Infusion Of Mixed Amino Acids Research Articles

Glucagon resistance in individuals with obesity and hepatic steatosis can be measured using the GLUSENTIC test and index.

Increased plasma levels of glucagon (hyperglucagonaemia) promote diabetes development but is also observed in patients with metabolic dysfunction-associated steatotic liver disease (MASLD). This may reflect hepatic glucagon resistance towards amino acid catabolism. A clinical test for measuring glucagon resistance has not been validated. We evaluated our glucagon sensitivity (GLUSENTIC) test, consisting of two study days: a glucagon injection and measurements of plasma amino acids, and an infusion of mixed amino acids and subsequent calculation of the GLUSENTIC index (primary outcome measure) from measurements of glucagon and amino acids. To distinguish glucagon-dependent from insulin-dependent actions on amino acid metabolism, we also studied patients with type 1 diabetes (T1D). The delta-decline in total amino acids was 49% lower in MASLD following exogenous glucagon (p=0.01), and the calculated GLUSENTIC index was 34% lower in MASLD (p<0.0001), but not T1D (p>0.99). In contrast, glucagon-induced glucose increments were similar in controls and MASLD (p=0.41). The GLUSENTIC test and index may be used to measure glucagon resistance in individuals with obesity and MASLD.

Development of a glucagon sensitivity test in humans: Pilot data and the GLUSENTIC study protocol

A physiological feedback system exists between hepatocytes and the alpha cells, termed the liver-alpha cell axis and refers to the relationship between amino acid-stimulated glucagon secretion and glucagon-stimulated amino acid catabolism. Several reports indicate that non-alcoholic fatty liver disease (NAFLD) disrupts the liver-alpha cell axis, because of impaired glucagon receptor signaling (glucagon resistance). However, no experimental test exists to assess glucagon resistance in humans. The objective was to develop an experimental test to determine glucagon sensitivity with respect to amino acid and glucose metabolism in humans. The proposed glucagon sensitivity test (comprising two elements: 1) i.v. injection of 0.2 mg glucagon and 2) infusion of mixed amino acids 331 mg/hour/kg) is based on nine pilot studies which are presented. Calculation of a proposed glucagon sensitivity index with respect to amino acid catabolism is also described. Secondly, we describe a complete study protocol (GLUSENTIC) according to which the glucagon sensitivity test will be applied in a cross-sectional study currently taking place. 65 participants including 20 individuals with a BMI 18.6–25 kg/m2, 30 individuals with a BMI ≥ 25–40 kg/m2, and 15 individuals with type 1 diabetes with a BMI between 18.6 and 40 kg/m2 will be included. Participants will be grouped according to their degree of hepatic steatosis measured by whole-liver magnetic resonance imaging (MRI). The primary outcome measure will be differences in the glucagon sensitivity index between individuals with and without hepatic steatosis. Developing a glucagon sensitivity test and index may provide insight into the physiological and pathophysiological mechanism of glucagon action and glucagon-based therapies.

The Safety and Efficacy of the Repeated PRRT with [90Y]Y/[177Lu]Lu-DOTATATE in Patients with NET.

Purpose The peptide receptor radionuclide therapy (PRRT) is a treatment option for patients with disseminated, inoperable G1 and G2 neuroendocrine tumours (NETs). The study aims to evaluate the safety, efficacy, and progression-free survival (PFS) of patients after retreatment (R-PRRT) and re-retreatment (RR-PRRT) with tandem isotopes [90Y]Y/[177Lu]Lu-DOTATATE. Material and Methods. Out of 99 treated patients with G1 and G2 NETs, 26 were included in the study and treated with the repeated PRRT (with 5 undergoing the re-repeated PRRT treatment) after an initial positive response to four PRRT cycles and later progression of the disease. [68Ga]Ga-DOTATATE PET/CT and CT/MRI procedures were performed before and after the treatment. Patients were treated with [90Y]Y/[177Lu]Lu-DOTATATE (1 : 1) with mixed amino acid infusion for kidney protection. Toxicity was evaluated using the CTCAE 3.0 criteria. Results The median follow-up was 88 months (the range: 42–164). The median cumulative administered activity was 22.2 GBq (the range: 17.8–30.7 GBq). Myelodysplastic syndrome occurred in one patient (3.8%), and grade 4 renal toxicity was also detected in one patient (3.8%). No other cases of grade 3 or 4 bone marrow and renal toxicity were observed. The median PFS rate was 31 months after the PRRT and 23 months following the R-PRRT. The OS rate from the diagnosis (OS-d) was 109 months and from the start of the PRRT (OS-t)-92.4 months. During the restaging, 3–6 months after the PRRT, PR, SD, and PD were observed in 19.2%, 80.8%, and 0% of the patients, respectively. After the R-PRRT, PR, SD, and PD were observed in 50%, 42.3%, and 7.7% of the patients, respectively. Conclusions The repeated therapy with [90Y]Y/[177Lu]Lu-DOTATATE is safe and effective for patients with disseminated, inoperable G1 and G2 neuroendocrine tumours.

Effect of peptide receptor radionuclide therapy (PRRT) with tandem isotopes - [90Y]Y/[177Lu]Lu-DOTATATE in patients with disseminated neuroendocrine tumours depending on [18F]FDG PET/CT qualification in Polish multicentre experience - do we need [18F]FDG PET/CT for qualification to PRRT?

Peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues is a treatment option for patients with disseminated neuroendocrine tumours (NET). The aim of the study was the evaluation of the role of [¹⁸F]FDG PET/CT in predicting response, progression-free survival (PFS) and overall survival (OS) after tandem therapy [⁹⁰Y]Y/[¹⁷⁷Lu]Lu-DOTATATE. Seventy-five patients with histopathologically proven NET G1 and G2 were included in the study. Before treatment [⁶⁸Ga]Ga-DOTATATE PET/CT and [¹⁸F]FDG PET/CT was performed. Patients were treated with [⁹⁰Y]Y/[¹⁷⁷Lu]Lu-DOTATATE (1:1) with mixed amino-acid infusion for kidney protection. Progression-free survival was 22.2 months for [¹⁸F]FDG-positive patients and 59.3 months for [¹⁸F]FDG-negative patients (p = 0.003). The OS from diagnosis (OS-D) and from the start of PRRT (OS-T) was not reached in [¹⁸F]FDG-negative patients, and in [¹⁸F]FDG-positive patients it was 71.8 months and 55.8 months, respectively. The observed overall one-year survival in [¹⁸F]FDG-positive vs. [¹⁸F]FDG-negative patients was 96.8% vs. 99.1%, two-year survival was 88.9% vs. 96%, and five-year survival was 58.8% vs. 88%, respectively. The one-year and two-year risk of progression was 15%vs. 58.9% in [¹⁸F]FDG-positive patients and 11% vs. 32% in [18F]FDG-negative patients. The objective response rate (ORR) [¹⁸F]FDG-positive vs. [¹⁸F]FDG-negative patients was 41.7% vs. 17%. [¹⁸F]FDG-positive patients have statistically significant shorter survival parameters than [¹⁸F]FDG-negative patients. The risk of progression in [¹⁸F]FDG-positive vs. [¹⁸F]FDG-negative patients in one-year follow-up is comparable, whereas in two-year follow-up it is nearly two times higher for [¹⁸F]FDG PET/CT-positive patients.

Effects of infusing milk precursors into the artery on rumen fermentation in lactating cows.

This experiment was conducted to investigate the effects of infusing milk precursors into the external pudic artery on rumen fermentation in lactating dairy cows. Eight multiparous Holstein cows were randomly assigned to Group A (experimental group) and Group B (control group) with 4 cows each. A 2 × 4 complex factor crossover design was used. Cows in Group A were fed corn straw as the only roughage, and cows in Group B were fed mixed roughage. The experiment was divided into two periods. In the first period, cows in Group A, received treatments: 1) a basal infusate as a control (CSC); 2) a milk fat precursor infusion including C16:0, C18:0, C18:1c9, C18:2c6, C18:3n3, acetic acid (CSF); 3) a milk protein precursor infusion including 16 amino acids (CSA); 4) the mixed infusion of milk fat and protein precursors (CSFA). And meanwhile, cows in Group B were infused the basal infusate as a control group. In the second period, the cows in both Groups A and B were crossed over, which cows in Group A were named as Group B and the cows originally in Group B were in Group A. The experimental results showed that cows in experimental group had higher ruminal pH compared with the control, and ruminal pH in CSC, CSF, CSA were significantly higher than those in their respective control group (P < 0.05). The concentration of ammonia nitrogen (NH3–N) was significantly higher in CSA and CSFA compared with Group B (P < 0.05). We also observed that the infusion of mixed amino acids significantly increased the bacterial protein (BCP) content in rumen (P < 0.05) and influenced the rumen acetic acid concentration as well as the acetic to propionic ratio (P < 0.05). Milk fat precursors infusion significantly affected butyric acid concentration (P < 0.05). In addition, the content of lipopolysaccharide (LPS) in CSA was significantly higher than that in the control group (P < 0.05). It is concluded that the milk precursors infused into external pudic artery caused feedback effects on ruminal fermentation under the corn straw roughage conditions. The milk protein precursor increased the ruminal pH, the contents of BCP and acetic acid, which adjust rumen fermentation and improve milk performance.

The curious case of anabolic resistance: old wives' tales or new fables?

the progressive loss of muscle mass, commonly termed sarcopenia, accompanies the process of healthy aging ([27][1]). The underlying basis of this condition, in a very simplistic view, would be at a more advanced age skeletal muscle proteins are being lost because of an imbalance between muscle

Insulin is required for amino acid stimulation of dual pathways for translational control in skeletal muscle in the late-gestation ovine fetus

During late gestation, amino acids and insulin promote skeletal muscle protein synthesis. However, the independent effects of amino acids and insulin on the regulation of mRNA translation initiation in the fetus are relatively unknown. The purpose of this study was to determine whether acute amino acid infusion in the late-gestation ovine fetus, with and without a simultaneous increase in fetal insulin concentration, activates translation initiation pathway(s) in skeletal muscle. Fetuses received saline (C), mixed amino acid infusion plus somatostatin infusion to suppress amino acid-stimulated fetal insulin secretion (AA+S), mixed amino acid infusion with concomitant physiological increase in fetal insulin (AA), or high-dose insulin infusion with euglycemia and euaminoacidemia (HI). After a 2-h infusion period, fetal skeletal muscle was harvested under in vivo steady-state conditions and frozen for quantification of proteins both upstream and downstream of mammalian target of rapamycin (mTOR). In the AA group, we found a threefold increase in ribosomal protein S6 kinase (p70(S6k)) and Erk1/2 phosphorylation; however, blocking the physiological rise in insulin with somatostatin in the AA+S group prevented this increase. In the HI group, Akt, Erk1/2, p70(S6k), and ribosomal protein S6 were highly phosphorylated and 4E-binding protein 1 (4E-BP1) associated with eukaryotic initiation factor (eIF)4E decreased by 30%. These data show that insulin is a significant regulator of intermediates involved in translation initiation in ovine fetal skeletal muscle. Furthermore, the effect of amino acids is dependent on a concomitant increase in fetal insulin concentrations, because amino acid infusion upregulates p70(S6k) and Erk only when amino acid-stimulated increase in insulin occurs.

Letter to the Editor on the Journal Club article by Barker and Traber

Letter to the Editor on the Journal Club article by Barker and Traber

Absence of amino acid-induced glomerular hyperfiltration in dopamine D3 receptor knockout mice

Pharmacological inhibition of receptors of the dopamine D2-like family has been shown to abolish the glomerular hyperfiltration in response to amino acids. To further discriminate between the receptor subtypes within the D2-like family, we investigated the effects of amino acid infusion on renal function in dopamine D3 receptor knockout (-/-) mice. In clearance experiments pentobarbital-anesthetized D3 receptor (-/-) and wild-type (+/+) mice were infused with Ringer solution at baseline, followed by a continuous infusion of mixed amino acids (10%). Baseline glomerular filtration rate (GFR), assessed by renal clearance of [3H]-inulin, was the same in D3 receptor (-/-) mice (0.56+/-0.08 ml/min per g kidney weight) and wild-type animals (0.56+/-0.04 ml/min per g kw). During infusion of amino acids, GFR was significantly elevated by 50% in D3 receptor (+/+) mice. In contrast, this amino acid-induced response of GFR was abolished in D3 receptor (-/-) mice. Baseline urinary water and sodium excretion was not significantly different in both groups of mice. As observed in GFR, these renal excretory parameters were significantly elevated during amino acid infusion in D3 receptor (+/+) but not in D3 receptor (-/-) mice. Time controls, constantly infused with Ringer solution, did not show significant changes in GFR, renal water or sodium excretion during the entire experiment, indicating stable experimental conditions. Taken together, the data underline the involvement of dopamine D2-like receptors in the renal response to amino acid infusion and, in addition, attribute this effect to the dopamine D3 receptor subtype.

Arginine and Mixed Amino Acids Increase Protein Accretion in the Growth-Restricted and Normal Ovine Fetus by Different Mechanisms

Protein metabolism may be perturbed in intrauterine growth restriction (IUGR). Arginine is indispensable for growth and nitrogen balance in young mammals. Fetuses with IUGR therefore may benefit from arginine supplementation. The purpose of this study was to determine 1) the effects of IUGR on protein metabolism in the ovine fetus and 2) the effects of arginine or mixed amino acid (AA) infusion on protein metabolism in these fetuses. Pregnant ewes and their fetuses were catheterized at 110 d gestation and randomly assigned to control or IUGR groups. IUGR was induced by repetitive placental embolization. Parameters of fetal protein metabolism were determined from [ring-(2)H(5)]phenylalanine kinetics at baseline and in response to a 4-h infusion of either arginine or an isonitrogenous AA mixture. There were no differences in protein metabolism between control and IUGR groups either at baseline or in response to arginine or AA treatment. Both arginine and AA infusion increased fetal protein accretion in both groups. Arginine did this by decreasing protein turnover, synthesis, and breakdown. AAs increased protein turnover and synthesis while decreasing protein breakdown. AA infusion resulted in a significantly higher increase in protein accretion than arginine infusion. Thus, in the ovine fetus, placental embolization has no clear effect on protein metabolism. Arginine and AAs both stimulate protein accretion but do so in distinctly different ways. Mixed AA infusion has a greater effect on protein accretion than arginine alone and therefore may be a better strategy for stimulating fetal growth.

Human soleus and vastus lateralis muscle protein metabolism with an amino acid infusion

The calf muscles, compared with the thigh, are less responsive to resistance exercise in ambulatory and bed-rested individuals, apparently due to muscle-specific differences in protein metabolism. We chose to evaluate the efficacy of using amino acids to elevate protein synthesis in the soleus, because amino acids have been shown to have a potent anabolic effect in the vastus lateralis. Mixed muscle protein synthesis in the soleus and vastus lateralis was measured before and after infusion of mixed amino acids in 10 individuals (28 +/- 1 yr). Phosphorylation of ribosomal protein p70 S6 kinase (p70S6K; Thr389) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1; Thr37/46) was also evaluated at rest and after 3 h of amino acid infusion. Basal protein synthesis was similar (P = 0.126), and amino acids stimulated protein synthesis to a similar extent (P = 0.004) in the vastus lateralis (0.043 +/- 0.011%/h) and soleus (0.032 +/- 0.017%/h). Phosphorylation of p70S6K (P = 0.443) and 4E-BP1 (P = 0.192) was not increased in either muscle; however, the soleus contained more total (P = 0.002) and phosphorylated (P = 0.013) 4E-BP1 than the vastus lateralis. These data support the need for further study of amino acid supplementation as a means to compensate for the reduced effectiveness of calf resistance exercise in ambulatory individuals and those exposed to extended periods of unloading. The greater 4E-BP1 in the soleus suggests that there is a muscle-specific distribution of general translational initiation machinery in human skeletal muscle.

Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study.

To test the hypothesis that muscle protein synthesis (MPS) is regulated by the concentration of extracellular amino acids, we investigated the dose-response relationship between the rate of human MPS and the concentrations of blood and intramuscular amino acids. We increased blood mixed amino acid concentrations by up to 240 % above basal levels by infusion of mixed amino acids (Aminosyn 15, 44-261 mg kg-1 h-1) in 21 healthy subjects, (11 men 10 women, aged 29 +/- 2 years) and measured the rate of incorporation of D5-phenylalanine or D3-leucine into muscle protein and blood and intramuscular amino acid concentrations. The relationship between the fold increase in MPS and blood essential amino acid concentration ([EAA], mM) was hyperbolic and fitted the equation MPS = (2.68 x [EAA])/(1.51 + [EAA]) (P < 0.01). The pattern of stimulation of myofibrillar, sarcoplasmic and mitochondrial protein was similar. There was no clear relationship between the rate of MPS and the concentration of intramuscular EAAs; indeed, when MPS was increasing most rapidly, the concentration of intramuscular EAAs was below basal levels. We conclude that the rates of synthesis of all classes of muscle proteins are acutely regulated by the blood [EAA] over their normal diurnal range, but become saturated at high concentrations. We propose that the stimulation of protein synthesis depends on the sensing of the concentration of extracellular, rather than intramuscular EAAs.

Renal effects of amino acid infusion in cardiac surgery

Objective: To evaluate effects of amino acids on renal function and oxygen consumption and the role of individual amino acids on renal blood flow (RBF) changes. Design: Prospective, randomized, controlled study. Setting: Operating room in cardiothoracic surgery department, university hospital. Participants: Twenty-two male patients submitted to elective first-time coronary artery bypass surgery. Interventions: A catheter was placed in the left renal vein for thermodilution RBF measurements and blood sampling. In 11 patients, a balanced mixed amino acid infusion was infused (200 mL/hr) for 30 minutes immediately after the operation. Measurements and Main Results: RBF and glomerular filtration rate increased during amino acid infusion compared with the control group. Renal oxygen consumption in-creased in the amino acid group and correlated with the increase in RBF ( r = 0.70, p < 0.001). Amino acid infusion induced two- to fourfold increases in plasma concentrations of individual amino acid concentrations and promoted renal extraction of aspartate, glutamate, glycine, and histidine. No correlation was observed between arterial concentration or uptake of individual amino acids and RBF. Conclusions: The increase in RBF from a mixed amino acid infusion was associated with increased glomerular filtration rate and renal consumption of oxygen. Changes in RBF of a mixed amino acid infusion could not be linked to plasma level or renal uptake of any individual amino acids.

Insulin and amino acid infusion after cardiac operations: Effects on systemic and renal perfusion

Objective: The purpose of this study was to answer two questions: (1) Does a mixed amino acid infusion enhance systemic and renal perfusion in the early postoperative period after heart operations? (2) Does the addition of insulin (glucose-insulin-potassium solution) provide additional effects to those of an amino acid infusion? Methods: Thirty-three male patients undergoing coronary artery bypass grafting (mean age 65.9 ± 1.2 years) were included in a prospective, controlled, randomized study. Eleven patients (AA group) received infusion of mixed amino acids (11.4 gm), 11 patients (AA + GIK group) received infusion of mixed amino acids (11.4 gm) and insulin solution (225 IU insulin, glucose with glucose clamp technique, and potassium), and 11 patients served as control subjects. Results: Amino acid infusion alone had no effect on systemic vascular resistance or cardiac index but increased renal blood flow 51% ± 11% (from 114 ± 13 to 172 ± 24 ml · min –1 · m –2 in one kidney, p < 0.05 vs the control group). Insulin solution in addition to amino acid infusion reduced systemic vascular resistance 24% ± 3% (from 1280 ± 85 to 960 ± 57 dyn · sec · cm –5, p < 0.05 vs the control and AA groups) and increased cardiac index 13% ± 3% (from 2.3 ± 0.2 to 2.6 ± 0.2 L · min –1 · m –2, p < 0.05 vs the control and AA groups). Insulin had no significant additive effect on renal blood flow. Conclusions: Our data imply that (1) infusion of mixed amino acids enhances renal blood flow after cardiac operations but has no effect on systemic perfusion and (2) the addition of insulin solution improves systemic perfusion. The combined treatment may potentially reduce the risk of renal hypoperfusion injury in the postoperative period after coronary artery bypass grafting. (J Thorac Cardiovasc Surg 1997;113:594-602)

Renal hemodynamics in renal transplant recipients. The role of reduced kidney mass and cyclosporine administration.

It has been hypothesized that both the cyclosporine (CsA) treatment and the reduction of renal mass may affect the renal hemodynamic regulation in kidney transplant recipients. To address this question, we evaluated the renal hemodynamic response to hyperaminoacidemia (i.v. mixed amino acid infusion 3.3 mg/kg/minute for 150 minutes) in four study groups: (1) 16 renal transplant recipients (Tx), (2) 6 uninephrectomized (Nx) subjects, (3) 7 subjects treated with CsA for chronic uveitis (CsA), and (4) 9 normal controls (NC). In response to amino acid administration (AA), glomerular filtration rate (GFR) rose significantly in NC subjects (80 +/- 6 vs. 91 +/- 6 ml/minute; P<0.01) and Nx patients (57 +/- 3 vs. 68 +/- 7 ml/minute; P<0.01) and failed to increase in Tx recipients (39 +/- 3 vs. 37 +/- 3 ml/minute) and CsA-treated patients (58 +/- 3 vs. 53 +/- 4 ml/minute). Renal plasma flow (RPF) did not change in Tx recipients (243 +/- 27 vs. 235 +/- 25 ml/minute) but rose significantly in all other groups (257 +/- 17 vs. 344 +/- 33 in NX, 364 +/- 6l vs. 441 +/- 55 in CsA, 412 +/- 49 vs. 472 +/- 72 ml/min in NC subjects; P<0.05 vs. basal). Basal renal vascular resistances were significantly higher in Tx (0.29 +/- 0.04 mmHg/mlxmin; P<0.01 vs. all other groups) than in Nx (0.21 +/- 0.01 mmHg/mlxmin), CsA (0.23 +/- 0.04 mmHg/mlxmin) (both P<0.01 vs. NC subjects), and NC subjects (0.13 +/- 0.02 mmHg/mlxmin). Renal vascular resistance failed to decline in Tx (0.31 +/- 0.04 mmHg/mlxmin) during AA infusion but declined significantly in all other groups. In Tx, basal GFR was positively correlated to renal allograft volume (r=0.547, P<0.03); however, no relationship was found between the latter and basal RPF or the AA induced changes in GFR. In summary, the present study demonstrates that in kidney transplant recipients and in CsA-treated subjects, the renal functional reserve to hyperaminoacidemia is impaired. This is at variance to what is observed in normal controls and uninephrectomized subjects. In renal transplant recipients, basal but not amino acid stimulated GFR correlates with renal allograft volume. We conclude that basal GFR is related to renal volume in Tx and that the response to hyperaminoacidemia seems to be affected by chronic CsA administration.

Glucose but Not a Mixed Amino Acid Infusion Regulates Plasma Insulin-Like Growth Factor-I Concentrations in Fetal Sheep

The influence of fetal glucose and amino acid supply on the regulation of fetal plasma IGF-I levels was investigated in fetuses from starved ewes. Paired maternal and fetal blood samples were taken during an initial 2-d control period, after 48 h of maternal starvation, during a 24-h fetal infusion of glucose (n = 6) or an amino acid mixture (Synthamin 17, n = 5) with continued starvation, and after 48 h of maternal refeeding. After 48 h of starvation, maternal and fetal plasma IGF-I, insulin, and blood glucose fell significantly in both groups compared with control values (IGF-I for glucose group: maternal, -18.53 +/- 6.60; fetal, -5.23 +/- 1.81 nmol/L; amino acid group: maternal, -18.2 +/- 6.97, fetal, -5.12 +/- 1.61 nmol/L; both p < 0.05). Fetal glucose but not mixed amino acid infusion raised fetal plasma IGF-I, insulin, and blood glucose to near control values (glucose group fetal IGF-I, -1.77 +/- 1.98; amino acid group, -5.93 +/- 2.22 nmol/L; both p < 0.05). Maternal plasma IGF-I remained depressed during glucose infusion (-16.33 +/- 8.32 nmol/L), but continued to fall in the amino acid group (-21.41 +/- 8.20 nmol/L, p < 0.05). After 48 h of maternal refeeding, all values had returned to near control values for both groups (glucose group IGF-I: maternal, -5.2 +/- 3.86; fetal, 0.01 +/- 2.2; nmol/L; amino acid group: maternal, -11.66 +/- 3.2; fetal, -0.70 +/- 2.61 nmol/L). We conclude that in the ovine fetus glucose may have a more important role than amino acids in the regulation of fetal plasma IGF-I.

Response of muscle protein synthesis to parenteral administration of amino acid mixtures in growing rats.

Previous studies have shown that 1-hour infusions of neither glucose nor an amino acid mixture alone stimulates muscle protein synthesis in postabsorptive rats (Biosci Rep 1986;6:177-183). We have therefore investigated whether longer periods of infusion are required to initiate responses. The effects of intravenous infusions of various substrates for 6 hours on rates of skeletal muscle protein synthesis in fasted rats were studied. Fractional rates of protein synthesis (ks, the percent of muscle protein renewed each day) and the ribonucleic acid activity (kRNA, the amount of protein synthesis per unit of ribonucleic acid [milligrams of protein per day per milligram of ribonucleic acid]) were measured with a flooding dose of L-[4-3H]phenylalanine. Infusion of mixed amino acids (15 mg of nitrogen per hour per rat) increased ks and kRNA by approximately 50% in comparison with saline-infused rats, but rates of muscle protein synthesis remained below those observed in fed animals. Doubling the infusion rate of amino acids to 30 mg of nitrogen per hour per rat had no additional effect on ks and kRNA, and addition of glucose (0.3 g/h per rat) or glucose plus lipid also had no significant additive effect. However, infusion of glucose alone (0.3 g/h per rat) increased ks and kRNA by 25% when compared with saline-infused controls. Infusion of the branched-chain amino acids in the same amounts as present in the amino acid mixture increased ks and kRNA by 20% compared with saline-infused controls, but the rate remained significantly lower than that obtained by infusion of mixed amino acids.

Stimulation of protein synthesis in pig skeletal muscle by infusion of amino acids during constant insulin availability.

Incorporation of L-[1-13C]leucine into muscle protein and leg exchange of L-[15N]phenylalanine were used to assess the effects over 240 min of amino acid supply on leg protein turnover in anesthetized, overnight-fasted (Landrace x Great White) female pigs. In all pigs, plasma insulin and glucagon stability was ensured by infusion of somatostatin (8 micrograms.kg-1.h-1), insulin (6 mU.kg-1.h-1), and glucagon (72 ng.kg-1.h-1). Mixed amino acid infusion (260 mg.kg-1.h-1) caused a 2- to 2.5-fold elevation of arterial plasma phenylalanine and leucine; in a control group (no amino acid infusion), an increase in phenylalanine and leucine concentration was observed as a result of the hormone clamp. Plasma insulin and glucagon concentrations were steady and not significantly different between control and amino acid-infused groups during the final 240 min, but plasma glucose fell (P less than 0.05) in both groups (4.57 +/- 0.17 to 3.15 +/- 0.73 mM). Muscle protein synthetic rate (estimated from the change in L-[1-13C]leucine incorporation compared with labeling of [13C]leucyl-tRNA) was greater in amino acid-infused (0.076%/h) than in control (0.053%/h) pigs. In the control group, leg amino acid balance was negative (Phe alone, -10.2 +/- 9.4 nmol Phe.100 g-1.min-1; total amino acids, -0.27 +/- 1.04 micrograms amino N.100 g-1.min-1), but during amino acid infusion, balance was positive (Phe alone, +33.6 +/- 8.8 nmol Phe.100 g-1.min-1; total amino acids, +58.2 +/- 4.9 micrograms amino N.100 g-1.min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated Dietary Protein Intake Impairs the Renal Hemodynamic Response to Hyperaminoacidemia in Patients with Primary Glomerular Diseases

We have evaluated the renal hemodynamic response to a mixed amino acid infusion in 7 control subjects and in 8 patients with primary glomerulonephritis (GN). In order to evaluate the role of dietary protein intake in this response, GN patients were maintained for 3 weeks on two separate dietary regimens providing 130 +/- 5 g of protein/day (study 1) and 60 +/- 3 g of protein/day (study 2), respectively. Normal subjects were studied while consuming a free diet. In GN patients, following the reduction in dietary protein intake basal RPF and GFR decreased from 589 +/- 109 to 422 +/- 81 ml/1.73 m2/min (p less than 0.01, vs. study 1) and from 75 +/- 7 to 70 +/- 8 ml/1.73 m2/min (p = NS). Filtration fraction rose from 0.14 +/- 0.02 to 0.19 +/- 0.03 (p less than 0.05). In study 1, during amino acid infusion GFR and RPF did not change significantly from baseline (75 +/- 7 vs. 66 +/- 8 ml/1.73 m2/min at 180 min and 589 +/- 109 vs. 567 +/- 102 ml/1.73 m2/min, respectively). These results are at variance with data obtained in normal controls in whom both GFR and RPF rose significantly following hyperaminoacidemia. In contrast, when dietary protein intake was reduced, a normal renal hemodynamic response to amino acid infusion was restored (GFR went from 70 +/- 8 to 90 +/- 18 ml/1.73 m2/min and RPF from 422 +/- 81 to 517 +/- 90 ml/1.73 m2/min, both p less than 0.05 vs. basal), both absolute and percentage increases were similar to what was observed in controls.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of amino acid infusion on leg protein turnover assessed by L‐[15N]phenylalanine and L‐[1‐13C]leucine exchange

A stable isotope technique depending on the use of [15N]phenylalanine and [1-13C]leucine to assess exchange was utilized to measure the components of protein turnover of the human leg and the effects of amino acid infusion. Eight healthy subjects (28.5 +/- 2.5 years) were studied when post-absorptive in the basal state and again during infusion of a mixed amino acid solution (55 g l-1, 1.52 ml kg-1 h-1). During the basal period leucine oxidation by the leg was 4.4 +/- 2.0 nmol 100 g-1 min-1 and this increased threefold during amino acid infusion (13.6 +/- 3.1 nmol 100 g-1 min-1, mean +/- SEM, P = 0.003). Amino acid infusion abolished the net negative balance between incorporation of leucine into, and release from, protein (basal, -31.8 +/- 5.8; during infusion, +3.1 +/- 7.1 nmol 100 g-1 P = 0.001). Phenylalanine exchange showed a similar pattern (basal, -13.7 +/- 1.8; during infusion, -0.8 +/- 3.0 nmol 100 g-1 min-1, P = 0.003). Basal entry of leucine into leg protein (i.e. protein synthesis) was 70.0 +/- 10.8 nmol 100 g-1 min-1 and this increased during amino acid infusion to 87.3 +/- 14.1 nmol 100 g-1 min-1 (P = 0.11). Phenylalanine entry to protein also increased with amino acid infusion (29.1 +/- 4.5 vs. 38.3 +/- 5.8 nmol 100 g-1 min-1, P = 0.09). Release from protein of leucine (101.8 +/- 9.1 vs. 84.2 +/- 9.1 nmol 100 g-1 min-1, P = 0.21) and of phenylalanine (42.8 +/- 4.2 vs. 39.1 +/- 4.2 nmol 100 g-1 min-1, P = 0.50) was unchanged by amino acid infusion. The results suggest that, in the post-absorptive state in man, infusion of mixed amino acids, without additional energy substrates; reverses negative amino acid balance by a mechanism which includes stimulation of muscle protein synthesis but which does not alter protein breakdown. Interpretation of the results obtained concurrently on whole-body protein turnover suggests that the increase in muscle protein synthesis contributes substantially to the whole-body increase, but the fall in whole-body breakdown with exogenous amino acids is independent of changes in muscle.