Low dose recombinant human insulin-like growth factor-I fails to affect protein anabolism but inhibits islet cell secretion in humans.

The in vivo effects of recombinant human insulin-like growth factor-I (rhIGF-I) on whole body protein metabolism were studied to ascertain whether rhIGF-I has comparable effects as those reported with rhGH use in humans. The doses of rhIGF-I chosen achieved similar plasma IGF-I concentrations as those achieved after 7 days of rhGH injections. Eight normal volunteers were studied using [1-13C]- and [1-14C]leucine tracers, before, 4 h, and 28 h after a continuous infusion of rhIGF-I at 5 micrograms kg-1 h-1 (n = 6) and 10 micrograms kg-1 h-1 (n = 2). Two additional subjects were studied in a protein catabolic state after 7 days of high dose (0.8 mg kg-1 day-1) glucocorticosteroid administration. Plasma concentrations of rhIGF-I were similar using either 5 or 10 micrograms kg-1 h-1 and increased to values approximately 300% above baseline by 28 h of infusion. No decrease in the plasma glucose concentration was observed during the 28-h infusion; however, plasma insulin, C-peptide, and glucagon concentrations significantly decreased, whereas plasma free fatty acids were not affected. No changes were observed in the rate of proteolysis (as estimated by the rate of leucine appearance), the rate of leucine oxidation, or the rate of protein synthesis in the absence or presence of glucocorticosteroid treatment. Plasma concentrations of insulin-like growth factor binding protein-3 did not change during the rhIGF-I infusion whereas they increased 50% in subjects who received rhGH, and in whom rhGH caused a potent protein anabolic effect. These results suggest that rhIGF-I may have a somatostatin-like effect. In addition, we found that rhIGF-I infusion is insufficient to promote protein anabolism. This may be due to the failure of rhIGF-I alone to induce a pivotal GH-dependent cofactor(s) necessary for IGF-I to elicit an anabolic effect on protein metabolism in humans.     

High dose growth hormone exerts an anabolic effect at rest and during exercise in endurance-trained athletes

The anabolic actions of GH in GH-deficient adults and children are well documented. Replacement with GH in such individuals promotes protein synthesis and reduces irreversible loss of protein through oxidation. Although GH is known to be self-administered by athletes, its protein metabolic effects in this context are unknown. This study was designed to determine whether 4 wk of high dose recombinant human GH (r-hGH) administration altered whole body leucine kinetics in endurance-trained athletes at rest and during and after 30 min of exercise at 60% of maximal oxygen uptake. Eleven endurance-trained male athletes were studied, six randomized to receive r-hGH (0.067 mg/kg.d), and five to receive placebo. Whole body leucine turnover was measured at rest and during and after exercise, using a 5-h primed constant infusion of 1-[(13)C]leucine, from which rates of leucine appearance (an index of protein breakdown), leucine oxidation, and nonoxidative leucine disposal (an index of protein synthesis) were estimated. Under resting conditions, r-hGH administration increased rate of leucine appearance and nonoxidative leucine disposal, and reduced leucine oxidation (P < 0.01). This effect was apparent after 1 wk, and was accentuated after 4 wk, of r-hGH administration (P < 0.05). During and after exercise, GH attenuated the exercise-induced increase in leucine oxidation (P < 0.05). There were no changes observed in placebo-treated subjects compared with the baseline study. We conclude that GH administration to endurance-trained male athletes has a net anabolic effect on whole body protein metabolism at rest and during and after exercise.     

Growth hormone blunts protein oxidation and promotes protein turnover to a similar extent in abdominally obese and normal-weight women

Abdominally obese individuals have reduced 24-h plasma GH concentrations. Their normal plasma IGF-I levels may reflect GH hypersensitivity. Alternatively, obesity-associated hyposomatotropism may cause less biological effect in target tissues. We therefore determined whole-body responsiveness to the anabolic effects of GH in abdominally obese (OB) and normal weight (NW) premenopausal women. A 1-h iv infusion of GH or placebo was randomly administered to six NW (body mass index, 21.1 +/- 1.9 kg/m(2)) and six OB (body mass index, 35.5 +/- 1.5 kg/m(2)) women in a cross-over design. Endogenous insulin, glucagon and GH secretion was suppressed by infusion of somatostatin. Whole-body protein turnover was measured using a 10-h infusion of [(13)C]-leucine. GH administration induced a similar plasma GH peak in NW and OB women (49.8 +/- 10.4 vs. 45.1 +/- 5.6 mU/liter). GH, compared with placebo infusion, increased nonoxidative leucine disposal, P < 0.0001) and endogenous leucine appearance (R(a), P = 0.0004) but decreased leucine oxidation (P = 0.0051). All changes were similar in both groups. Accordingly, whole-body GH responsiveness, defined as the maximum response of nonoxidative leucine disposal, leucine R(a), and oxidation per unit of GH, was not different in OB and NW women (0.25 +/- 0.18 vs. 0.19 +/- 0.17 micro mol/kg.h, 0.21 +/- 0.23 vs. 0.13 +/- 0.17 micro mol/kg.h, and -0.10 +/- 0.08 vs. -0.08 +/- 0.05 micro mol/kg.h, respectively). These results indicated that whole-body tissue responsiveness to the net anabolic effect of GH is similar in OB and NW women. Hence, we inferred that hyposomatotropism may promote amino acid oxidation and blunt protein turnover in abdominal obesity. However, hyposomatotropism cannot account for all anomalous features of protein metabolism in abdominally obese humans.     

Recombinant human growth hormone and recombinant human insulin-like growth factor I diminish the catabolic effects of hypogonadism in man: metabolic and molecular effects

Severe gonadal androgen deficiency can have profound catabolic effects in man. Hypogonadal men develop a loss of lean body mass, increased adiposity, and decreased muscle strength despite normal GH and insulin-like growth factor I (IGF-I) concentrations. We designed these studies to investigate whether GH or IGF-I administration to male subjects with profound hypogonadism can diminish or abolish the catabolic effects of testosterone deficiency. Moreover, we also examined the nature of the interactions among GH, IGF-I, and androgens in specific genes of the im system. A group of 13 healthy subjects (mean age, 22 +/- 1 yr) was studied at baseline (D1) and 10 weeks after being made hypogonadal using a GnRH analog (GnRHa; D2). At 6 weeks from baseline they were started on either recombinant human (rh) IGF-I (60 microg/kg, sc, twice daily) or rhGH (12.5 microg/kg, sc, daily) for 4 weeks. On each study day subjects had infusions of L-[(13)C]leucine; indirect calorimetry; isokinetic dynamometry of the knee extensors; determination of body composition (dual energy x-ray absortiometry) and hormone and growth factor concentrations, as well as percutaneous muscle biopsies. Their data were compared with those of previously studied male subjects who received only GNRHA: Administration of rhIGF-I and rhGH to the hypogonadal men had similar effects on whole body metabolism, with maintenance of protein synthesis rates, fat oxidation rates, and fat-free mass compared with the eugonadal state, preventing the decline observed with hypogonadism alone. This was further amplified by the molecular assessment of important genes in muscle function. During rhIGF-I treatment, im expression of IGF-I declined, and IGF-binding protein-4 increased, similar to the changes during GnRHa alone. However, rhGH administration was associated with a marked increase in IGF-I and androgen receptor messenger ribonucleic acid concentrations in skeletal muscle with a reciprocal decline in IGF-binding protein-4 expression in the hypogonadal men. The gene expression for myostatin did not change. These effects were accompanied by a much greater increase in plasma IGF-I concentrations after rhIGF-I (225 +/- 32 vs. 768 +/- 117 microg/L) compared with the concentrations achieved during rhGH (217 +/- 20 vs. 450 +/- 19 microg/L). We conclude that 1) rhGH and rhIGF-I both may be beneficial in preserving lean body mass and sustaining rates of protein synthesis during states of severe androgen deficiency in man; 2) GH may affect the im IGF system via an a paracrine, local production of IGF-I; 3) androgens may be necessary for the full anabolic effect of GH/IGF-I in man. These hormones, particularly GH, may play a role in the treatment of hypogonadal men rendered hypogonadal pharmacologically or those unable to take full testosterone replacement. The latter requires further study.     

Short- and long-term effects of growth hormone (GH) replacement on protein metabolism in GH-deficient adults

Reduced fat-free mass (FFM) in GH-deficient (GHD) adults is improved by GH replacement, but the protein metabolic changes are unclear. Using iv [(2)H(3)]leucine and oral l-[(13)C(1)]leucine infusions and dual emission x-ray absorptiometry, we compared leucine kinetics and body composition in eight GHD adults and eight healthy controls in the fasted and fed states, before and after 2 wk and 6 months of GH replacement. Leucine kinetics were not different between pretreatment GHD subjects and controls. After 2 wk of GH treatment, leucine oxidation decreased in the GHD subjects compared with baseline values [fasted, 41 +/- 6 vs. 30 +/- 5 micromol/kg FFM.h (P < 0.01); fed, 49 +/- 3 vs. 41 +/- 3.6 micromol/kg FFM.h (P < 0.05)], leucine balance improved [fasted, -14 +/- 4 vs. -3.5 +/- 3 micromol/kg FFM.h (P < 0.01); fed, 65 +/- 10 vs. 72 +/- 7 micromol/kg FFM.h (P = 0.07)], and protein synthesis increased [fasted, 116 +/- 5 vs. 131 +/- 6 micromol/kg FFM.h (P < 0.05); fed, 103 +/- 6 vs. 116 +/- 6 micromol/kg FFM.h (P < 0.05)]. After 6 months of GH treatment, these changes were not maintained in the fed state. The five GHD subjects with decreased FFM at baseline showed a significant increase after 6 months of GH treatment (P < 0.05). GH replacement in GHD acutely improves protein balance by stimulating synthesis and inhibiting catabolism. After 6 months, protein kinetics reached a new homeostasis to maintain the net gain in FFM.     

Administration of recombinant human growth hormone on alternate days is sufficient to increase whole body protein synthesis and lipolysis in growth hormone deficient adults

OBJECTIVE: At present, the duration of the effect of recombinant human growth hormone (rhGH) on the rates of protein synthesis and lipolysis in GH deficient (GHD) adults is unknown. This study was designed to establish the frequency of rhGH administration necessary to provide the beneficial metabolic effects of the hormone in GHD adults. DESIGN AND PATIENTS: Two different studies (A and B) were performed in two groups of five GHD men. In study A, whole body protein and lipid kinetics was determined in the basal state (Bas), 12 (GH12h) and 36 (GH36h) h after the last of seven injections of rhGH (3.3 microg/kg), given at bedtime on alternate days. In study B, the same parameters were determined in the basal state (Bas), 60 (GH60h) and 84 (GH84h) h after the last of seven injections of rhGH (3.3 microg/kg), given at bedtime at 3 day intervals. MEASUREMENTS: The rates of protein metabolism were estimated by infusing [1-13C]leucine, and those of lipolysis by infusing [1,1,2,3, 3-D5]glycerol. RESULTS: Leucine oxidation decreased (P < 0.01) by approximately 30% after GH12h and GH36h but did not change after GH60h and GH84h. Non-oxidative leucine disposal increased after GH12h and GH36h by approximately 13% (P < 0.05) whereas it did not change after GH60h and GH84h. Glycerol appearance increased (P < 0. 01) by approximately 45% after GH12h and GH36h but did not change after GH60h and GH84h. CONCLUSIONS: The effects on protein and lipid metabolism following the injection of rhGH last longer than 36 and less than 60 h. In fact, rhGH administration on alternate days induced a sustained increase in the rates of protein synthesis and lipolysis of GHD adults, whereas a longer interval of administration (3 days) had no effect by 60 h.     

Insulin-like growth factor I and growth hormone (GH) treatment in GH-deficient humans: differential effects on protein, glucose, lipid, and calcium metabolism

We examined the effects of recombinant human (rh) insulin-like growth factor I (IGF-I) vs. rhGH in a variety of metabolic paths in a group of eight severely GH-deficient young adults using an array of contemporary tools. Protein, glucose, and calcium metabolism were studied using stable labeled tracer infusions of L-[1-13C]leucine, [6,6-2H2]glucose, and 42Ca and 44Ca; substrate oxidation rates were assessed using indirect calorimetry; muscle strength was determined by isokinetic and isometric dynamometry of the anterior quadriceps, as well as growth factors, hormones, glucose, and lipid concentrations in plasma before and after 8 weeks of rhIGF-I (60 microg/kg, sc, twice daily), followed by 4 weeks of washout, then 8 weeks ofrhGH (12.5 microg/kg-day, sc); the treatment order was randomized. In the doses administered, rhIGF-I and rhGH both increased fat-free mass and decreased the percent fat mass, with a more robust decrease in the percent fat mass after rhGH; both were associated with an increase in whole body protein synthesis rates and a decrease in protein oxidation. Neither hormone affected isokinetic or isometric measures of skeletal muscle strength. However, rhGH was more potent than rhIGF-I at increasing lipid oxidation rates and improving plasma lipid profiles. Both hormones increased hepatic glucose output, but rhGH treatment was also associated with decreased carbohydrate oxidation and increased glucose and insulin concentrations, indicating subtle insulin resistance. Neither hormone significantly affected bone calcium fluxes, supporting the concept that these hormones, by themselves, are not pivotal in bone calcium metabolism. In conclusion, rhIGF-I and rhGH share common effects on protein, muscle, and calcium metabolism, yet have divergent effects on lipid and carbohydrate metabolism in the GH-deficient state. These differences may allow for better selection of treatment modalities depending on the choice of desired effects in hypopituitarism.     

The effects of growth hormone and/or testosterone on whole body protein kinetics and skeletal muscle gene expression in healthy elderly men: a randomized controlled trial

CONTEXT: Alterations of protein turnover may contribute to the progressive decline of muscle mass with aging. OBJECTIVE: Our objective was to examine the effects of near-physiological recombinant human GH and/or testosterone (T) administration to older men on whole body protein kinetics and muscle gene expression. DESIGN, SETTINGS, AND PARTICIPANTS: A 6-month randomized, double-blind, placebo-controlled trial in 21 healthy elderly men aged 65-75 yr, was performed. Participants were randomized to receive placebo GH and placebo T, rhGH and placebo T (GH), T and placebo GH (T), or rhGH and T (GHT). INTERVENTIONS: The leucine rate of appearance (index of proteolysis), nonoxidative leucine disposal rate (an index of protein synthesis), and leucine oxidation rate were measured with an infusion of l-[1-(13)C] leucine. Muscle biopsies for the measurement of gene expression were performed. Body composition and aerobic capacity (maximal oxygen capacity) were measured. RESULTS: Serum IGF-I levels increased significantly with GH and GHT (P < 0.001) compared with placebo. T increased significantly only in the T group (P = 0.028). Leucine rate of appearance and nonoxidative leucine disposal rate increased with GH (P = 0.015, P = 0.019) and GHT (P = 0.017, P = 0.02), but leucine oxidation did not change significantly in any treatment group. Midthigh muscle mass and maximal oxygen capacity increased (P < 0.04) with GHT only. Expression of muscle function genes did not change significantly, but the within-group comparisons revealed a significant increase of androgen receptor expression in the GHT group (P = 0.001). CONCLUSION: This study showed that 6-month treatment with low-dose GH alone or with T in healthy elderly men produces comparable increments in whole body protein turnover and protein synthesis.     

生长激素对于GnRHa治疗中生长过度减速的特发性中枢性性早熟女孩的远期疗效观察

目的 观察重组人生长激素(rhGH)对于促性腺激素释放激素类似物(GnRHa)治疗中生长过度减速的特发性中枢性性早熟(ICPP)女孩的最终成年身高(FAH)的影响.方法 49例ICPP女孩接受GnRHa治疗,当身高增长速度减慢至4 cm/年以下时,其中26例联用rhGH治疗为联合治疗组,23例拒绝加用rhGH但继续使用GnRHa为单用组.比较2组治疗前后的预测成年身高(PAH)和FAH.结果 联合治疗组联用rhGH前半年,身高增长速度均小于4 cm/年[(3.2±1.0)cm/年],PAH无明显改善[联用rhGH前半年和rhGH开始时PAH分别为(152.5±4.0)cm和(152.6±3.7)cm];联用rhGH(11.4±5.4)个月后,身高增长速度增加至(6.7±2.0)cm/年,PAH增加至治疗结束时的(157.1±4.7)cm(均P<0.01);FAH[(157.5±4.5)cm]显著高于GnRHa开始时的PAH[(148.1±4.6)cm]和遗传靶身高[(154.4±4.6)cm,均P<0.01].单用组治疗结束时的PAH[(153.9±6.3)cm]较拒绝联用rhGH时的PAH[(153.1±6.2)cm]无差异;FAH[(154.7±5.5)cm]高于治疗开始时的PAH[(150.3±6.0)cm,P<0.01],但与遗传靶身高[(155.6±4.3)cm]无差异.结论 联用rhGH能够显著加快GnRHa治疗中生长过度减速的ICPP女孩的身高增长速度,进一步改善PAH和FAH.

Abstract：

Objective To evaluate the long-term final adult height outcome of combined treatment with gonadotropin-releasing hormone analogue(GnRHa)and recombinant human growth hormone(rhGH)in girls with idiopathic central precocious puberty(ICPP).Methods Out of 49 sirls with ICPP[treated with GnRHa at a dose of 60-80 μg/kg every 4 weeks for at least 6 months,whose height velocity fell below 4 cm/year and showed no improvement of predicted adult height(PAH)in 6 months],26 received(rhGH-combined group),in addition to chronological age,and duration of GnRHa treatment,who showed the same growth pattern but refused rhGH treatment,served to evaluate the efficacy of rhGH in addition.At the conclusion of the smdy,all the girls had been followed up for(3.3±1.9)years,and(3.2±0.9)years in rhGH-combined group and control group,respectively;and had achieved adult heisht.To compare the PAH with the final adult height(FAH)before and after treatment in the two groups.Results During rhGH treatment, height velocity of the rhGH-combined girls increased significantly[(6.7±2.0 vs <4)cm/year baseline],RhGH-combined gids showed an adult height far higher than pretreatment PAH [(157.5±4.5 vs 148.1±4.6)cm,P<0.01],and target height[(154.4±4.6)cm] was,significantly excceded.The control group reached an adult heisht also significandy higher than pretreatment PAH[(154.7±5.5vs 150.3±6.0)cm,P<0.01],while target height[(155.6±4.3)cm]was just reached but not significantlyexcceded.The gain in height obtained,calculated between pretreatment PAH and final heisat,(9.4±4.9)cm in rhGH-combined group was much more than that(4.3±4.2)cm in the control group(P<0.01).Conclusion RhGH may accelerate the linear growth and improve adult height of GnRHa-treated ICPP girls.     

Anabolic effect of biosynthetic growth hormone in cystic fibrosis patients

The purpose of this study was to determine whether GH treatment of cystic fibrosis (CF) patients can result in an anabolic effect, i.e., increased weight gain, improved growth rate, nitrogen retention, and improved pulmonary function. Nine prepubertal endocrinologically normal CF patients (3 girls, 6 boys; chronological age (CA) 5.5–9.8 years, and bone age (BA) 4.5–9.0 years), received recombinant human growth hormone (rhGH) 0.3 mg/kg/week subcutaneously for a period of 12 months (N = 8) or 9 months (N = 1). Normal glucose tolerance was determined before treatment. Pulmonary function studies and anthropometric measurements were done every 3 months. Thyroid status, somatomedin C (SmC), BA, and routine chemistries were evaluated every 6 months. The pretreatment growth velocity averaged 5.7 ± 0.3 (SE) cm/year and significantly increased to 7.8 ± 0.4 (SE) cm/year during therapy, (P < 0.01). Standard deviation scores (SDS) for height significantly increased during rhGH therapy as compared with pretreatment, (P < 0.05). Weight of the patients during rhGH therapy did not significantly change during or after rhGH therapy. After therapy, all patients showed a significant increase in arm muscle area (AMA) and a significant decrement in arm fat area (AFA) (P < 0.01). Net nitrogen anabolism was negative in all subjects before therapy but became more positive in five patients during rhGH therapy. Three patients achieved positive nitrogen retention. SmC values significantly increased from a mean value of 0.62 ± 0.1 (SE) U/ml to 1.6 ± 0.6 (SE) U/ml after therapy. BA advanced 1.0 ± 0.1 SE per year after treatment. Of the seven patients able to perform adequate pulmonary function testing, improvement occurred in FVC, FEV₁₀, and PEFR in 5, 5, and 4 patients, respectively, but these changes did not reach statistical significance. We conclude that biosynthetic rhGH therapy had a significant anabolic effect in CF patients as shown by increased growth velocity, SmC values, increased protein and decreased fat stores, and a positive or less negative net nitrogen retention in five of the patients. Pediatr Pulmonol. 1996;22:90–95. © 1996 Wiley-Liss, Inc.     

Dietary carbohydrate deprivation increases 24-hour nitrogen excretion without affecting postabsorptive hepatic or whole body protein metabolism in healthy men

Because insulin is an important regulator of protein metabolism, we hypothesized that physiological modulation of insulin secretion, by means of extreme variations in dietary carbohydrate content, affects postabsorptive protein metabolism. Therefore, we studied the effects of three isocaloric diets with identical protein content and low-carbohydrate/high-fat (2% and 83% of total energy, respectively), intermediate-carbohydrate/intermediate-fat (44% and 41% of total energy, respectively), and high-carbohydrate/low-fat (85% and 0% of total energy, respectively) content in six healthy men. Whole body protein metabolism was assessed by 24-h urinary nitrogen excretion, postabsorptive leucine kinetics, and fibrinogen and albumin synthesis by infusion of [1-(13)C]leucine and [1-(13)C]valine. The low-carbohydrate/high-fat diet resulted in lower absorptive and postabsorptive plasma insulin concentrations, and higher rates of nitrogen excretion compared with the other two diets: 15.3 +/- 0.9 vs. 12.1 +/- 1.1 (P = 0.03) and 10.8 +/- 0.5 g/24 h (P = 0.005), respectively. Postabsorptive rates of appearance of leucine and of leucine oxidation were not different among the three diets. In addition, dietary carbohydrate content did not affect the synthesis rates of fibrinogen and albumin. In conclusion, eucaloric carbohydrate deprivation increases 24-h nitrogen loss but does not affect postabsorptive protein metabolism at the hepatic and whole body level. By deduction, dietary carbohydrate is required for an optimal regulation of absorptive, rather than postabsorptive, protein metabolism.     

Relationship of plasma leucine and α-ketoisocaproate during a L-[1-13C]leucine infusion in man: A method for measuring human intracellular leucine tracer enrichment

The keto analog of leucine, α-ketoisocaproate (KIC), is formed intracellularly from leucine and is released, in part, into the systemic circulation. Therefore, KIC can be used to estimate intracellular leucine tracer enrichment in man during labeled-leucine tracer experiments without requiring tissue biopsy samples. This approach was studied in young, healthy, male adults maintained on different dietary protein intakes from generous (1.5 g kg^?1d^?1) to deficient (0.0 g kg^?1d^?1) for 5–7 day periods. At the end of each dietary period, the volunteers were given a primed, continuous infusion of L-[1-¹³C]leucine either after an overnight fast (postabsorptive state) or while being fed hourly aliquots of the same diet. The plasma concentrations of all 3 branched-chain amino and keto acid pairs were measured from early morning blood samples taken from 4 subjects at 4 different levels of protein intake. Leucine concentration showed a weak correlation, and valine concentration showed a strong correlation with protein intake; isoleucine and the 3 keto acids did not. However, each branched-chain amino acid concentration was strongly correlated with its corresponding keto acid concentration. In plasma samples obtained during the L-[1-¹³C]leucine infusions, the ratio of [1-¹³C]KIC to [1-¹³C]leucine enrichment ratio remained relatively constant (77 ± 1% over the wide range of dietary protein intakes and for both the fed and postabsorptive states. For the tissues from which the plasma KIC originates, the rate of plasma leucine into cells will account for approximately 77% of the intracellular leucine flux with the remaining 23% coming primarily from leucine release via protein breakdown. The constant nature of the plasma KIC to leucine ¹³C enrichment ratio implies that relative changes in leucine kinetics will appear the same under many dietary circumstances regardless of whether plasma leucine or KIC enrichments are used for the calculations.     

Thyroid hormone transport by the heterodimeric human system L amino acid transporter

Transport of thyroid hormone across the cell membrane is required for thyroid hormone action and metabolism. We have investigated the possible transport of iodothyronines by the human system L amino acid transporter, a protein consisting of the human 4F2 heavy chain and the human LAT1 light chain. Xenopus oocytes were injected with the cRNAs coding for human 4F2 heavy chain and/or human LAT1 light chain, and after 2 d were incubated at 25 C with 0.01-10 microM [(125)I]T(4), [(125)I]T(3), [(125)I]rT(3), or [(125)I]3,3'-diiodothyronine or with 10-100 microM [(3)H]arginine, [(3)H]leucine, [(3)H]phenylalanine, [(3)H]tyrosine, or [(3)H]tryptophan. Injection of human 4F2 heavy chain cRNA alone stimulated the uptake of leucine and arginine due to dimerization of human 4F2 heavy chain with an endogenous Xenopus light chain, but did not affect the uptake of other ligands. Injection of human LAT1 light chain cRNA alone did not stimulate the uptake of any ligand. Coinjection of cRNAs for human 4F2 heavy chain and human LAT1 light chain stimulated the uptake of phenylalanine > tyrosine > leucine > tryptophan (100 microM) and of 3,3'-diiodothyronine > rT(3) approximately T(3) > T(4) (10 nM), which in all cases was Na(+) independent. Saturation analysis provided apparent Michaelis constant (K(m)) values of 7.9 microM for T(4), 0.8 microM for T(3), 12.5 microM for rT(3), 7.9 microM for 3,3'-diiodothyronine, 46 microM for leucine, and 19 microM for tryptophan. Uptake of leucine, tyrosine, and tryptophan (10 microM) was inhibited by the different iodothyronines (10 microM), in particular T(3). Vice versa, uptake of 0.1 microM T(3) was almost completely blocked by coincubation with 100 microM leucine, tryptophan, tyrosine, or phenylalanine. Our results demonstrate stereospecific Na(+)-independent transport of iodothyronines by the human heterodimeric system L amino acid transporter.     

Impact of growth hormone and dehydroepiandrosterone on protein metabolism in glucocorticoid-treated patients

CONTEXT: Chronic pharmacological glucocorticoid (GC) use causes substantial morbidity from protein wasting. GH and androgens are anabolic agents that may potentially reverse GC-induced protein loss. OBJECTIVE: Our objective was to assess the effect of GH and dehydroepiandrosterone (DHEA) on protein metabolism in subjects on long-term GC therapy. DESIGN: This was an open, stepwise GH dose-finding study (study 1), followed by a randomized cross-over intervention study (study 2). SETTING: The studies were performed at a clinical research facility. PATIENTS AND INTERVENTION: In study 1, six subjects (age 69+/-4 yr) treated with long-term (>6 months) GCs (prednisone dose 8.3+/-0.8 mg/d) were studied before and after two sequential GH doses (0.8 and 1.6 mg/d) for 2 wk each. In study 2, 10 women (age 71+/-3 yr) treated with long-term GCs (prednisone dose 5.4+/-0.5 mg/d) were studied at baseline and after 2-wk treatment with GH 0.8 mg/d, DHEA 50 mg/d, or GH and DHEA (combination treatment). MAIN OUTCOME MEASURE: Changes in whole body protein metabolism were assessed using a 3-h primed constant infusion of 1-[13C]leucine, from which rates of leucine appearance, leucine oxidation, and leucine incorporation into protein were estimated. RESULTS: In study 1, GH 0.8 and 1.6 mg/d significantly reduced leucine oxidation by 19% (P=0.03) and 31% (P=0.02), and increased leucine incorporation into protein by 10% (P=0.13) and 19% (P=0.04), respectively. The lower GH dose did not cause hyperglycemia, whereas GH 1.6 mg/d resulted in fasting hyperglycemia in two of six subjects. In study 2, DHEA did not significantly change leucine metabolism alone or when combined with GH. Blood glucose was not affected by DHEA. CONCLUSION: GH, at a modest supraphysiological dose of 0.8 mg/d, induces protein anabolism in chronic GC users without causing diabetes. DHEA 50 mg/d does not enhance the effect of GH. GH may safely prevent or reverse protein loss induced by chronic GC therapy.     

In vivo and in vitro effects of growth hormone on the incorporation of [14C]leucine into protein of liver and muscle of the eel

In vivo administration of ovine GH (2 micrograms/g body wt) increased [14C]leucine incorporation into protein of the liver, skeletal muscle, and opercular muscle of hypophysectomized eels. Addition of ovine GH into the medium (5 micrograms/ml) slightly increased [14C]leucine incorporation into protein of liver slices during 5 hr in vitro incubation, but did not affect protein synthesis from [14C]leucine in opercular muscle. In vivo pretreatment with ovine GH (2 micrograms/g body wt) 48 hr prior to tissue preparation clearly increased [14C]leucine incorporation into protein of liver slices in vitro. However, no statistically significant change was observed for in vitro incorporation of [14C]leucine into protein of opercular muscle of hypophysectomized eels which had been previously treated with ovine GH (2 micrograms/g body wt). These results indicate that ovine GH has a protein anabolic action in the liver and muscle of the eel and that compared to mammals a rather long lag period is needed to elicit such protein anabolic actions of GH in these animals.     

Metabolic effects of oral versus transdermal estrogen in growth hormone-treated girls with turner syndrome

BACKGROUND: Transdermal (TD) estrogen is often preferred over the oral route in postmenopausal and GH-deficient women taking estrogen, but this has not been studied in detail in girls. OBJECTIVE: Our objective was to study the metabolic effects of oral vs. TD estrogen in GH-treated girls with Turner syndrome. DESIGN AND METHODS: Eleven girls with Turner syndrome, mean age 13.4 +/- 0.5 (se) yr, on GH for at least 6 months were recruited. Studies included [(13)C]leucine and d5-glycerol infusions, indirect calorimetry, dual-emission x-ray absorptiometry, and hormone and substrate measurements. They received, in random order, 17beta-estradiol orally (0.5, 1, and 2 mg for 2 wk each) and TD (0.025, 0.0375, and 0.05 mg for 2 wk each), and studies were repeated after each 6-wk course with 4 wk washout in between. RESULTS: Rates of whole-body protein turnover, oxidation and synthesis, lipolysis, lipid and carbohydrate oxidation, and resting energy expenditure were unaffected by either form of estrogen; nor were lipids, insulin, and fibrinogen concentrations affected. Plasma IGF-I concentrations did not change clinically significantly with either form of estrogen, despite higher estrogen concentrations after oral estrogen. Estradiol concentrations did not correlate with any variables measured. CONCLUSIONS: In GH-treated girls with Turner syndrome, neither oral nor TD estrogen adversely affected rates of protein turnover, lipolysis, and lipid oxidation rates or plasma lipids, fibrinogen, or fasting insulin concentrations. There was no clinically significant change in IGF-I concentrations after either form of estrogen. In aggregate, these data suggest that the route of delivery of estrogen does not adversely affect these metabolic effects of GH in young girls with Turner syndrome.     

Lack of Effect of Acute Enteral Arginine Infusion on Whole-Body and Intestinal Protein Metabolism in Humans

Arginine is a conditionally essential amino acid and exerts anabolic effects. We studied the effects of enteral arginine on whole-body and duodenal protein metabolism. Eight healthy fasted volunteers received randomly a 5-hr enteral infusion of either arginine (Arg; 20 g) or an isonitrogenous amino acid mixture (AA) and an IV infusion of [¹³C]leucine. Duodenal biopsies were performed. Whole-body protein turnover and duodenal protein synthesis (FSR) were calculated from GC/MS-assessed enrichment. The mRNA levels for major components of proteolytic pathways, ubiquitin, cathepsin D, and m-calpain, were evaluated by RT-PCR. Results were compared using paired Wilcoxon test. Endogenous, oxidative, and nonoxidative leucine fluxes were not different after Arg and AA infusions, respectively. Duodenal mucosal protein FSR (71% ± 26% vs 81% ± 30%/day) and mRNA levels of ubiquitin, cathepsin D, and m-calpain were also similar after Arg and AA infusions. We conclude that in healthy subjects, arginine infusion exerts no effect on whole-body and duodenal protein metabolism. Whether arginine might specifically affect these parameters in catabolic or inflammatory situations remains to be determined. The first two authors contributed equally to this work.     

Influence of adiposity in the blunted whole-body protein anabolic response to insulin with aging

BACKGROUND: Although insulin resistance of glucose is often reported with aging, that of protein metabolism is still debated. We tested if the insulin sensitivity of protein metabolism parallels that of glucose and is altered with aging. METHODS: Whole-body (13)C-leucine and (3)H-glucose kinetics were measured in the postabsorptive state and during an hyperinsulinemic, euglycemic, isoaminoacidemic clamp in 12 young men (age: 27 +/- 1 years; body mass index [BMI]: 23 +/- 1 kg/m(2)), 11 young women (age: 25 +/- 1 years; BMI: 21 +/- 1 kg/m(2)), 9 elderly men (age: 70 +/- 1 years; BMI: 26 +/- 1 kg/m(2)), and 10 elderly women (age: 69 +/- 2 years; BMI: 23 +/- 1 kg/m(2)). RESULTS: Postabsorptive leucine flux rates adjusted for fat-free mass (FFM) were not different between elderly and young participants. During the clamp, leucine flux and protein synthesis rates increased less in the elderly participants, and protein breakdown decreased equally. Thus, the net anabolic (protein balance) response to hyperinsulinemia was lower in elderly versus young participants (p =.007) and was highly correlated with the clamp glucose rate of disposal (r = 0.671, p <.001), indicating insulin resistance of protein concurrent with that of glucose. From regression analysis, FFM explained 73% of the variance in the anabolic response. Age explained an additional 3%, but was accounted for by markers of adiposity. FFM and percent body fat collectively explained 79% of the variance. CONCLUSION: Both reduction in absolute FFM and increased adiposity, intrinsic to the aging process, are associated with an altered anabolic action of insulin in stimulating protein synthesis. This alteration may contribute to the progressive muscle loss with aging.     

Estrogen supplementation reduces whole body leucine and carbohydrate oxidation and increases lipid oxidation in men during endurance exercise

Healthy active men exhibit higher rates of carbohydrate (CHO) and leucine oxidation and lower rates of lipid oxidation compared with their female counterparts both at rest and during moderate intensity endurance exercise. We postulated that this reduced dependence on amino acids as a fuel source in women was due to the female sex hormone estrogen. In a randomized, double-blind, placebo-controlled, cross-over design, we investigated the effect of supplementing 12 recreationally active men with estrogen on whole body substrate oxidation and leucine kinetics at rest and during moderate intensity endurance exercise. Subjects cycled for 90 min at an intensity of 65% maximum O(2) consumption after 8 d of either estrogen supplementation (2 mg 17beta-estradiol/d) or placebo (polycose). After a 2-wk washout period, they repeated the test after 8 d of the alternate treatment. On the test day, after a primed continuous infusion of l-[(13)C]leucine, O(2) consumption, CO(2) production, steady-state breath (13)CO(2), and plasma alpha-[(13)C]ketoisocaproate enrichments were measured at rest and at 60, 75, and 90 min during exercise in the postabsorptive state. Exercise increased energy expenditure more than 5-fold, CHO oxidation more than 6-fold, lipid oxidation more than 4-fold, and leucine oxidation 2.2-fold (all P < 0.0001), whereas it decreased the ratio of lipid to CHO oxidation by 50-70% (P = 0.003) compared with values at rest. Estrogen supplementation decreased respiratory exchange ratio during exercise (P = 0.03). Estrogen supplementation significantly decreased CHO oxidation by 5-16% (P = 0.04) and leucine oxidation by 16% (P = 0.01), whereas it significantly increased lipid oxidation by 22-44% (P = 0.024) at rest and during exercise. We conclude that estrogen influences fuel source selection at rest and during endurance exercise in recreationally active men, characterized by a reduced dependence on amino acids and CHO and an increased reliance on lipids as a fuel source.     

Does enteral glutamine modulate whole-body leucine kinetics in hypercatabolic dogs in a fed state?

To determine whether enteral glutamine alters whole-body leucine metabolism in a state of hypercatabolism, 6 dogs adapted to a normocaloric, low-protein diet received intramuscular dexamethasone (0.44 mg. kg(-1). d(-1)) for 1 week, during 2 separate study periods. On the last day of each period, intravenous infusions of L-[1-(13)C]leucine and L-[2-(15)N]glutamine were performed to assess whole-body leucine and glutamine metabolism, and duodenal biopsies were obtained to determine gut protein fractional synthesis rate (FSR), while dogs were receiving enteral nutrition. The nutrient mixture supplied 6.2 kcal. h(-1) nonprotein energy per kg(0.75) of body weight (84% glucose, 16% fat) and 0.2 g amino acid per kg(-0.75). h(-1); the nutrient mixture was glutamine-free on the "control day," and supplemented with 1,150 micromol. kg(-1). h(-1) natural L-glutamine on the "glutamine day." Glutamine supplementation induced an approximately 56% rise in plasma glutamine appearance rate (P <.05), and was associated with an approximately 26% reduction in leucine oxidation (P <.05) with no change in leucine release from protein breakdown or nonoxidative leucine disposal, an index of whole-body protein synthesis. Glutamine supplementation improved net leucine balance (protein synthesis-protein breakdown) (-26 +/- 4 v -48 +/- 11 micromol. kg(-1). h(-1); P <.05). In addition, glutamine enhanced intestinal protein FSR by approximately 22% in the 4 dogs where it was assessed. We conclude that, in hypercatabolic adult dogs in the fed state, enteral glutamine supplementation acutely decreases leucine oxidation and improves net leucine balance, and may thus preserve body protein.