Effects of dietary intake and hemodialysis on protein turnover in uremic children

Amino acid repletion does not decrease muscle protein catabolism during hemodialysis

Intradialytic protein catabolism is attributed to loss of amino acids in the dialysate. We investigated the effect of amino acid infusion during hemodialysis (HD) on muscle protein turnover and amino acid transport kinetics by using stable isotopes of phenylalanine, leucine, and lysine in eight patients with end-stage renal disease (ESRD). Subjects were studied at baseline (pre-HD), 2 h of HD without amino acid infusion (HD-O), and 2 h of HD with amino acid infusion (HD+AA). Amino acid depletion during HD-O augmented the outward transport of amino acids from muscle into the vein. Increased delivery of amino acids to the leg during HD+AA facilitated the transport of amino acids from the artery into the intracellular compartment. Increase in muscle protein breakdown was more than the increase in synthesis during HD-O (46.7 vs. 22.3%, P < 0.001). Net balance (nmol.min(-1).100 ml (-1)) was more negative during HD-O compared with pre-HD (-33.7 +/- 1.5 vs. -6.0 +/- 2.3, P < 0.001). Despite an abundant supply of amino acids, the net balance (-16.9 +/- 1.8) did not switch from net release to net uptake. HD+AA induced a proportional increase in muscle protein synthesis and catabolism. Branched chain amino acid catabolism increased significantly from baseline during HD-O and did not decrease during HD+AA. Protein synthesis efficiency, the fraction of amino acid in the intracellular pool that is utilized for muscle protein synthesis decreased from 42.1% pre-HD to 33.7 and 32.6% during HD-O and HD+AA, respectively (P < 0.01). Thus amino acid repletion during HD increased muscle protein synthesis but did not decrease muscle protein breakdown.

Amino Acid and protein kinetics in renal failure: an integrated approach

Even apparently healthy patients on dialysis have significant loss of lean body mass. Patients with chronic renal failure without coexisting metabolic acidosis or inflammation have decreased protein turnover, with balanced reduction in protein synthesis and breakdown. However, regional and whole-body protein kinetic studies indicate that hemodialysis (HD) induces net increase in protein breakdown. Whole-body protein turnover studies show that HD is associated with decreased protein synthesis, but proteolysis is not increased. Muscle protein kinetics studies, however, identify enhanced muscle protein breakdown with inadequate compensatory increases in synthesis as the cause of the catabolism. Transmembrane amino acid-transport kinetics studies show that the outward transport is increased more than the inward transport of amino acids during HD. Altered intracellular amino acid transport kinetics and protein turnover during HD could be caused by the loss of amino acids in the dialysate or cytokine activation. Cytokines may be released from peripheral blood mononuclear cells and skeletal muscle during HD. Preliminary evidence indicates that intradialytic increase in cytokines activates the ubiquitin-proteasome pathway. An intradialytic increase in albumin and fibrinogen synthesis is facilitated by interleukin-6 and the constant supply of amino acids derived from skeletal muscle catabolism. Protein anabolism can be induced in end-stage renal disease patients by repletion of amino acids, and perhaps treatment with recombinant human insulin-like growth factor.

Protein turnover and amino acid transport kinetics in end-stage renal disease

Protein and amino acid metabolism is abnormal in end-stage renal disease (ESRD). Protein turnover is influenced by transmembrane amino acid transport. The effect of ESRD and hemodialysis (HD) on intracellular amino acid transport kinetics is unknown. We studied intracellular amino acid transport kinetics and protein turnover by use of stable isotopes of phenylalanine, leucine, lysine, alanine, and glutamine before and during HD in six ESRD patients. Data obtained from amino acid concentrations and enrichment in the artery, vein, and muscle compartments were used to calculate intracellular amino acid transport and muscle protein synthesis and catabolism. Fractional muscle protein synthesis (FSR) was estimated by the precursor product approach. Despite a significant decrease in the plasma concentrations of amino acids in the artery and vein during HD, the intracellular concentrations remained stable. Outward transport of the amino acids was significantly higher than the inward transport during HD. FSR increased during HD (0.0521 +/- 0.0043 vs. 0.0772 +/- 0.0055%/h, P < 0.01). Results derived from compartmental modeling indicated that both protein synthesis (118.3 +/- 20.6 vs. 146.5 +/- 20.6 nmol.min-1.100 ml leg-1, P < 0.01) and catabolism (119.8 +/- 18.0 vs. 174.0 +/- 14.2 nmol.min-1.100 ml leg-1, P < 0.01) increased during HD. However, the intradialytic increase in catabolism exceeded that of synthesis (57.8 +/- 13.8 vs. 28.0 +/- 8.5%, P < 0.05). Thus HD alters amino acid transport kinetics and increases protein turnover, with net increase in protein catabolism.

Protein metabolism in patients with chronic renal failure: role of uremia and dialysis

Individuals with chronic renal failure (CRF) have a high prevalence of protein-energy malnutrition. There are many causes for this condition, chief among which is probably reduced nutrient intake from anorexia. In nondialyzed patients with CRF, energy intake is often below the recommended amounts; in maintenance dialysis patients, both dietary protein and energy intake are often below their needs. Although a number of studies indicate that rats with CRF have increased protein catabolism in comparison to control animals, more recent evidence suggests that increased catabolism in CRF rats is largely if not entirely due to acidemia, particularly if these animals are compared to pair-fed control rats. Studies in humans with advanced CRF also indicate that acidemia can cause protein catabolism. Indeed, nitrogen balance studies and amino acid uptake and release and isotopic kinetic studies indicate that in nondialyzed individuals with CRF, who are not acidemic, both their ability to conserve body protein when they ingest low protein diets and their dietary protein requirements appear to be normal. For patients undergoing maintenance hemodialysis or chronic peritoneal dialysis, dietary protein requirements appear to be increased. The increased need for protein is due, in part, to the losses into dialysate of such biologically valuable nitrogenous compounds as amino acids, peptides, and proteins. However, the sum of the dietary protein needs for CRF patients (of about 0.60 g/kg/day) and the dialysis losses of amino acids, peptides and proteins do not equal the apparent dietary protein requirements for most maintenance dialysis patients. This discrepancy may be due to a chronic state of catabolism in the clinically stable maintenance dialysis patient that is not present in the clinically stable nondialyzed individual who has advanced CRF. Possible causes for such a low grade catabolic state include resistance to anabolic hormones (for example, insulin, IGF-1) and a chronic inflammatory state associated with increased levels of pro-inflammatory cytokines.

Protein and energy intake, nitrogen balance and nitrogen losses in patients treated with continuous ambulatory peritoneal dialysis

The aim of this investigation was to analyze factors which influence the dietary protein intake (DPI), the energy intake and the utilization of ingested protein, and to determine the relationship between various types of nitrogen losses in stable continuous ambulatory peritoneal dialysis (CAPD) patients. We performed 23 nitrogen balance (NB) studies of 6 to 11 days duration in 12 CAPD patients. One study was performed in all patients 3.4 +/- 1.2 months after starting CAPD (early studies). The study was then repeated in nine patients after 12.1 +/- 2.6 months, and two of these patients were studied again after 16 and 24 months, respectively (late studies). Before each NB study, the dietary intakes prior to the study were assessed in diaries and interviews. During a few days preceding the NB periods and during the NB periods each patient received an individualized diet composed so as to resemble the patients' spontaneously chosen diet regarding DPI and dietary energy intake (DEI). Total nitrogen, protein, urea and creatinine were analyzed in the dialysate and urine collected daily. Total nitrogen was also analyzed in the feces, collected over the whole NB period. Total nitrogen appearance (TNA), non-protein nitrogen appearance (NPNA) and urea nitrogen appearance (UNA) were calculated by correcting total nitrogen output, non-protein nitrogen output, that is, TNA minus the total protein losses (PL) and urea nitrogen output for changes in total body urea nitrogen. Glucose was determined in the collected dialysate and the daily glucose absorption was calculated. DPI varied between 0.62 and 2.09 g/kg/day, DEI between 21 and 42 kcal/kg/day and the peritoneal energy (glucose) intake (PEI) between 4 and 13 kcal/kg/day. DPI (but not DEI) correlated with Kt/V(urea) and Kt/VCr and with total and renal clearances for urea and creatinine. NB (not corrected for "unmeasured" nitrogen losses) was positive in most studies, and it correlated with DPI and the total energy intake (TEI) in the early studies, but only with TEI in the late studies. DPI correlated with TNA, NPNA, UNA, non-protein-non-urea nitrogen loss and fecal nitrogen loss. UNA was highly correlated with TNA and NPNA (r = 0.95). We used data from 33 NB studies in CAPD patients (our present data combined with data from the literature) to calculate regression equations describing the relationship between TNA and NPNA, respectively, and UNA.(ABSTRACT TRUNCATED AT 400 WORDS)

A model for determination of total body protein synthesis based upon compartmental analysis of the plasma [15N] glycine decay curve

A model for whole body glycine nitrogen flux based on the compartmental analysis of plasma [15N] glycine decay curves is described and used for the measurements of (1) total body glycine nitrogen flux and the components of this flux in three healthy young adults and (2) total body protein synthesis based on the conversion of 15N to excretory products, ie, the sum of urinary [15N] urea and 15NH3 and the amount of labeled urea remaining in the body at five hours following administration of [15N] glycine. The mean glycine nitrogen flux was 3.93 +/- 0.42 mg N X kg-1 X h-1 (SEM). The major components of this flux are de novo synthesis of glycine, which accounts for 18% to 27%, and release from protein breakdown, which accounts for 62% to 73%. The outward pathways of glycine from the total body free glycine pool are conversion to other amino acids and oxidation to excretory end products (30% to 42%) and incorporation into protein, which accounts for 45% to 61% of glycine N loss from the metabolic pool. The mean rate of total body protein synthesis as determined by compartmental analysis was 3.56 g protein X kg-1 X day-1. The results that were obtained for protein synthesis and whole body glycine kinetics accord well with previous studies in normal adults, using the stochastic model.

Cerebral cortical atrophy in pediatric patients with end-stage renal disease

Computed tomography of the head in 15 children with end-stage renal disease revealed cerebral cortical atrophy in eight and ventricular enlargement in an additional two. These changes were not necessarily associated with any clinical signs or symptoms. Although high doses of steroids may have been responsible for some of the lesions, alternate etiologies are required to explain the abnormality in others cases. The mean length of time the patients received chronic hemodialysis was twice as long in patients with atrophy as in those with normal scans. However, possible causative factor/s related to dialysis could not be identified. The high incidence of cerebral atrophy is cause for concern and indicates the need for further study of this phenomenon.