in two respects. A depleted muscle can no longer provide enough substrates for the splanchnic organs to maintain intestinal integrity and to maintain a high immunocompetence. In addition, a depleted muscle will be restored back to normal only very slowly; in elderly patients restoration may not even occur at all. The effects of an attenuation of muscle depletion on rehabilitation time have yet to be evaluated. An understanding of protein metabolism may be the key to better patient care in the ICU in the future.

Application of proton NMR spectroscopy to measurement of whole-body RNA degradation rates: effects of surgical stress in human patients

The urinary catabolites, N2,N2-dimethylguanosine (DMG), pseudouridine (PSU) and 7-methylguanine (m7-Gua) are formed from post-transcriptional methylation of RNA bases and are not reincorporated into RNA upon its degradation. Their quantitative urinary excretion may be used to determine rates of whole body degradation of individual RNA species since DMG occurs exclusively in tRNA, PSU occurs in rRNA and tRNA and m7-Gua occurs in all RNA species. Conventional HPLC analysis has several drawbacks since pre-analytical steps may involve selective losses and, under certain conditions, other urinary analytes may co-elute. In the present paper, we report analysis of these compounds by high-field 1H-nuclear magnetic resonance (1H-NMR) spectroscopy. Urinary concentrations of these metabolites were found to be in agreement with previously published HPLC and ELISA determinations. However, NMR analysis required minimal sample preparation (other than lyophilisation and reconstitution) and was capable of the simultaneous determination of other relevant analytes such as creatinine. This technique was therefore applied to urine samples from patients who had undergone surgical stress and insulin-like growth factor-1 (IGF-I) therapy. Surgical stress increased the excretion of DMG and m7-Gua. Degradation rates for tRNA and mRNA were also higher in surgically stressed subjects when compared with controls but degradation rates of rRNA decreased by approx. 30%. However, injection of IGF-I (40 micrograms/kg s.c.) had no significant effect on the excretion of these nucleosides. These data indicated that IGF-I therapy has no marked effects on RNA turnover following trauma. We suggest that this technique can be applied to study of RNA metabolism in any surgical or medical condition. Furthermore, since only 0.6 ml of urine is required, studies in neonates seem to be feasible.

Muscle protein synthesis rate decreases 24 hours after abdominal surgery irrespective of total parenteral nutrition

BACKGROUND

Muscle protein synthesis rate is known to decrease postoperatively as a part of the catabolic response to trauma. Conventional total parenteral nutrition (TPN) in the postoperative period does not seem to counteract the decrease in protein synthesis. However, it is still unclear if ongoing TPN given continuously after surgery would inhibit this fall in muscle protein synthesis.

METHODS

The rate of protein synthesis in skeletal muscle was determined before and 24 hours after open cholecystectomy, used as a standardized human model of trauma. Patients (n = 14) were randomized to receive either TPN continuously throughout the postoperative period or saline as postoperative fluid therapy. The protein synthesis rate was calculated from the increase in enrichment of labeled phenylalanine in protein after an IV flooding dose of [2H5] phenylalanine, 45 mg/kg body weight.

RESULTS

The fractional synthesis rate decreased by 31% from 1.74 +/- 0.13% to 1.15 +/- 0.10% per 24 hours in the saline group (p < .02) and by 23% from 1.59 +/- 0.10% to 1.22 +/- 0.07% per 24 hours in the group receiving TPN (p < .01), showing no significant difference between the two groups.

CONCLUSION

A continuous and ongoing infusion of conventional TPN started immediately after surgery did not counteract the obligatory decline of muscle protein synthesis, observed 24 hours postoperatively.

Altered expression of eukaryotic initiation factor 2B in skeletal muscle during sepsis

Sepsis causes an inhibition of protein synthesis in skeletal muscles composed of fast-twitch fibers, in part, as a result of a decreased activity of the eukaryotic initiation factor 2B (eIF-2B). In the present study, we investigated the expression of two subunits of eIF-2B, i.e., the beta- and epsilon-subunits during sepsis. The expression of both beta- and epsilon-subunits of eIF-2B in gastrocnemius was decreased approximately 50% from control values during the first 5 days after induction of sepsis. The decreased expression of eIF-2B epsilon during sepsis correlated with similar reductions in eIF-2B epsilon mRNA. Restoration of protein synthesis (10 days postsurgery) was associated with a return of eIF-2B epsilon expression to values observed in control rats. Expression of eIF-2B epsilon was not altered in heart during sepsis or in gastrocnemius from nonseptic abscess animals. Amrinone, which ameliorated the inhibition of protein synthesis during sepsis, also prevented the fall in eIF-2B epsilon protein after 5 days of infection. The data provide evidence that expression of eIF-2B epsilon is markedly influenced in gastrocnemius during the course of the septic episode and support the concept that this change is a mechanism responsible for the inhibition of protein synthesis observed under this condition.

Laparoscopic cholecystectomy does not prevent the postoperative protein catabolic response in muscle

OBJECTIVE

The authors determined the effect of laparoscopic cholecystectomy on protein synthesis in skeletal muscle. In addition to a decrease in muscle protein synthesis, after open cholecystectomy, the authors previously demonstrated a decrease in insulin sensitivity. This study on patients undergoing laparoscopic and open surgery, therefore, included simultaneous measurements of protein synthesis and insulin sensitivity.

SUMMARY BACKGROUND DATA

Laparoscopy has become a routine technique for several operations because of postoperative benefits that allow rapid recovery. However, its effect on postoperative protein catabolism has not been characterized. Conventional laparotomy induces a drop in muscle protein synthesis, whereas degradation is unaffected.

METHODS

Patients were randomized to laparoscopic or open cholecystectomy, and the rate of protein synthesis in skeletal muscle was determined 24 hours postoperatively by the flooding technique using L-(2H5)phenylalanine, during a hyperinsulinemic normoglycemic clamp to assess insulin sensitivity.

RESULTS

The protein synthesis rate decreased by 28% (1.77 +/- 0.11%/day vs. 1.26 +/- 0.08%/day, p < 0.01) in the laparoscopic group and by 20% (1.97 +/- 0.15%/day vs. 1.57 +/- 0.15%/day, p < 0.01) in the open cholecystectomy group. In contrast, the fall in insulin sensitivity after surgery was lower with laparoscopic (22 +/- 2%) compared with open surgery (49 +/- 5%).

CONCLUSIONS

Laparoscopic cholecystectomy did not avoid a substantial decline in muscle protein synthesis, despite improved insulin sensitivity. The change in the two parameters occurred independently, indicating different mechanisms controlling insulin sensitivity and muscle protein synthesis.

Effect of prednisone on protein metabolism in Duchenne dystrophy

Prednisone improves strength in Duchenne dystrophy and changes the natural history of the disease. We studied the in vivo effects of prednisone (0.75 mg.kg-1.day-1) on muscle and whole body protein metabolism in six patients with Duchenne dystrophy and three patients with Becker dystrophy. Patients were admitted to the Clinical Research Center for study and consumed a constant flesh-free diet. Strength was measured by manual and quantitative muscle testing. Fractional muscle protein breakdown was estimated by the ratio of 3-methylhistidine to creatinine excretion determined in three consecutive 24-h urine collections. Whole body protein kinetics were studied in the postabsorptive state using a primed continuous infusion of L-[1-13C]leucine. Fractional muscle protein synthesis was determined from tracer incorporation into noncollagen muscle protein obtained by needle biopsy. After 6-8 wk of prednisone treatment, average muscle strength increased by 15% (P < 0.04), and 24-h creatinine excretion (an index of muscle mass) increased by 21% (P = 0.002). 3-Methylhistidine excretion decreased by 10%, but the change was not statistically significant. The ratio of 3-methylhistidine to creatinine excretion decreased by 26% (P < 0.04). Fractional muscle protein synthesis and whole body protein synthesis and breakdown did not change significantly. We conclude that the beneficial effect of prednisone on strength in Duchenne dystrophy appears to be associated with an increase in muscle mass, which may be mediated by inhibition of muscle proteolysis rather than stimulation of muscle protein synthesis.

Long-term effects of postoperative total parenteral nutrition supplemented with glycylglutamine on subjective fatigue and muscle protein synthesis

Seventeen patients undergoing elective open cholecystectomy were given conventional total parenteral nutrition either with (nine patients) or without (eight) glutamine supplementation of 20 g/day for 3 days after surgery and thereafter ordinary food for the following 27 days. Muscle protein synthesis, as assessed by the total concentration of ribosomes, decreased in control patients on day 3 following surgery and remained low on days 10, 20 and 30 (P < 0.05). In patients who received glutamine the total ribosome concentration was maintained on the third day after operation. Concurrently, the subjective feeling of fatigue increased on days 3 and 10 after surgery and the nitrogen balance was negative after operation in both groups, without any difference related to glutamine supplementation. Intravenous glutamine after surgery counteracts a decline in muscle protein synthesis only for as long as it is provided.

Effect of surgical trauma on muscle protein synthesis in the rat

The rate of protein synthesis in skeletal muscle was measured in vivo in rats at various times during the first 2 days after abdominal surgery. Protein synthesis in abdominal muscle at the site of the wound was slightly reduced 2 h after operation, had returned to normal by 24 h and was massively increased by 48 h after surgery. In contrast, there was no change at any time in the rate of protein synthesis in either the gastrocnemius muscle or abdominal muscle distant from the wound site. Surgery had no effect on the weight or protein content of the gastrocnemius muscle, although urinary nitrogen excretion was increased relative to food intake, indicating the presence of a net catabolic response. Changes in whole-body protein turnover in response to uncomplicated abdominal surgery are thus likely to reflect the anabolic processes of wound healing and repair as well as any catabolic response in uninjured tissues.

Forearm 3-methylhistidine efflux in myotonic dystrophy

Myotonic dystrophy is associated with progressive muscular atrophy. To define the mechanism of muscle wasting in this disease, we studied myofibrillar proteolysis in vivo in 8 men moderately affected with myotonic dystrophy, and compared the results with those of 10 normal men. Myofibrillar proteolysis was estimated by measuring the 3-methylhistidine arteriovenous difference (A-V) and efflux (Q) across the forearm in the postabsorptive state. Plasma 3-methylhistidine concentrations were determined by high-performance liquid chromatography with postcolumn o-phthalaldehyde derivatization and fluorescence detection. Plasma flow to the forearm muscles (F) was estimated to represent 85% of total forearm plasma flow as determined by the indicator-dilution technique. Forearm 3-methylhistidine efflux was calculated as: Q = F(A-V). Mean muscle mass (24-hour creatinine excretion), lean body mass, and forearm volume were decreased in the patients with myotonic dystrophy, confirming the presence of muscle atrophy. Mean forearm 3-methylhistidine arteriovenous difference and efflux were not significantly different in the two groups. We conclude that myofibrillar protein degradation is not increased in myotonic dystrophy, even when measured in a muscle compartment selectively affected by wasting. Muscle atrophy in myotonic dystrophy is probably the result of defective anabolism rather than accelerated catabolism.

Stress hormones alter the pattern of free amino acids in human skeletal muscle

Stress hormones were infused for 6 h in healthy volunteers (n = 32). Free amino acid concentrations were determined in plasma and in skeletal muscle biopsy specimens. A triple hormone combination of adrenaline, cortisol, and glucagon raised the level of alanine in muscle, while glutamine, glutamate, the branched chain amino acids, the aromatic amino acids, and the basic amino acids decreased. Adrenaline alone partly reproduced this pattern, while a 6-h infusion of cortisol left the muscle free amino acids unaffected. In plasma all individual amino acids except alanine and glutamate decreased in the subjects receiving adrenaline or a triple-hormone combination. Altered plasma amino acid concentrations did not necessarily reflect changes in the tissue amino acid content. It is concluded that an infusion of a triple combination of stress hormones into healthy volunteers produces changes in muscle amino acid metabolism similar to those seen immediately after surgical trauma.

Protein and amino acid metabolism in cancer cachexia: investigative techniques and therapeutic interventions

Cancer cachexia is a complex syndrome characterized primarily by diminished nutrient intake and progressive tissue depletion that is manifest clinically as anorexia and host weight loss. The gradual loss of host protein stores is central to this process. This review outlines the techniques that have been used to evaluate human amino acid metabolism, their application in patients with cancer cachexia, and possible therapeutic interventions designed to overcome alterations in host protein and amino acid metabolism associated with malignant cachexia. The techniques of nitrogen balance and 3-methylhistidine excretion provide indirect estimates of overall nitrogen metabolism and skeletal muscle myofibrillar protein breakdown. Measurement of circulating amino acid concentrations, particularly when combined with assessment of arterial-venous differences and regional amino acid balance allows for investigation of interorgan amino acid metabolism. One of the most significant advances in in vivo amino acid metabolic research has been the development of labeled amino acid tracer studies to evaluate whole body and regional amino acid kinetics. The use of stable and unstable amino acid isotopes in these techniques is reviewed in detail. Virtually all of these techniques have now been employed in the evaluation of human cancer cachexia. The results of studies evaluating amino acid concentrations, regional amino acid balance, and 3-methylhistidine excretion are summarized. The use of regional and whole body kinetic studies in cancer cachexia are reviewed extensively. Most investigators have observed increased rates of whole body protein turnover, synthesis, and catabolism in both weight-stable and weight-losing cancer patients. Some studies have suggested a relationship between the extent of disease and the degree of aberration in amino acid kinetic parameters. Investigators have attempted to reverse some of these alterations by provision of substrate (nutritional support) or administration of specific pharmacologic or anabolic agents such as hydrazine sulfate, insulin, growth hormone, and beta-2 agonists. The role of total parenteral nutrition (TPN) in cancer and its effects on protein and amino acid kinetics and tumor growth are addressed. The possible benefits of specific amino acid nutritional formulations with increased branched chain amino acids, arginine, and glutamine are reviewed. Although many of these approaches appear promising, significant impact on clinically definable parameters remains to be demonstrated. A better understanding of the underlying protein catabolic mechanisms of cancer cachexia will likely lead to more effective therapies to reverse the protein calorie malnutrition associated with cancer cachexia.

Biosynthetic human growth hormone preserves both muscle protein synthesis and the decrease in muscle-free glutamine, and improves whole-body nitrogen economy after operation

As a reproducible human trauma model, patients (n = 17) undergoing elective cholecystectomy were studied for 3 postoperative days. They were randomly allocated to receive either recombinant human growth hormone (hGH; 0.3 U/kg/24 hours) or placebo together with total parenteral nutrition, including 0.2 gN/kg/24 hours and 135 kJ/kg/24 hours. Before operation and on the third postoperative day, percutaneous muscle biopsies were performed to determine the concentration and size distribution of ribosomes and the free amino acid concentrations. The significant postoperative decrease in the total ribosome concentration (15.3 +/- 6.4%) and the polyribosome concentration (20.9 +/- 6.5%) in the control group was impeded in the group receiving synthetic hGH. Muscle free glutamine decreased by 35.6 +/- 4.2% in the control group and to a lesser extent in the group that was given hGH after operation (p less than 0.05). The protein content of skeletal muscle was unchanged. The cumulated nitrogen balance for the study period was negative in the control group (-7.09 +/- 0.71 gN), but was not different from zero in the hGH group (-2.32 +/- 1.66 gN). It is concluded that synthetic hGH administered after operation has beneficial effects on the whole-body nitrogen economy, as indicated by the unchanged capacity for protein synthesis in skeletal muscle, the preserved levels of muscle free glutamine, and improvement in the whole-body nitrogen balance. The effects of hGH on skeletal muscle protein and amino acid metabolism can explain the postoperative nitrogen-sparing effect attributed to hGH.

Uncomplicated surgery, but not general anesthesia, decreases muscle protein synthesis

The impact of anesthesia and surgery on protein metabolism is not well characterized. The single effect of general anesthesia and the combined effects of surgery and general anesthesia on protein synthesis in skeletal muscle were studied in metabolically healthy patients (n = 14) undergoing elective abdominal surgery. The rate of muscle protein synthesis was calculated from the increase in enrichment of [1-13C]leucine in protein during 90 min after an intravenous infusion of [1-13C]leucine (0.05 g/kg, 20 atom percent excess). The 1-13C enrichments of plasma leucine and plasma alpha-ketoisocaproate were used to indicate the enrichment of muscle free leucine. The protein synthesis rate was unaffected by general anesthesia; however, at the end of surgery, a 31.5% decline was seen from 2.19(2.13,2.33)%/24 h before anesthesia to 1.50(1.21,1.77)%/24 h (P less than 0.05) immediately after cholecystectomy while the patients were still under general anesthesia.

Effect of surgical injury and intraoperative hypothermia on whole body protein metabolism

Whole body protein turnover and urinary nitrogen and 3-methyl-L-histidine (3-MH) excretion were measured before and after cardiac surgery using cardiopulmonary bypass in 20 male patients randomized to an intraoperative blood temperature of 28 or 20 degrees C. Rates of whole body protein synthesis (WBPS) and breakdown (WBPB) were determined from the 15N isotopic enrichment of urinary urea, ammonium, and from a calculated end-product average (EPA) after a primed 24-h infusion of [15N]glycine. In the postoperative period, there was a 40% increase in median nitrogen excretion in the 28 degrees C group (P less than 0.03) and a 22% increase in the 20 degrees C group (P = 0.10). There was a 79% increase in the median postoperative 3-MH excretion in the 28 degrees C group (P = 0.01) and a 66% increase in the median postoperative 3-MH excretion in the 20 degrees C group (P less than 0.01). Postoperatively, there was a 23% fall in the median value of WBPS in the 28 degrees C group (P less than 0.01) and an 11% fall in the 20 degrees C group [not significant (NS)] measured by 15N enrichment of urinary urea. In contrast, when WBPS was measured from isotopic enrichment of urinary ammonium, there was an increase in the median value of the postoperative rates of WBPS in both groups, by 20% in the 28 degrees C group (P = NS) and 29% in the 20 degrees C group (P = 0.03). There was no significant change in the rate of WBPS and WBPB, judged by the postoperative EPA in either group.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged fasting as conditioned by prior protein depletion: effect on urinary nitrogen excretion and whole-body protein turnover

To study the influence of previous dietary protein depletion on nitrogen (N) loss and protein turnover during a total fast, we measured plasma leucine kinetics and urinary N and 3-methylhistidine (3MH) excretion in obese and normal subjects. In one study, 10 moderately obese women fasted for 2 weeks after adaptation either to a normal maintenance intake of 80 g protein and 150% of estimated resting energy expenditure (control group), or to 10 days of a 950-kcal, 200-g carbohydrate, 4-g protein diet (depletion group), with measurement of postabsorptive (or fasting) plasma leucine turnover on the maintenance diet and after 3 and 10 days of fasting. As measured after 10 days of fasting, body N loss was blunted by 17% when preceded by the protein-deficient diet. Plasma leucine flux and oxidation of the control group increased in early fasting and decreased by 10 days, in accordance with previous reports. Results for the depletion group were similar in absolute magnitude, despite the preceding protein-deficient diet. In a second study of five normal men, leucine kinetics were measured on a maintenance diet, after 10 days of a protein-free diet, and after 3 days of fasting. After protein depletion, leucine flux decreased by 19% (P less than .05). After 3 days fasting, leucine flux was 16% higher than on the maintenance diet (P less than .05), but 44% higher than the value on the protein-free diet 3 days earlier (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Anticatabolic properties of branched chain amino-acids in post-operative patients. A prospective study

The effect of infusion of branched chain amino-acids (BCAA) on post-operative protein metabolism was analysed in 19 elective surgical patients treated for the first 5 post-operative days with a nutritional regimen of 30 kcal kg(-1) day(-1) and 2 g of amino-acids kg(-1) day(-1). The patients were divided into three groups whose only difference was the amount of BCAA delivered. Our results showed that an increased BCAA input improved nitrogen balance and reduced protein catabolism as estimated by the excretion of 3-methyl-histidine. Since nitrogen retention was maximal during the first 3 post-operative days and the reduction in 3-methylhistidine excretion was observed only on post-operative days 4 and 5, a dual action of BCAA on improving protein synthesis and reducing catabolism is postulated, even though the reduction in catabolism seems to be the main action. This dual action may reflect the unique role of BCAA, which is both 'nutritional' (as they constitute 40% of total amino-acid daily requirements of the healthy subject) and 'pharmacological (as they reduce protein catabolism and improve synthesis in muscle and liver with a dose-dependent effect). Of the three BCAA, isoleucine and leucine seemed to have an 'anticatabolic' effect, whereas an analysis of literature data showed that valine probably has none.

Elective abdominal surgery depresses muscle protein synthesis and increases subjective fatigue: effects lasting more than 30 days

Ten patients without metabolic disease undergoing elective cholecystectomy were studied before surgery and on days 3, 10, 20 and 30 after operation. Percutaneous muscle biopsies were taken and protein synthesis was determined from the total concentration and size distribution of ribosomes. The subjective feeling of fatigue was estimated using a visual analogue scale. The nitrogen balance was calculated at 20 days following surgery. The mean (s.e.m.) total concentration of ribosomes per milligram of DNA decreased by 27.5(6.6) per cent (P less than 0.01), 44.5(6.5) per cent (P less than 0.001), 48.3(8.9) per cent (P less than 0.001) and 45.0(8.2) per cent (P less than 0.01) on days 3, 10, 20 and 30, respectively. By 30 days after surgery no sign of restoration of normality was seen. The relative proportion of polyribosomes had decreased by 20.4(6.4) per cent (P less than 0.05) on the third postoperative day and by 20.4(3.9) per cent (P less than 0.01) on the tenth postoperative day and was restored to the preoperative level by day 20. The subjective fatigue score increased after operation and five of nine patients had not regained their preoperative scores 30 days after surgery. The daily nitrogen balance was negative for 5 days. The cumulated nitrogen losses were not restored until after 18 days following surgery. Elective abdominal surgery caused a sustained depression of protein synthesis for over 30 days, a longer period than previously presumed. These results show that long-term follow-up is required when the effect of different postoperative nutritional regimens are to be evaluated.

Protein and amino acid metabolism during early starvation as reflected by excretion of urea and methylhistidines

Endogenous excretion of nitrogenous products was studied during early starvation in six healthy, nonobese subjects after six days on a well-defined diet, designed to achieve net protein balance and an adequate calorie supply. The diet contained 0.5 g myofibrillar-free protein and 35 kcal/kg body weight. The subjects then fasted for three days. Urine was collected for 24-hour periods and analyzed for urea, ammonia, 3-methylhistidine, and 1-methylhistidine. Blood glucose and serum urea levels were measured daily. In a second group of subjects, muscle biopsies for determination of free amino acid concentrations were taken in the overnight fasted state and after three days of fasting. During the period with a balanced diet, urea production fell initially and stabilized after two to three days at a level of 146 +/- 15 mmol/24 h. During the period of fasting, serum urea increased from 3.0 +/- 0.4 to a maximum value of 6.2 +/- 0.7 mmol/L and urea production rose markedly, to a peak of 293 +/- 16 mmol/24 h. Ammonia excretion was 24 +/- 2 mmol/24 h before and 71 +/- 13 mmol/24 h after three days of fasting. 3-Methylhistidine excretion was stable before fasting and then rose from 154 +/- 17 to 198 +/- 17 mumol/24 h. 1-Methylhistidine excretion was unchanged during fasting. Blood glucose levels were stable at 4.8 +/- 0.2 mmol/L before fasting and then fell to 3.7 +/- 0.3 mmol/L. Intracellular concentrations of amino acids in skeletal muscle decreased markedly during fasting; after three days of fasting the glutamine concentration had fallen by 34%.(ABSTRACT TRUNCATED AT 250 WORDS)