Leucine Balance Research Articles

Differential responsiveness of protein synthesis and degradation to amino acid availability in humans.

We investigated the effects of graded hyperaminoacidemia on protein metabolism in eight healthy, young (25 +/- 2 years), normal weight (BMI = 25 +/- 1 kg/m2), overnight-fasted human subjects. A balanced amino acid solution was infused for 180 min at five different rates: 0.5 (study I), 1.0 (study II), 2.0 (study III), 4.0 (study IV). and 6.0 (study V) mg x kg-1 x min-1 on separate days in random order. Studies were performed with [1-14C]leucine infusion and indirect calorimetry to calculate leucine oxidation (LOX), nonoxidative leucine disposal (NOLD) (an index of protein synthesis), and endogenous leucine flux (ELF) (an index of proteolysis). Basal total plasma amino acid concentrations averaged 1.85 +/- 0.1 mmol/l and increased to 2.27 +/- 0.1, 2.70 +/- 0.2, 3.84 +/- 0.2, 5.87 +/- 0.4, and 7.52 +/- 0.3 mmol/l in studies I-V, respectively. ELF decreased from a basal value of 2.27 +/- 0.2 to 2.12 +/- 0.2, 1.97 +/- 0.1, 1.73 +/- 0.2, 1.67 +/- 0.3, and 1.65 +/- 0.1 micromol x kg-1 x min-1 in studies I-V, respectively (P < 0.05 for study I vs. basal, P < 0.01 for studies II-V vs. basal, and NS for studies IV and V vs. study III). LOX increased from a basal value of 0.31 +/- 0.04 to 0.38 +/- 0.05, 0.41 +/- 0.02, 0.64 +/- 0.04, 1.11 +/- 0.07, and 1.56 +/- 0.05 micromol x kg-1 x min-1 in studies I-V (all P < 0.01 vs. basal; P < 0.05-0.01 for each study vs. preceding study). Basal NOLD averaged 1.96 +/- 0.2 and did not change significantly in studies I and II (2.03 +/- 0.2 and 2.10 +/- 0.1 micromol x kg-1 x min-1). In contrast, a significant increase in NOLD was observed in studies III, IV, and V (to 2.3 +/- 0.15, 2.74 +/- 0.2, and 3.25 +/- 0.7 micromol x kg-1 x min-1, respectively; all P < 0.01 vs. basal; P < 0.05-0.01 for each study vs. preceding study). The net leucine balance (difference between ELF and NOLD)(-0.31 +/- 0.06 micromol x kg-1 x min-1) became less negative in study I (P < 0.01 vs. basal) and positive during studies II-V when the rise in plasma total amino acid levels was >/= 50% above basal level (P < 0.01 vs. each preceding study). In conclusion, NOLD, ELF, and LOX exhibit a differential responsiveness to acute changes in substrate availability: 1) small increments (25-50%) in plasma amino acid levels inhibit ELF and stimulate LOX but have no effect on NOLD; 2) stimulation of NOLD is observed only with increments in plasma amino acid levels >/= 100% above basal values; and 3) increments in plasma amino acid concentrations > 100% above basal values cause a progressive dose-related increase in LOX and NOLD but do not induce any further inhibition of ELF.

Influences of dietary energy and protein on leucine kinetics during feeding in healthy adults.

Ten adult men were infused with L-[1-13C]leucine for 9 h commencing in the postabsorptive state (PA, 0-3 h), during the half-hourly feeding of low-protein meals (LP, protein = 2% calories, 3-6 h), and during feeding isoenergetic high-protein meals (HP, protein = 14% calories, 6-9 h). Leucine oxidation and turnover (protein synthesis and degradation) were determined from plasma alpha-[1-13C]ketoisocaproate enrichment and expired 13CO2 excretion measured during the third hour of each 3-h period. Plasma insulin increased markedly with feeding to a level that was maintained with both diets. The negative postabsorptive leucine balance became less negative during the LP meals (P < 0.01) and was positive with the HP meals (P < 0.01). The significant responses to feeding (all P < 0.01) were for oxidation -13% (PA-LP), +50% (LP-HP), and +29% (PA-HP); for degradation -24% (PA-LP), -30% (LP-HP), and -47% (PA-HP); and for synthesis -14% (PA-LP), +29% (LP-HP), and +11% (PA-HP). These data support a feeding mechanism involving both an insulin-mediated, protein-conserving influence of dietary energy that inhibits degradation, lowers amino acid levels, and reduces oxidation, and amino acid-mediated augmentation of the inhibition of degradation, a stimulation of synthesis, and an increase in oxidation when leucine dietary supply exceeds the capacity for its net deposition.

Effects of dietary protein restriction on regional amino acid metabolism in insulin-dependent diabetes mellitus.

We studied subjects with insulin-dependent diabetes mellitus (IDDM) and controls by administering primed continuous infusions of L-[1-13C,15N)]leucine and L-[2,3-13C2]alanine to measure whole body and forearm metabolism of these amino acids during ample protein intake and again after 4 wk of moderately restricted protein intake. Decreased rates of whole body protein degradation, leucine transamination, leucine oxidation, and increased forearm alanine release produced by dietary protein restriction occurred equivalently in IDDM subjects under short-term tightly managed glycemia and in controls. Dietary protein restriction did not affect whole body alanine appearance or forearm leucine appearance, disposal, or balance in IDDM subjects or controls. IDDM subjects differed from controls only in that normal forearm leucine balance was maintained at higher rates of leucine appearance and disposal. We conclude that IDDM subjects adapt normally to dietary protein restriction. Undernutrition during moderate protein deprivation in these patients likely occurs during episodes of poor glycemic control.

Immediate commencement of amino acid supplementation in preterm infants: Effect on serum amino acid concentrations and protein kinetics on the first day of life

To determine whether the general reluctance to begin amino acid administration to preterm infants from birth onward might lead to loss of lean body mass and impairment of growth, we measured amino acid levels and protein kinetics in 18 preterm infants. Nine infants received amino acids (1.15 ± 0.06 gm · kg -1 · day -1) and glucose (6.05 ± 1.58 gm · kg -1 · day -1), whereas the other nine infants received only glucose (6.48 ± 1.30 gm · kg -1 · day -1) from birth onward. Protein kinetics on the first postnatal day were measured with a stable isotope dilution technique with [1- 13C]leucine as a tracer. No statistically significant differences were noted in blood pH, base excess, urea concentration, or glucose levels. Both total amino acid concentration and total essential amino acid concentration were significantly lower and were below the reference range in the nonsupplemented group. Plasma amino acid levels of five essential amino acids (methio-nine, cystine, isoleucine, leucine, arginine) were below the reference range in the nonsupplemented group, whereas only cystine was below the reference range in the supplemented group. Nitrogen retention was improved significantly by the administration of amino acids (-110 ± 44 mg nitrogen per kilogram per day in the glucose-only group vs +10 ± 127 mg nitrogen per kilogram per day in the group given glucose and amino acids; p = 0.001); leucine oxidation was not significantly increased in the supplemented group (41 ± 13 μmol · kg -1 · hr -1 vs 46 ± 16 μmol · kg -1 · hr -1). Leucine balance also improved significantly (-41 ± 13 μmol ·kg -1 · hr -1 vs -8 ± 16 μmol · kg -1 · hr -1; p = 0.01) because of a combination of an increased amount of leucine being used for protein synthesis and a lower amount of leucine coming from protein breakdown. Plasma cystine concentration, the only amino acid below the reference range in the supplemented group, was highly predictive for protein synthesis in that group. We conclude that the administration of amino acids to preterm infants from birth onward seems safe and prevents the loss of protein mass. (J P EDIATR 1995;127:458-65)

A model of skeletal muscle leucine kinetics measured across the human forearm.

We propose a new six-compartment model of intracellular muscle kinetics of leucine and of its transamination product alpha-ketoisocaproic acid (KIC) by combining systemic tracer infusions of [14C]- and [15N]leucine with the arterial-deep venous catheterization of the human forearm. Venous [14C]KIC specific activity (SA) is taken as representative of intracellular [14C]leucine SA, whereas net [15N]leucine disposal is used to calculate leucine inflow and outflow across forearm cell membrane(s). In post-absorptive normal subjects, model-derived rates of intracellular leucine release from and incorporation into protein were approximately 32% (P = 0.03) and approximately 37% greater (P = 0.025), respectively, than those calculated using a conventional arteriovenous approach. Forearm fasting proteolysis exceeded protein synthesis (P < 0.025), whereas leucine oxidation was greater than zero (P < 0.01), suggesting a net negative leucine (i.e., protein) balance. Leucine inflow from blood to cell represented approximately 30% of arterial leucine delivery; therefore approximately 70% of arterial leucine bypassed intracellular metabolism. This model provides a comprehensive description of regional leucine and KIC kinetics and new estimates of protein degradation and synthesis across the human forearm.

Comparative Evaluation of the Quantitative Utilization of Parenterally and Enterally Administered Leucine and L‐[1–13C,15N]Leucine Within the Whole Body and the Splanchnic Region

The purpose of this study was to quantify the kinetic aspects of leucine metabolism in the whole body and within the splanchnic region when an amino acid mixture is administered by the enteral route as compared with administration by the parenteral route. Seven chronically catheterized dogs were studied during an intragastric infusion of L-[1-13C,15N]leucine with 0.24 g of complete amino acid mixture (kg-1.h-1) for 6 hours. The results are compared with those previously reported for nine dogs studied under essentially identical experimental conditions except that the tracer and amino acid mixtures were given by vein. At the whole body level, the various parameters of leucine metabolism (flux, oxidation, and disappearance into and release from proteins) were not significantly different among animals that were infused enterally and those that were infused parenterally. Leucine metabolism within the splanchnic region and gut was more extensive for enteral administration than for parenteral administration. For the splanchnic region, 31.4%, 27.9%, and 6.0% of enterally administered leucine was used for protein synthesis, deamination, and oxidization, respectively, compared with respective mean values of 19.5% (p < .001), 13.4% (p < .001), and 4.1% (p < .05) for parenterally administered leucine. For liver, 4.8% of enterally administered leucine was oxidized compared with 2.3% (p < .001) of parenterally administered leucine. These findings are qualitatively similar to those reported previously by us when comparing enteral and parenteral amino acid intakes but with an IV tracer infusion. Enteral administration seems to be more effective than an acute parenteral feeding regimen, at least in maintaining leucine (protein) balance in gut tissues.

Whole-body protein turnover from leucine kinetics and the response to nutrition in human immunodeficiency virus infection

Whole-body protein metabolism was investigated in human immunodeficiency virus (HIV) infection by primed constant infusion of L-[1-13C]leucine in 8 control and 22 HIV-infected subjects (8 stage II; 14 stage IV disease), in postabsorptive and fed states. Postabsorptive leucine flux was increased 25% in subjects with stage IV HiV infection vs that in control subjects (130 +/- 13 vs 103 +/- 10 mumol leucine.kg-1.h-1, P < 0.001); both leucine disposal by protein synthesis (111.6 +/- 12.1 vs 82.3 +/- 9.2, P < 0.001) and release by protein degradation (129.7 +/- 13.1 vs 103.4 +/- 10.2, P < 0.001) were increased. No difference in leucine balance or oxidation was found but fat oxidation was greater in subjects with HIV infection (61.1 +/- 13.0% of energy) than in control subjects (47.6 +/- 13.7% of energy, P < 0.025). Stage II subjects had intermediate values of leucine flux, not significantly different from those of control subjects. Provision of parenteral nutrition for 4 h increased leucine flux with a switch in leucine balance from net loss to net gain; this response was quantitatively similar in all groups. HIV infection increases whole-body protein turnover but does not quantitatively impair the acute anabolic response to intravenous nutrition.

Leucine kinetics are different during feeding with whole protein or oligopeptides

To determine if the molecular form of nitrogen intake affects protein metabolism during feeding, 12 normal volunteers received, by continuous nasogastric infusion, a protein or a peptide-based diet. Leucine kinetics (oral [13C]leucine and intravenous [2H3]leucine) were measured during the following three consecutive periods: first carbohydrates and lipids alone, then with either whole casein or oligopeptides in a randomized crossover design, with these two latter periods being isonitrogenous, isocaloric, and of identical amino acid compositions. Leucine concentration, turnover, oxidation, and nonoxidative disposal increased during nitrogen administration (all P < 0.01) and were higher with oligopeptides than with casein (242 +/- 44 vs. 188 +/- 31 mumol/l; 2.75 +/- 0.45 vs. 2.23 +/- 0.31; 1.14 +/- 0.19 vs. 0.82 +/- 0.22 mumol.kg-1.min-1, all P < 0.001; 1.64 +/- 0.32 vs. 1.44 +/- 0.33 mumol.kg-1.min-1, P < 0.05, respectively). Endogenous leucine production was less inhibited by oligopeptides than by casein (0.82 +/- 0.41 vs. 0.38 +/- 0.31 mumol.kg-1.min-1, P < 0.001), whereas splanchnic extractions were similar. Finally, leucine balance was more positive with casein than with oligopeptides (P < 0.001). In conclusion, the response of leucine kinetics to feeding is modified by the molecular form of nitrogen intake, with the oligopeptides inducing a higher oxidation and protein synthesis and a lesser inhibition of protein breakdown.

Glutamine metabolism in healthy adult men: response to enteral and intravenous feeding

To assess the effect of feeding on glutamine kinetics, six healthy men received 4-h intravenous infusions of L-[2-15N]glutamine and L-[1-13C]leucine on 3 separate days: 1) in the postabsorptive state, 2) over the course of an 8-h nasogastric infusion of a small peptide-based nutrient mixture, and 3) during an 8-h isonitrogenous, isoenergetic intravenous infusion (1.5 g amino acid.kg-1.d-1; 130 kJ.kg-1.d-1, or 31 kcal.kg-1.d-1; 58% carbohydrate and 42% fat). Regardless of the route, nutrition increased leucine appearance rate (Ra) and oxidation, stimulated protein synthesis, and improved leucine balance; apparent rates of protein breakdown decreased during enteral nutrition only. Glutamine Ra increased 16.8% (NS) and 26.2% (P < 0.01) with parenteral and enteral feeding, respectively, over postabsorptive values. The present findings are consistent with a major role of glutamine in interorgan nitrogen transport in the fed state and further suggest that increased availability of precursors may stimulate glutamine synthesis de novo, and enteral infusion of peptide-bound amino acids may be an effective route to provide free glutamine to the rest of the body.

Acute growth hormone effects on amino acid and lipid metabolism.

The anabolic actions of GH are well known, although specific tissue responses and the mechanism of nitrogen conservation are less well understood. This study was designed to examine the acute metabolic effects of GH on whole body and regional protein metabolism, using an experimental protocol which controlled for confounding perturbations in other hormones by a simultaneous infusion of somatostatin. Control subjects received replacement doses of insulin, glucagon, and GH for the entire 7-h study period, whereas GH subjects received an identical protocol, except for an increased dose of GH sufficient to increase serum concentrations into the high-physiological range (12-20 ng/mL) for the final 3.5 h of the study (P < 0.001). Thirteen young, healthy male subjects were studied in the postabsorptive period; five served as control subjects and eight as treatment (GH) subjects. Each received continuous iv infusions of somatostatin, L-[13-C]leucine, and L-[2H5]phenylalanine throughout the study. Femoral arterial and venous sampling allowed for simultaneous measurements across the leg and in the whole body. C-Peptide levels were suppressed throughout the infusion; insulin, glucagon, insulin-like growth factor I, cortisol, epinephrine, norepinephrine, and glucose concentrations were not different between groups. Glycerol concentrations increased 3-fold in GH subjects during the final 3.5-h period (P = 0.04). Concentrations of several amino acids declined through the study, but no differences were observed between treatment groups. Leucine oxidation was reduced in GH compared to control subjects (P = 0.04). No changes in CO2 production or whole body leucine or phenylalanine flux were observed, whereas nonoxidative disposal of leucine was marginally higher in GH compared to control subjects (P = 0.07). By contrast, rates of appearance and disappearance of both leucine and phenylalanine across the leg all were relatively lower in GH compared to control subjects; leucine balance across the leg was reduced by GH (P = 0.03), whereas phenylalanine balance was not influenced by GH. Our data thus demonstrate an acute stimulatory effect of GH on lipolysis, a decrease in leucine oxidation, and no stimulation of muscle protein synthesis in spite of enhanced protein synthesis in nonmuscle tissue.

Validation of the tracer-balance concept with reference to leucine: 24-h intravenous tracer studies with L-[1-13C]leucine and [15N-15N]urea

The validity of tracer-derived estimates of whole-body leucine balance was investigated. Seven healthy young adult subjects received an adequate protein diet for 6 d; at 1800 on the last day, L-[1-13C]leucine and [15N-15N]urea were given as primed, continuous intravenous infusions for 24 h. Subjects were in a fasting state for the first 12 h and at 0600 on day 7 they then received hourly 10 equal meals to achieve a fed state. Total leucine intake (diet plus tracer) was 89.4 mg.kg-1.d-1. Mean daily leucine oxidation was equivalent to 89.5 +/- 3.3 mg leucine/kg. The predicted daily oxidation rate, from measurements made during the last hour of the fast and the fifth hour of the fed period, was 91.2 +/- 5.8 mg/kg (P = 0.25 from measured). Measured and predicted whole-body leucine balances were 0.76 +/- 2.99 and -0.98 +/- 5.54 mg/kg, respectively (P = 0.25). Urea production exceeded urea excretion by 20%; daily protein oxidation was the same when estimated from leucine oxidation or nitrogen excretion. Thus, the tracer-balance concept is valid, and reliable predictions of total daily leucine oxidation and whole-body leucine balance can be obtained from short-term measurements of leucine oxidation during fasted and fed states.

The 24-h pattern and rate of leucine oxidation, with particular reference to tracer estimates of leucine requirements in healthy adults

Daily leucine oxidation and derived values for whole-body leucine balance, obtained by continuous measurement throughout a 24-h period, were compared with those predicted from short-term measurements during fasted and fed states in five healthy adults studied during two 6-d experimental diet periods, each immediately followed by a 24-h continuous intravenous tracer infusion of L-[1-13C]leucine. Leucine intake was either 14 or 38.3 mg.kg-1.d-1. Mean measured daily leucine oxidation (mg leucine.kg-1.d-1) was 27.8 and 45.2 for the 14- and 38.3-mg intakes, respectively. Oxidation rates predicted by extrapolation of rates measured during the final hour of fasting (15 h after last meal) and the 5th h of feeding were approximately 12% higher (P < 0.01) than measured rates for both diets. For the prediction based on the 12th h of fasting and 5th h of feeding, it was 4% higher or 0.4% lower than measured rates for the 38.3- and 14- mg intakes, respectively. Hence, relatively small differences exist between measured vs predicted estimates of daily leucine oxidation and balance. These studies support previous conclusions that the current, international requirement value for leucine in healthy adults is far too low.

Nitrogen homoeostasis in man: diurnal changes in nitrogen excretion, leucine oxidation and whole body leucine kinetics during a reduction from a high to a moderate protein intake.

1. The adaptation of the diurnal cycle of nitrogen (N) homeostasis during a change in protein intake was investigated with diurnal measurements of N and leucine balance and turnover during a reduction from a high to a moderate protein intake in normal adults. 2. In experiment 1, during a 9 day period after a reduction from 1.82 to 0.77 g of protein day-1 kg-1, N excretion fell slowly at a similar rate in fed and fasted states so that the lowered intake was unable to replete any of the postabsorptive losses for 3 days. There was a marked negative N balance, which persisted throughout the study, although with a significant reduction in N losses in both fed and fasted states on day 4, balances during days 4-9 (-32.8 +/- 28.3 mg of N day-1 kg-1) were less negative than during days 1-3 (-79.1 +/- 60.4 mg of N day-1 kg-1). 3. In experiment 2, during a 14 day period after a reduction from 1.89 to 0.77 g of protein day-1 kg-1, [1-13C]leucine oxidation and turnover were measured by primed intravenous infusion, during fasting and feeding in subjects before and on days 3, 7 and 14 after the dietary change. Leucine oxidation fell by 32% (P < 0.05) on day 3 in the fed state and by 12% (P < 0.05) during fasting, falling further in each case by day 7 with improved balance.(ABSTRACT TRUNCATED AT 250 WORDS)

Requirements for indispensable amino acids in adult humans: longer-term amino acid kinetic study with support for the adequacy of the Massachusetts Institute of Technology amino acid requirement pattern

Twenty young men received an L-amino acid diet, supplying 140 mg N.kg-1 x d-1 and patterned as in the Egg diet for 1 wk, and then for 3 wk either a pattern based on international recommendations (modified FAO diet; n = 7), our new amino acid requirement pattern (MIT diet; n = 7), or the egg pattern (Egg diet; n = 6). At the end of the initial week, at 1 and 3 wk with the three experimental diets, and after 3 d after return to the Egg diet, an 8-h continuous intravenous infusion with [1-13C]leucine (3 h fast, 5 h fed while subjects received hourly meals supplying the equivalent of one-twelfth the daily intake) was conducted. After 3 wk with the different diets, mean daily leucine balances were lower (P < 0.01) with the FAO diet (-160 mumol.kg-1 x d-1) than with the MIT diet (-15 mumol.kg-1 x d-1). Together with changes in plasma amino acid profiles [eg, methionine increased (P < 0.05) during feeding with the FAO and Egg diets but not with the MIT diet; increased proline concentrations during the fed state (P < 0.05) with the FAO diet but not with the MIT or Egg diets] we interpret these findings to indicate that the FAO diet is not capable of maintaining amino acid homeostasis, as is the case with the MIT and Egg diets.

Leucine metabolism at graded amino acid intakes in children receiving parenteral nutrition

To assess the response of protein turnover to graded levels of amino acid (AA) intakes, leucine kinetics were determined in six 8- to 16-yr-old patients in a stable nutritional status receiving home parenteral nutrition (PN) for short-bowel syndrome or intestinal pseudo-obstruction syndrome. Although daily energy intake was kept constant at 68.7 +/- 13 kcal/kg lean body mass (LBM) with 25.4 +/- 3.6% lipid, patients were given, for three consecutive 7-day periods, 0.7, 1.5, or 2.5 g AA.kg LBM-1.day-1, with the order of the regimens being randomized. On day 7 of each period, a 4-h infusion of L-[1-13C]leucine was performed during intravenous feeding; plasma [13C]ketoisocaproate and expired 13CO2 enrichments were used to assess whole body leucine turnover (Ra), oxidation rate (Ox), nonoxidative disposal [an estimate of protein synthesis (S)], and leucine derived from protein breakdown (B). Urine collection (24 h) was performed for determination of nitrogen excretion. Results indicate a dose-dependent rise in plasma leucine concentration, Ra, and Ox but no significant change in B. There was a significant increase of S (P = 0.04 analysis of variance) with increased AA intakes as well as net leucine balance (P = 0.02). Results are consistent with improved leucine balance, when leucine intake increases, despite increased leucine oxidation. The net protein gain observed with higher AA intakes may suggest a beneficial effect for children receiving long-term PN.

The effect of hemodialysis on protein metabolism. A leucine kinetic study.

To assess the effect of hemodialysis on protein metabolism, leucine flux was measured in seven patients before, during, and after high efficiency hemodialysis using cuprophane dialyzers and bicarbonate dialysate during a primed-constant infusion of L-[1-13C]leucine. The kinetics [mumol/kg per h, mean +/- SD] are as follows: leucine appearance into the plasma leucine pool was 86 +/- 28, 80 +/- 28, and 85 +/- 25, respectively, before, during, and after dialysis. Leucine appearance into the whole body leucine pool, derived from plasma [1-13C]alpha-ketoisocaproate enrichment, was 118 +/- 31, 118 +/- 31, and 114 +/- 28 before, during, and after dialysis, respectively. In the absence of leucine intake, appearance rate reflects protein degradation, which was clearly unaffected by dialysis. Leucine oxidation rate was 17.3 +/- 7.8 before, decreased to 13.8 +/- 7.8 during, and increased to 18.9 +/- 10.3 after dialysis (P = 0.027). Leucine protein incorporation was 101 +/- 26 before, was reduced to 89 +/- 23 during, and returned to 95 +/- 23 after dialysis (P = 0.13). Leucine net balance, the difference between leucine protein incorporation and leucine release from endogenous degradation, was -17.3 +/- 7.8 before, decreased to -28.5 +/- 11.0 during, and returned to -18.9 +/- 10.3 after dialysis (P < 0.0001). This markedly more negative leucine balance during dialysis was accountable by dialysate leucine loss, which was 14.4 +/- 6.2 mumol/kg per h. These data suggest that hemodialysis using a cuprophane membrane did not acutely induce protein degradation. It was, nevertheless, a net catabolic event because protein synthesis was reduced and amino acid was lost into the dialysate.

An isotopic study of the effect of dietary carbohydrate on the metabolic fate of dietary leucine and phenylalanine

The fate of dietary leucine and phenylalanine was studied in five healthy, young adult men, by using a dual, stable isotope-tracer infusion approach to estimate amino acid fluxes, splanchnic (Sp) uptake, and dietary of absorbed amino acid to the peripheral circulation. Subject received two, 4-h tracer infusions of [1-13C]leucine and [15N]phenylalanine infused through a feeding tube placed in the duodenum, and [5,5,5-2H3]leucine, [ring-2H5]phenylalanine, and [6,6-2H2]glucose infused simultaneously by vein. In one experiment subjects received an amino acid mixture (83 mg amino acid.kg-1.h-1) via the feeding tube and in the other experiment amino acids were supplied with carbohydrate (CHO) (167 mg.kg-1.h-1). Sp uptake of dietary leucine decreased with added dietary CHO (29% of ingested leucine for amino acids alone vs 20% with CHO; P < 0.05) but was not different for phenylalanine (P > 0.05). Addition of CHO decreased both release of leucine via protein breakdown and leucine oxidation and increased body leucine balance (P < 0.05).

The role of the central nervous system in modulating glucose and protein metabolism during insulin-induced hypoglycemia

We previously established that insulin-induced hypoglycemia is associated with enhanced proteolysis with the GI tract being the major contributor to this response. In the present study we examined whether brain hypoglycemia modulates the changes in amino acid and glucose kinetics during insulin-induced hypoglycemia. Studies were performed in 24 h faster conscious dogs chronically fitted with catheters in the femoral artery, portal vein, hepatic vein, femoral vein and in both carotid and vertebral arteries, and that were also fitted with permanent tracheostomies. Following a 30 min basal period insulin was infused i.v. at 300 mU/kg·h to achieve systemic hypoglycemia averaging 42±2mg/dl. In group 1 ( n = 6) both central and systemic (global) hypoglycemia was allowed to develop. The rate of net hepatic glucose output (NHGO) increased by 50% above basal. Plasma leucine increased from 120 ± 12 μmol/1 basally to 154 ± 30 μmol/1 during the last hour of hypoglycemia. The rate of leucine appearance into the plasma compartment ( R u) increased from 154 ± 30 to 200 ± 36 μmol/kg·h and its rate of oxidation increased from 22 ± 5 to 51 ± 8 μmol/kg·h, while its non-oxidative rate of disposal (141 ± 12 μmol/kg·h) did not change. Net leucine balance across the gut was neutral (6 ± 5 μmol/kg·h) and switched to net output of 96 ± 24 μmol/kg·h. In group II ( n = 5) 10% dextrose in water was infused into both carotid and vertebral arteries to prevent profound CNS glucopenia. This was associated with a drop in NHGO. Plasma leucine dropped from 143 ± 9 to 39 ± 7 μmol/I, leucine R u dropped from 165 ± 15 to 60 ± 9 μmol/kg·h, leucine oxidation dropped from 34 ± 9 to 9 ± 2 μmol/kg·h and non-oxidative R u dropped from 129 ± 15 to 54 ± 6 μmol/kg·h (all P < 0.001); net leucine balance across the gut was neutral and did not change (5 ± 6 to 16 ± 7 μmol/kg·h). We conclude that the proteolytic response of insulin-induced hypoglycemia, and particularly that of the gl tract can be totally attributed to CNS glucopenia. Systemic glucopenia plays a very minor role in this response.

Growth hormone stimulates skeletal muscle protein synthesis and antagonizes insulin's antiproteolytic action in humans.

We examined the effects of a combined, local intra-arterial infusion of growth hormone (GH) and insulin on forearm glucose and protein metabolism in seven normal adults. GH was infused into the brachial artery for 6 h with a dose that, in a previous study, stimulated muscle protein synthesis (phenylalanine Rd) without affecting systemic GH, insulin, or insulinlike growth factor I concentrations. For the last 3 h of the GH infusion, insulin was coinfused with a dose that, in the absence of infused GH, suppressed forearm muscle proteolysis by 30-40% without affecting systemic insulin levels. Measurements of forearm glucose, amino acid balance, and [3H]phenylalanine and [14C]leucine kinetics were made at 3 and 6 h of the infusion. Glucose uptake by forearm tissues in response to GH and insulin did not change significantly between 3 and 6 h. By 6 h, the combined infusion of GH and insulin promoted a significantly more positive net balance of phenylalanine, leucine, isoleucine, and valine (all P less than 0.05). The change in net phenylalanine balance was due to a significant increase in phenylalanine Rd (51%, P less than 0.05) with no observable change in phenylalanine Ra. For leucine, a stimulation of leucine Rd (50%, P less than 0.05) also accounted for the change in leucine net balance, with no suppression of leucine Ra. The stimulation of Rd, in the absence of an observed effect on Ra, suggests that GH blunts the action of insulin to suppress proteolysis in addition to blunting insulin's action on Rd.

Glucose and amino acid metabolism in chronic renal failure: effect of insulin and amino acids.

The effects of hyperinsulinemia and hyperaminoacidemia on glucose and amino acid metabolism were examined in 16 control and 13 chronic renal failure (CRF) patients under two conditions: 1) euglycemic hyperinsulinemia and 2) amino acid infusion. All studies were performed with continuous indirect calorimetry and [1-14C]leucine infusion. In CRF patients insulin-mediated whole body glucose metabolism was reduced by 35% (4.41 +/- 0.50 vs. 6.76 +/- 0.73 mg.kg-1.min-1, P less than 0.01), primarily due to a decrease in nonoxidative glucose disposal (1.70 +/- 0.70 vs. 4.32 +/- 0.60 mg.kg-1.min-1, P less than 0.01); glucose oxidation was similar in both groups. In the postabsorptive state total leucine turnover (1.56 +/- 0.06 vs. 1.75 +/- 0.06), leucine oxidation (0.25 +/- 0.01 vs. 0.30 +/- 0.01), and nonoxidative leucine disposal (1.29 +/- 0.06 vs. 1.40 +/- 0.07 mumol.kg-1.min-1) were reduced in CRF vs. control subjects (all P less than 0.05). In response to hyperinsulinemia, endogenous leucine flux (index of proteolysis), leucine oxidation, nonoxidative leucine disposal (NOLD) (index of protein synthesis), and net leucine flux into protein were similar in CRF and control subjects. In contrast, the ability of hyperaminoacidemia to enhance NOLD (1.54 +/- 0.11 vs. 2.10 +/- 0.10 mumol.kg-1.min-1, P less than 0.01) and net leucine balance (0.27 +/- 0.05 vs. 0.41 +/- 0.05, P less than 0.05) was reduced in CRF patients.(ABSTRACT TRUNCATED AT 250 WORDS)