Phenylbutyrate-induced glutamine depletion in humans: effect on leucine metabolism.

The present study was designed to determine whether sodium phenylbutyrate (phi B) acutely induces a decrease in plasma glutamine in healthy humans, and, if so, will decrease estimates of whole body protein synthesis. In a first group of three healthy subjects, graded doses (0, 0.18, and 0.36 g.kg-1.day-1) of phi B were administered for 24 h before study: postabsorptive plasma glutamine concentration declined in a dose-dependent manner, achieving an approximately 25% decline for a dose of 0.36 g phi B.kg-1.day-1. A second group of six healthy adults received 5-h infusions of L-[1-14C]leucine and L-[1-13C]glutamine in the postabsorptive state on two separate days: 1) under baseline conditions and 2) after 24 h of oral treatment with phi B (0.36 g.kg-1.day-1) in a randomized order. The 24-h phenylbutyrate treatment was associated with 1) an approximately 26% decline in plasma glutamine concentration from 514 +/- 24 to 380 +/- 15 microM (means +/- SE; P < 0.01 with paired t-test) with no change in glutamine appearance rate or de novo synthesis; 2) no change in leucine appearance rate (Ra), an index of protein breakdown (123 +/- 7 vs. 117 +/- 5 mumol.kg-1.h-1; not significant); 3) an approximately 22% rise in leucine oxidation (Ox) from 23 +/- 2 to 28 +/- 2 mumol.kg-1.h-1 (P < 0.01), resulting in an approximately 11% decline in nonoxidative leucine disposal (NOLD = Ra-Ox), an index of protein synthesis, from 100 +/- 6 to 89 +/- 5 mumol.kg-1.h-1 (P < 0.05). The data suggest that, in healthy adults, 1) large doses of oral phenylbutyrate can be used as a "glutamine trap" to create a model of glutamine depletion; 2) a moderate decline in plasma glutamine does not enhance rates of endogenous glutamine production; and 3) a short-term depletion of plasma glutamine decrease estimates of whole body protein synthesis.

Glutamine metabolism in very low birth weight infants.

To quantitate glutamine kinetics in premature infants and determine whether glutamine affects leucine metabolism. 11 very low birth weight (< 1250 g) neonates received 4-h i.v. infusions of L-[2H3]leucine and L-[13C5]glutamine, along with orogastric infusion of L-[I-13C]leucine and L-[I-13C]glutamine on the 10th d of life and in the fed state. Patients were receiving parenteral nutrition and were randomized to receive either hypocaloric, enteral preterm formula alone (controls; n = 5), or glutamine (0.2 g.kg-1.d-1 on the day of the study) supplemented formula (GL.n; n = 6). The rates of appearance (Ra) of leucine and glutamine, and their rates of splanchnic extraction were determined from isotopic enrichments in plasma at steady state. Leucine release from protein breakdown did not differ between groups (123 +/- 51 versus 162 +/- 94 mumol.kg-1h-1 in the controls and GLN group, respectively). Glutamine de novo synthesis accounted for > 80% of overall glutamine Ra, and was similar in both groups (626 +/- 177 versus 525 +/- 86 mumol.kg-1.h-1; NS); 46 +/- 16% and 53 +/- 31% of the enteral glutamine underwent first-pass splanchnic extraction in the controls and GLN group, respectively. These findings indicate that the pathways of glutamine de novo synthesis and glutamine utilization in the splanchnic bed are functional in very low birth weight humans by the 10th d of life. Glutamine supplementation provided at low doses on a hypocaloric regimen results in no apparent differences in flux of glutamine or leucine.

Protein and energy metabolism in prepubertal children with sickle cell anemia.

We hypothesized that, in children with homozygous sickle cell anemia (HbSS), the shortened life-span of erythrocytes places an increased demand on protein stores, accelerates whole body protein turnover, and consequently, energy expenditure, as well as the rate of utilization of glutamine, a major fuel for reticulocytes. Eight (11.2 +/- 0.4 y old) children with HbSS who were free of infection of vaso-occlusive disease, and seven (11.3 +/- 0.4 y old) healthy black children were therefore studied in the postabsorptive state. Each received a continuous 4-h infusion of L-[1-(13)C]leucine to determine the rate of leucine oxidation, leucine rate of appearance, and nonoxidative leucine disposal, indicators of whole body protein breakdown and synthesis, respectively. Infusion of L-[2-(15)N]glutamine was used to assess rates of glutamine utilization. Resting energy expenditure and cardiac output were measured using indirect calorimetry and echocardiography, respectively. Compared with control subjects, HbSS children had a 58 and 65% higher leucine rate of appearance and nonxidative leucine disposal, respectively (both p < 0.001), 47% higher rates of whole body glutamine utilization (p < 0.01), 19% higher resting energy expenditure (p < 0.05), and 66% higher cardiac output (p < 0.01). In conclusion, children with HbSS show evidence of hypermetabolism with regard to protein, energy, and glutamine utilization. Both increased Hb synthesis and increased cardiac workload may contribute to excess protein and energy utilization. Whatever the mechanism of hypermetabolism, the data suggest that children with HbSS may have greater protein and energy requirements than the general population.

Response of glutamine metabolism to exogenous glutamine in humans.

To determine whether exogenous glutamine affects whole body glutamine metabolism, preliminary experiments were performed to verify that L-[1-13C]-, L-[U-14C]-, and L-[3,4-3H]glutamine given simultaneously by vein provided similar estimates of glutamine appearance rates [Ra; 355 +/- 24, 373 +/- 19, and 393 +/- 24 (SE) mumol.kg-1.h-1, respectively, P = NS] in six healthy men; glutamine oxidation accounted for 32 +/- 3 and 51 +/- 5% (P < 0.01) of glutamine Ra when it was measured using L-[U-14C]- and L-[1-13C]glutamine, respectively. Five subjects received two 5-h intravenous infusions of L-[3,4-3H]glutamine and a simultaneous nasogastric infusion of L-[1-13C]glutamine on 2 separate days in the postabsorptive state, along with saline on 1 day and natural L-glutamine (856 +/- 45 mumol.kg-1.h-1) on another day in a randomized order. Splanchnic glutamine extraction (determined from [13C]glutamine appearance into systemic blood) reached 74 +/- 4 and 53 +/- 5% during the enteral infusion of tracer alone and in combination with a large load of glutamine, respectively. Glutamine infusion was associated with increased plasma glutamine concentration (from 630 +/- 50 to 1,297 +/- 75 microM), Ra (from 258 +/- 20 to 589 +/- 45 mumol.kg-1.h-1), and oxidation (from 179 +/- 20 to 477 +/- 47 mumol.kg-1.h-1, all P < 0.01), no change in glutamine release from proteolysis, and a decline in glutamine de novo synthesis (from 156 +/- 15 to 93 +/- 13 mumol.kg-1.h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Use of [13C]bicarbonate infusion for measurement of CO2 production.

To determine whether infusion of 13C-labeled bicarbonate can be used to measure rates of CO2 production (VCO2), seven healthy adults received 6-h primed continuous intravenous infusions of NaH13CO3 and L-[1-14C]leucine in the post-absorptive state while VCO2 was measured by indirect calorimetry. Indirect calorimetry and the use of specific activity and rate of 14CO2 expired yielded identical values of VCO2: 8.97 +/- 0.82 and 8.80 +/- 0.83 mmol/min, respectively (P = NS). The concentration of NaH13CO3 in the infusates and the 13C enrichment in breath CO2 were determined using gas chromatography-isotope ratio mass spectrometry. The rate of appearance of CO2 measured using the NaH13CO3 infusion rate and the steady-state breath 13CO2 enrichments was 11.41 +/- 1.56 mmol/min, which was higher (P < 0.001) than that determined by either of the other two methods. When corrected for the recovery of labeled CO2 during the infusion of NaH13CO3 by use of published values, rate of appearance of CO2 was 9.24 +/- 0.78 mmol/min, which did not differ from VCO2 determined using the other two methods. We conclude that infusion of NaH13CO3 can be used to determine VCO2. This method should be useful to study the oxidation of substrates in populations such as ventilator-dependent neonates, in whom indirect calorimetry is laborious and inaccurate.