Enhanced response of muscle protein synthesis and plasma insulin to food intake in suckled rats

Branched-chain amino acid supplementation does not enhance lean tissue accretion in low birth weight neonatal pigs, despite lower Sestrin2 expression in skeletal muscle

Postnatal muscle growth is impaired in low birth weight (L) neonatal pigs. Leucine supplementation has been established as a dietary intervention to enhance muscle growth in growing animals. The aim of this study was to investigate the efficacy of supplementing L neonatal pig formulas with branched-chain amino acids (B) to enhance the rate of protein accretion. Twenty-four 3-day old pigs were divided into two groups low (L) and normal birth weight (N) based on weight at birth. Pigs were assigned to a control (C) or 1% branched-chain amino acids (B) formulas, and fed at 250 mL·kg body weight -1·d-1 for 28 d. Body weight of pigs in the L group was less than those in the N group (P < 0.01). However, fractional body weight was greater for L pigs compared with their N siblings from day 24 to 28 of feeding regardless of formula (P < 0.01). In addition, feed efficiency (P < 0.0001) and efficiently of protein accretion (P < 0.0001) were greater for L than N pigs regardless of supplementation. Pigs fed the B formula had greater plasma leucine, isoleucine, and valine concentrations compared with those fed the C formula (P < 0.05). Longissimus dorsi Sestrin2 protein expression was less for pigs in the L group compared with those in the N group (P < 0.01), but did not result in a corresponding increase in translation initiation signaling. Longissimus dorsi mRNA expression of BCAT2 was less for LB pigs compared with those in the LC group, and was intermediate for NC and NB pigs (P < 0.05). Hepatic mRNA expression of BCKDHA was greater for pigs in the L compared with those in the N groups (P < 0.05). However, plasma branched-chain keto-acid concentration was reduced for C compared with those in the B group (P < 0.05). These data suggest that branched-chain amino acid supplementation does not improve lean tissue accretion of low and normal birth weight pigs, despite a reduction in Sestrin2 expression in skeletal muscle of low birth weight pigs. The modest improvement in fractional growth rate of low birth weight pigs compared with their normal birth weight siblings was likely due to a more efficient dietary protein utilization.

A narrative review of skeletal muscle atrophy in critically ill children: pathogenesis and chronic sequelae

Muscle wasting is now recognized as a growing, debilitating problem in critically ill adults, resulting in long-term deficits in function and an impaired quality of life. Ultrasonography has demonstrated decreases in skeletal muscle size during pediatric critical illness, although variations exist. However, muscle protein turnover patterns during pediatric critical illness are unclear. Understanding muscle protein turnover during critical illness is important in guiding interventions to reduce muscle wasting. The aim of this review was to explore the possible protein synthesis and breakdown patterns in pediatric critical illness. Muscle protein turnover studies in critically ill children are lacking, with the exception of those with burn injuries. Children with burn injuries demonstrate an elevation in both muscle protein breakdown (MPB) and synthesis during critical illness. Extrapolations from animal models and whole-body protein turnover studies in children suggest that children may be more dependent on anabolic factors (e.g., nutrition and growth factors), and may experience greater muscle degradation in response to insults than adults. Yet, children, particularly the younger ones, are more responsive to anabolic agents, suggesting modifiable muscle wasting during critical illness. There is a lack of evidence for muscle wasting in critically ill children and its correlation with outcomes, possibly due to current available methods to study muscle protein turnover in children-most of which are invasive or tedious. In summary, children may experience muscle wasting during critical illness, which may be more reversible by the appropriate anabolic agents than adults. Age appears an important determinant of skeletal muscle turnover. Less invasive methods to study muscle protein turnover and associations with long-term outcome would strengthen the evidence for muscle wasting in critically ill children.

Intermittent Bolus Feeding Enhances Organ Growth More Than Continuous Feeding in a Neonatal Piglet Model

BACKGROUND

Orogastric tube feeding is frequently prescribed for neonates who cannot ingest food normally. In a piglet model of the neonate, greater skeletal muscle growth is sustained by upregulation of translation initiation signaling when nutrition is delivered by intermittent bolus meals, rather than continuously.

OBJECTIVES

The objective of this study was to determine the long-term effects of feeding frequency on organ growth and the mechanism by which feeding frequency modulates protein anabolism in these organs.

METHODS

Eighteen neonatal pigs were fed by gastrostomy tube the same amount of a sow milk replacer either by continuous infusion (CON) or on an intermittent bolus schedule (INT). After 21 d of feeding, the pigs were killed without interruption of feeding (CON; n = 6) or immediately before (INT-0; n = 6) or 60 min after (INT-60; n = 6) a meal, and fractional protein synthesis rates and activation indexes of signaling pathways that regulate translation initiation were measured in the heart, jejunum, ileum, kidneys, and liver.

RESULTS

Compared with continuous feeding, intermittent feeding stimulated the growth of the liver (+64%), jejunum (+48%), ileum (+40%), heart (+64%), and kidney (+56%). The increases in heart, kidney, jejunum, and ileum masses were proportional to whole body lean weight gain, but liver weight gain was greater in the INT-60 than the CON, and intermediate for the INT-0 group. For the liver and ileum, but not the heart, kidney, and jejunum, INT-60 compared with CON pigs had greater fractional protein synthesis rates (22% and 48%, respectively) and was accompanied by an increase in ribosomal protein S6 kinase 1 and eukaryotic initiation factor 4E binding protein 1 phosphorylation.

CONCLUSIONS

These results suggest that intermittent bolus compared with continuous orogastric feeding enhances organ growth and that in the ileum and liver, intermittent feeding enhances protein synthesis by stimulating translation initiation.

The daytime feeding frequency affects appetite-regulating hormones, amino acids, physical activity, and respiratory quotient, but not energy expenditure, in adult cats fed regimens for 21 days

The effects of feeding frequency on postprandial response of circulating appetite-regulating hormones, insulin, glucose and amino acids, and on physical activity, energy expenditure, and respiratory quotient were studied in healthy adult cats. Two experiments were designed as a 2 x 3 replicated incomplete Latin square design. Eight cats, with an average body weight (BW) of 4.34 kg ± 0.04 and body condition score (BCS) of 5.4 ± 1.4 (9 point scale), were fed isocaloric amounts of a commercial adult maintenance canned cat food either once (0800 h) or four times daily (0800 h, 1130 h, 1500 h, 1830 h). Study 1 consisted of three 21-d periods. On day 14, two fasted and 11 postprandial blood samples were collected over 24 hours to measure plasma concentrations of ghrelin, GLP-1, GIP, leptin, PYY, insulin and amino acids, and whole blood glucose. Physical activity was monitored from day 15 to 21 of each period. In Study 2 indirect calorimetry was performed on the last day of each period. Body weight was measured weekly and feed intake recorded daily in both experiments. No effect of feeding regimen on BW was detected. Cats eating four times daily had lesser plasma concentrations of GIP and GLP-1 (P<0.05) and tended to have lesser plasma PYY concentrations (P<0.1). Plasma leptin and whole blood glucose concentrations did not differ between regimens (P>0.1). Cats fed once daily had a greater postprandial plasma amino acid response, and greater plasma ghrelin and insulin concentrations (P<0.05). Physical activity was greater in cats fed four times (P<0.05), though energy expenditure was similar between treatments at fasting and in postprandial phases. Finally, cats eating one meal had a lower fasting respiratory quotient (P<0.05). Overall, these data indicate that feeding once a day may be a beneficial feeding management strategy for indoor cats to promote satiation and lean body mass.

Prematurity blunts the feeding-induced stimulation of translation initiation signaling and protein synthesis in muscle of neonatal piglets

Postnatal growth of lean mass is commonly blunted in preterm infants and may contribute to short- and long-term morbidities. To determine whether preterm birth alters the protein anabolic response to feeding, piglets were delivered at term or preterm, and fractional protein synthesis rates (K_s) were measured at 3 days of age while fasted or after an enteral meal. Activation of signaling pathways that regulate protein synthesis and degradation were determined. Relative body weight gain was lower in preterm than in term. Gestational age at birth (GAB) did not alter fasting plasma glucose or insulin, but when fed, plasma insulin and glucose rose more slowly, and reached peak value later, in preterm than in term. Feeding increased K_s in longissimus dorsi (LD) and gastrocnemius muscles, heart, pancreas, and kidney in both GAB groups, but the response was blunted in preterm. In diaphragm, lung, jejunum, and brain, feeding increased K_s regardless of GAB. Liver K_s was greater in preterm than term and increased with feeding regardless of GAB. In all tissues, changes in 4EBP1, S6K1, and PKB phosphorylation paralleled changes in K_s. In LD, eIF4E·eIF4G complex formation, phosphorylation of TSC2, mTOR, and rpS6, and association of mammalian target of rapamycin (mTOR1) complex with RagA, RagC, and Rheb were increased by feeding and blunted by prematurity. There were no differences among groups in LD protein degradation markers. Our results demonstrate that preterm birth reduces weight gain and the protein synthetic response to feeding in muscle, pancreas, and kidney, and this is associated with blunted insulin- and/or amino acid-induced translation initiation signaling.

Intermittent bolus feeding promotes greater lean growth than continuous feeding in a neonatal piglet model

Background

Orogastric tube feeding is indicated in neonates with an impaired ability to ingest food normally and can be administered with an intermittent bolus or continuous feeding schedule.

Objectives

The objectives were to 1) compare the long-term effect of continuous with intermittent feeding on growth using the newborn pig as a model, 2) determine whether feeding frequency alters lean tissue and fat mass gain, and 3) identify the signaling mechanisms by which protein deposition is controlled in skeletal muscle in response to feeding frequency.

Design

Neonatal pigs were fed the same amount of a balanced formula by orogastric tube either as an intermittent bolus meal every 4 h (INT) or as a continuous infusion (CON). Body composition was assessed at the start and end of the study by dual-energy X-ray absorptiometry, and hormone and substrate profiles, muscle mass, protein synthesis, and indexes of nutrient and insulin signaling were measured after 21 d.

Results

Body weight, lean mass, spine length, and skeletal muscle mass were greater in the INT group than in the CON group. Skeletal muscle fractional protein synthesis rates were greater in the INT group after a meal than in the CON group and were associated with higher circulating branched-chain amino acid and insulin concentrations. Skeletal muscle protein kinase B (PKB) and ribosomal protein S6 kinase phosphorylation and eukaryotic initiation factor (eIF) 4E-eIF4G complex formation were higher, whereas eIF2α phosphorylation was lower in the INT group than in the CON group, indicating enhanced activation of insulin and amino acid signaling to translation initiation.

Conclusions

These results suggest that when neonates are fed the same amounts of nutrients as intermittent meals rather than continuously there is greater lean growth. This response can be ascribed, in part, to the pulsatile pattern of amino acids, insulin, or both induced by INT, which enables the responsiveness of anabolic pathways to feeding to be sustained chronically in skeletal muscle.

Leucine is a major regulator of muscle protein synthesis in neonates

Approximately 10% of infants born in the United States are of low birth weight. Growth failure during the neonatal period is a common occurrence in low birth weight infants due to their inability to tolerate full feeds, concerns about advancing protein supply, and high nutrient requirements for growth. An improved understanding of the nutritional regulation of growth during this critical period of postnatal growth is vital for the development of strategies to improve lean gain. Past studies with animal models have demonstrated that muscle protein synthesis is increased substantially following a meal and that this increase is due to the postprandial rise in amino acids as well as insulin. Both amino acids and insulin act independently to stimulate protein synthesis in a mammalian target of rapamycin-dependent manner. Further studies have elucidated that leucine, in particular, and its metabolites, α-ketoisocaproic acid and β-hydroxy-β-methylbutyrate, have unique anabolic properties. Supplementation with leucine, provided either parenterally or enterally, has been shown to enhance muscle protein synthesis in neonatal pigs, making it an ideal candidate for stimulating growth of low birth weight infants.

Ribosome abundance regulates the recovery of skeletal muscle protein mass upon recuperation from postnatal undernutrition in mice

Nutritionally-induced growth faltering in the perinatal period has been associated with reduced adult skeletal muscle mass; however, the mechanisms responsible for this are unclear. To identify the factors that determine the recuperative capacity of muscle mass, we studied offspring of FVB mouse dams fed a protein-restricted diet during gestation (GLP) or pups suckled from postnatal day 1 (PN1) to PN11 (E-UN), or PN11 to PN22 (L-UN) on protein-restricted or control dams. All pups were refed under control conditions following the episode of undernutrition. Before refeeding, and 2, 7 and 21 days later, muscle protein synthesis was measured in vivo. There were no long-term deficits in protein mass in GLP and E-UN offspring, but in L-UN offspring muscle protein mass remained significantly smaller even after 18 months (P < 0.001). E-UN differed from L-UN offspring by their capacity to upregulate postprandial muscle protein synthesis when refed (P < 0.001), a difference that was attributable to a transient increase in ribosomal abundance, i.e. translational capacity, in E-UN offspring (P < 0.05); translational efficiency was similar across dietary treatments. The postprandial phosphorylation of Akt and extracellular signal-regulated protein kinases were similar among treatments. However, activation of the ribosomal S6 kinase 1 via mTOR (P < 0.02), and total upstream binding factor abundance were significantly greater in E-UN than L-UN offspring (P < 0.02). The results indicate that the capacity of muscles to recover following perinatal undernutrition depends on developmental age as this establishes whether ribosome abundance can be enhanced sufficiently to promote the protein synthesis rates required to accelerate protein deposition for catch-up growth.

Enteral leucine supplementation increases protein synthesis in skeletal and cardiac muscles and visceral tissues of neonatal pigs through mTORC1-dependent pathways

INTRODUCTION

Leucine (Leu) activates mammalian target of rapamycin (mTOR) to upregulate protein synthesis (PS).

RESULTS

PS in skeletal muscles, heart, liver, pancreas, and jejunum, but not kidney, were greater in low protein supplemented with Leu (LP+L) than LP, but lower than high protein (HP). In longissimus dorsi muscle, protein kinase B phosphorylation was similar in LP and LP+L, but lower than HP. Although less than HP, p70 ribosomal S6 kinase 1 (S6K1) and eukaryotic initiation factor (eIF) 4E binding protein 1 (4EBP1) association with regulatory associated protein of mammalian target of rapamycin was greater in LP+L than LP, resulting in higher S6K1 and 4EBP1 phosphorylation. Feeding LP+L vs. LP decreased 4EBP1·eIF4E and increased eIF4E·eIF4G formation, but not to HP. Similar results were obtained for S6K1 and 4EBP1 phosphorylation in gastrocnemius, masseter, heart, liver, pancreas, and jejunum, but not kidney. eIF2α and elongation factor 2 phosphorylation was unaffected by treatment. DICUSSION: Our results suggest that enteral Leu supplementation of a low protein diet enhances PS in most tissues through mTOR complex 1 pathways.

METHODS

To examine enteral Leu effects on PS and signaling activation, 5-d-old piglets were fed for 24 h diets containing: (i) LP, (ii) LP+L, or (iii) HP.

Anabolic signaling and protein deposition are enhanced by intermittent compared with continuous feeding in skeletal muscle of neonates

Orogastric tube feeding is indicated for neonates with impaired ability to ingest and can be administered by intermittent bolus or continuous schedule. Our aim was to determine whether feeding modalities affect muscle protein deposition and to identify mechanisms involved. Neonatal pigs were overnight fasted (FAS) or fed the same amount of food continuously (CON) or intermittently (INT; 7 × 4 h meals) for 29 h. For 8 h, between hours 20 and 28, pigs were infused with [(2)H(5)]phenylalanine and [(2)H(2)]tyrosine, and amino acid (AA) net balances were measured across the hindquarters. Insulin, branched-chain AA, phenylalanine, and tyrosine arterial concentrations and whole body phenylalanine and tyrosine fluxes were greater for INT after the meal than for CON or FAS. The activation of signaling proteins leading to initiation of mRNA translation, including eukaryotic initiation factor (eIF)4E·eIF4G complex formation in muscle, was enhanced by INT compared with CON feeding or FAS. Signaling proteins of protein degradation were not affected by feeding modalities except for microtubule-associated protein light chain 3-II, which was highest in the FAS. Across the hindquarters, AA net removal increased for INT but not for CON or FAS, with protein deposition greater for INT. This was because protein synthesis increased following feeding for INT but remained unchanged for CON and FAS, whereas there was no change in protein degradation across any dietary treatment. These results suggest that muscle protein accretion in neonates is enhanced with intermittent bolus to a greater extent than continuous feeding, mainly by increased protein synthesis.

Intermittent bolus feeding has a greater stimulatory effect on protein synthesis in skeletal muscle than continuous feeding in neonatal pigs

Orogastric tube feeding, using either continuous or intermittent bolus delivery, is common in infants for whom normal feeding is contraindicated. To compare the impact of different feeding strategies on muscle protein synthesis, after withholding food overnight, neonatal pigs received a complete formula orally as a bolus feed every 4 h or were continuously fed. Protein synthesis rate and translational mechanisms in skeletal muscle were examined after 0, 24, and 25.5 h. Plasma amino acid and insulin concentrations increased minimally and remained constant in continuously fed compared to feed-deprived pigs; however, the pulsatile meal feeding pattern was mimicked in bolus-fed pigs. Muscle protein synthesis was stimulated by feeding and the greatest response occurred after a bolus meal. Bolus but not continuous feeds increased polysome aggregation, the phosphorylation of protein kinase B, tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kDa, eukaryotic initiation factor (eIF) 4E binding protein (4EBP1), and rp S6 kinase and enhanced dissociation of the 4EBP1 ·eIF4E complex and formation of the eIF4E ·eIF4G complex compared to feed deprivation (P < 0.05). Activation of insulin receptor substrate-1, regulatory associated protein of mammalian target of rapamycin, AMP-activated protein kinase, eukaryotic elongation factor 2, and eIF2α phosphorylation were unaffected by either feeding modality. These results suggest that in neonates, intermittent bolus feeding enhances muscle protein synthesis to a greater extent than continuous feeding by eliciting a pulsatile pattern of amino acid- and insulin-induced translation initiation.

Differential regulation of protein synthesis and mTOR signaling in skeletal muscle and visceral tissues of neonatal pigs after a meal

Protein synthesis (PS) increases after a meal in neonates, but the time course of the changes in PS in different tissues after a meal is unknown. We aimed to evaluate the changes in tissue PS, mammalian target of rapamycin complex 1 (mTORC1) activation, and proportion of ribosomal protein (rp) mRNAs in polysomes over 4 h after a bolus meal in neonatal pigs (n = 6/group; 5- to 7-d-old). The results show a more sustained increase in PS in glycolytic compared with mixed fiber type muscles and no changes in oxidative muscles. PS increased in liver, jejunum, and pancreas but not in kidney and heart. Feeding did not affect AMP-activated protein kinase or RAS-related GTP binding B activation. Phosphorylation of tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kD, mTOR, eukaryotic initiation factor 4E binding protein, and rp S6 kinase 1 increased in all tissues after feeding. The proportion of mRNAs encoding rp S4 and S8 in liver polysomes increased within 30 min postfeeding. These results suggest that feeding stimulates mTORC1 signaling in muscle and viscera, but mTORC1 activation alone is not sufficient to stimulate PS in all tissues.

Leucine supplementation of a low-protein meal increases skeletal muscle and visceral tissue protein synthesis in neonatal pigs by stimulating mTOR-dependent translation initiation

Protein synthesis and eukaryotic initiation factor (eIF) activation are increased in skeletal muscle of neonatal pigs parenterally infused with amino acids. Leucine appears to be the most effective single amino acid to trigger these effects. To examine the response to enteral leucine supplementation, overnight food-deprived 5-d-old pigs were gavage fed at 0 and 60 min a: 1) low-protein diet (LP); 2) LP supplemented with leucine (LP+L) to equal leucine in the high-protein diet (HP); or 3) HP diet. Diets were isocaloric and equal in lactose. Fractional protein synthesis rates and translation initiation control mechanisms were examined in skeletal muscles and visceral tissues 90 min after feeding. Protein synthesis rates in longissimus dorsi, gastrocnemius, and masseter muscles, heart, jejunum, kidney, and pancreas, but not liver, were greater in the LP+L group compared with the LP group and did not differ from the HP group. Feeding LP+L and HP diets compared with the LP diet increased phosphorylation of mammalian target of rapamycin (mTOR), 4E-binding protein 1, ribosomal protein S6 kinase-1, and eIF4G and formation of the active eIF4E·eIF4G complex in longissimus dorsi muscle. In all tissues except liver, activation of mTOR effectors increased in pigs fed LP+L and HP vs. LP diets. Our results suggest that leucine supplementation of a low-protein meal stimulates protein synthesis in muscle and most visceral tissues to a rate similar to that achieved by feeding a high-protein meal and this stimulation involves activation of mTOR downstream effectors.

Leucine and alpha-ketoisocaproic acid, but not norleucine, stimulate skeletal muscle protein synthesis in neonatal pigs

The branched-chain amino acid, leucine, acts as a nutrient signal to stimulate protein synthesis in skeletal muscle of young pigs. However, the chemical structure responsible for this effect has not been identified. We have shown that the other branched-chain amino acids, isoleucine and valine, are not able to stimulate protein synthesis when raised in plasma to levels within the postprandial range. In this study, we evaluated the effect of leucine, alpha-ketoisocaproic acid (KIC), and norleucine infusion (0 or 400 micromol kg(-1) h(-1) for 60 min) on protein synthesis and activation of translation initiation factors in piglets. Infusion of leucine, KIC, and norleucine raised plasma levels of each compound compared with controls. KIC also increased (P < 0.01) and norleucine reduced (P < 0.02) plasma levels of leucine compared with controls. Administration of leucine and KIC resulted in greater (P < 0.006) phosphorylation of eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1) and eIF4G, lower (P < 0.04) abundance of the inactive 4E-BP1.eIF4E complex, and greater (P < 0.05) active eIF4G.eIF4E complex formation in skeletal muscle compared with controls. Protein synthesis in skeletal muscle was greater (P < 0.02) in leucine- and KIC-infused pigs than in those in the control group. Norleucine infusion did not affect muscle protein synthesis or translation initiation factor activation. In liver, neither protein synthesis nor activation of translation initiation factors was affected by treatment. These results suggest that the ability of leucine to act as a nutrient signal to stimulate skeletal muscle protein synthesis is specific for leucine and/or its metabolite, KIC.

Differential effects of long-term leucine infusion on tissue protein synthesis in neonatal pigs

Leucine is unique among the amino acids in its ability to promote protein synthesis by activating translation initiation via the mammalian target of rapamycin (mTOR) pathway. Previously, we showed that leucine infusion acutely stimulates protein synthesis in fast-twitch glycolytic muscle of neonatal pigs but this response cannot be maintained unless the leucine-induced fall in amino acids is prevented. To determine whether leucine can stimulate protein synthesis in muscles of different fiber types and in visceral tissues of the neonate in the long-term if baseline amino acid concentrations are maintained, overnight fasted neonatal pigs were infused for 24 h with saline, leucine (400 micromol kg(-1) h(-1)), or leucine with replacement amino acids to prevent the leucine-induced hypoaminoacidemia. Changes in the fractional rate of protein synthesis and activation of mTOR, as determined by eukaryotic initiation factor 4E binding protein (4E-BP1) and S6 kinase 1 (S6K1) phosphorylation, in the gastrocnemius and masseter muscles, heart, liver, jejunum, kidney, and pancreas were measured. Leucine increased mTOR activation in the gastrocnemius and masseter muscles, liver, and pancreas, in both the absence and presence of amino acid replacement. However, protein synthesis in these tissues was increased only when amino acids were infused to maintain baseline levels. There were no changes in mTOR signaling or protein synthesis in the other tissues we examined. Thus, long-term infusion of leucine stimulates mTOR signaling in skeletal muscle and some visceral tissues but the leucine-induced stimulation of protein synthesis in these tissues requires sustained amino acid availability.

Stimulation of muscle protein synthesis by prolonged parenteral infusion of leucine is dependent on amino acid availability in neonatal pigs

The postprandial rise in amino acids, particularly leucine, stimulates muscle protein synthesis in neonates. Previously, we showed that a 1-h infusion of leucine increased protein synthesis, but this response was not sustained for 2 h unless the leucine-induced decrease in amino acids was prevented. To determine whether a parenteral leucine infusion can stimulate protein synthesis for a more prolonged, clinically relevant period if baseline amino acid concentrations are maintained, overnight food-deprived neonatal pigs were infused for 24 h with saline, leucine (400 mumol.kg(-1). h(-1)), or leucine with replacement amino acids. Amino acid replacement prevented the leucine-induced decrease in amino acids. Muscle protein synthesis was increased by leucine but only when other amino acids were supplied to maintain euaminoacidemia. Leucine did not affect activators of mammalian target of rapamycin (mTOR), i.e. protein kinase B, AMP-activated protein kinase, tuberous sclerosis complex 2, or eukaryotic elongation factor 2. There was no effect of treatment on the association of mTOR with regulatory associated protein of mammalian target of rapamycin (raptor), G-protein beta subunit-like protein, or rictor or the phosphorylation of raptor or proline-rich Akt substrate of 40 kDa. Phosphorylation of mTOR and its downstream targets, eukaryotic initiation factor (eIF) 4E binding protein and ribosomal protein S6 kinase, and the eIF4E . eIF4G association were increased and eIF2alpha phosphorylation was reduced by leucine and was not further altered by correcting for the leucine-induced hypoaminoacidemia. Thus, prolonged parenteral infusion of leucine activates mTOR and its downstream targets in neonatal skeletal muscle, but the stimulation of protein synthesis also is dependent upon amino acid availability.

Amino acids and insulin are regulators of muscle protein synthesis in neonatal pigs

The stage of development between birth and weaning in mammals is a period of very rapid growth that is crucial for the long-term well-being of the animal. The rate of protein deposition in neonatal animals is very high because dietary protein is efficiently utilized to increase body protein mass. Our studies in neonatal pigs have shown that this high efficiency of protein deposition is largely due to the marked increase in protein synthesis after feeding, and this response is particularly profound in the skeletal muscle. The enhanced stimulation of muscle protein synthesis in neonates after feeding is independently mediated by the rise in insulin and amino acids and this response declines with age. Intracellular signaling components that respond to the postprandial rise in amino acids and insulin have been identified and their activation has been shown to be elevated in skeletal muscle of neonatal pigs after a meal and to decrease with development. The enhanced activation of these components in the amino acid and insulin signaling pathways in neonatal muscle contributes to the high rate of muscle protein synthesis and rapid gain in skeletal muscle mass in newborn pigs, which are essential determinants of efficient growth during development.

Feeding rapidly stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing translation initiation

Food consumption increases protein synthesis in most tissues by promoting translation initiation, and in the neonate, this increase is greatest in skeletal muscle. In this study, we aimed to identify the currently unknown time course of changes in the rate of protein synthesis and the activation of factors involved in translation in neonatal muscle after a meal. After overnight food deprivation, 36 5- to 7-d-old piglets were administered a nutritionally complete bolus i.g. meal and were killed immediately before or 30, 60, 90, 120, or 240 min later. The increase in skeletal muscle protein synthesis peaked 30 min after the meal and this was sustained through 120 min, returning to baseline thereafter. The relative proportion of polysomes to nonpolysomes was higher only after 30 min. Protein kinase B phosphorylation peaked 30 min after feeding and returned to baseline by 90 min. The phosphorylation of mammalian target of rapamycin, eukaryotic initiation factor (eIF) 4E binding protein (4E-BP1), ribosomal protein S6, and eIF4G was increased within 30 min of feeding and persisted through 120 min, but all had returned to baseline by 240 min. The association of 4E-BP1.eIF4E was reduced and eIF4E.eIF4G increased 30 min after receiving a meal, remaining so for 120 min, before returning to baseline at 240 min. Thus, in neonates, food consumption rapidly increased skeletal muscle protein synthesis by enhancing translation initiation and this increase was sustained for at least 120 min after the meal but returned to baseline by 240 min after the feeding.

Positive net movements of amino acids in the hindlimb after overnight food deprivation contribute to sustaining the elevated anabolism of neonatal pigs

During the neonatal period, high protein breakdown rate is a metabolic process inherent to elevated rates of protein accretion in skeletal muscle. To determine the relationship between hindlimb net movements of essential and nonessential amino acids in the regulation of hindlimb protein breakdown during an overnight fasting-feeding cycle, we infused overnight-food-deprived 10- and 28-day-old piglets with [1-(13)C]phenylalanine and [ring-(2)H(4)]tyrosine over 7 h (during 3 h of fasting and then during 4 h of feeding). Extraction rates for aspartate and glutamate after an overnight fast were 15% and 51% in the 10-day-old compared with 6% and 25% in the 28-day-old (P < 0.05) piglets, suggesting an altered requirement for precursors of amino acids to shuttle nitrogen to the liver as early life progresses. This occurred simultaneously with marginal positive hindlimb net balance of essential amino acids after an overnight fast, with negative net release of many nonessential amino acids, such as alanine, asparagine, glutamine, glycine, and proline. This suggests that newborn muscle does not undergo significant protein mobilization after a short period of fasting in support of an elevated rate of protein accretion. Furthermore, tyrosine efflux from hindlimb breakdown between overnight fasting and feeding periods was not different in the 10-day-old piglets, for which tyrosine was limiting, but when tyrosine supply balanced requirements in the 28-day-old piglet, hindlimb efflux was increased (P = 0.01). The results of the present study indicate that proteolysis and net movements of amino acids are coordinated mechanisms that sustain the elevated rate of net protein accretion during overnight feeding-fasting cycles in the neonate.

Differential regulation of protein synthesis by amino acids and insulin in peripheral and visceral tissues of neonatal pigs

The high efficiency of protein deposition during the neonatal period is driven by high rates of protein synthesis, which are maximally stimulated after feeding. In the current study, we examined the individual roles of amino acids and insulin in the regulation of protein synthesis in peripheral and visceral tissues of the neonate by performing pancreatic glucose-amino acid clamps in overnight-fasted 7-day-old pigs. We infused pigs (n = 8-12/group) with insulin at 0, 10, 22, and 110 ng kg(-0.66) min(-1) to achieve approximately 0, 2, 6 and 30 muU ml(-1) insulin so as to simulate below fasting, fasting, intermediate, and fed insulin levels, respectively. At each insulin dose, amino acids were maintained at the fasting or fed level. In conjunction with the highest insulin dose, amino acids were also allowed to fall below the fasting level. Tissue protein synthesis was measured using a flooding dose of L: -[4-(3)H] phenylalanine. Both insulin and amino acids increased fractional rates of protein synthesis in longissimus dorsi, gastrocnemius, masseter, and diaphragm muscles. Insulin, but not amino acids, increased protein synthesis in the skin. Amino acids, but not insulin, increased protein synthesis in the liver, pancreas, spleen, and lung and tended to increase protein synthesis in the jejunum and kidney. Neither insulin nor amino acids altered protein synthesis in the stomach. The results suggest that the stimulation of protein synthesis by feeding in most tissues of the neonate is regulated by the post-prandial rise in amino acids. However, the feeding-induced stimulation of protein synthesis in skeletal muscles is independently mediated by insulin as well as amino acids.

Stimulation of muscle protein synthesis by somatotropin in pigs is independent of the somatotropin-induced increase in circulating insulin

Chronic treatment of growing pigs with porcine somatotropin (pST) promotes protein synthesis and doubles postprandial levels of insulin, a hormone that stimulates translation initiation. This study aimed to determine whether the pST-induced increase in skeletal muscle protein synthesis was mediated through an insulin-induced stimulation of translation initiation. After 7-10 days of pST (150 microg x kg(-1) x day(-1)) or control saline treatment, pancreatic glucose-amino acid clamps were performed in overnight-fasted pigs to reproduce 1) fasted (5 microU/ml), 2) fed control (25 microU/ml), and 3) fed pST-treated (50 microU/ml) insulin levels while glucose and amino acids were maintained at baseline fasting levels. Fractional protein synthesis rates and indexes of translation initiation were examined in skeletal muscle. Effectiveness of pST treatment was confirmed by reduced urea nitrogen and elevated insulin-like growth factor I levels in plasma. Skeletal muscle protein synthesis was independently increased by both insulin and pST. Insulin increased the phosphorylation of protein kinase B and the downstream effectors of the mammalian target of rapamycin, ribosomal protein S6 kinase, and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1). Furthermore, insulin reduced inactive 4E-BP1.eIF4E complex association and increased active eIF4E.eIF4G complex formation, indicating enhanced eIF4F complex assembly. However, pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of skeletal muscle protein synthesis in growing pigs is independent of the insulin-associated activation of translation initiation.

Activation by insulin and amino acids of signaling components leading to translation initiation in skeletal muscle of neonatal pigs is developmentally regulated

Insulin and amino acids act independently to stimulate protein synthesis in skeletal muscle of neonatal pigs, and the responses decrease with development. The purpose of this study was to compare the separate effects of fed levels of INS and AA on the activation of signaling components leading to translation initiation and how these responses change with development. Overnight-fasted 6- (n = 4/group) and 26-day-old (n = 6/ group) pigs were studied during 1) euinsulinemic-euglycemiceuaminoacidemic conditions (controls), 2) euinsulinemic-euglycemichyperaminoacidemic clamps (AA), and 3) hyperinsulinemic-euglycemic-euaminoacidemic clamps (INS). INS, but not AA, increased the phosphorylation of protein kinase B (PKB) and tuberous sclerosis 2 (TSC2). Both INS and AA increased protein synthesis and the phosphorylation of mammalian target of rapamycin (mTOR), ribosomal protein S6 kinase-1, and eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1), and these responses were higher in 6-day-old compared with 26-day-old pigs. Both INS and AA decreased the binding of 4E-BP1 to eIF4E and increased eIF4E binding to eIF4G; these effects were greater in 6-day-old than in 26-day-old pigs. Neither INS nor AA altered the composition of mTORC1 (raptor, mTOR, and GbetaL) or mTORC2 (rictor, mTOR, and GbetaL) complexes. Furthermore, neither INS, AA, nor age had any effect on the abundance of Rheb and the phosphorylation of AMP-activated protein kinase and eukaryotic elongation factor 2. Our results suggest that the activation by insulin and amino acids of signaling components leading to translation initiation is developmentally regulated and parallels the developmental decline in protein synthesis in skeletal muscle of neonatal pigs.

Amino acid availability and age affect the leucine stimulation of protein synthesis and eIF4F formation in muscle

We have previously shown that a physiological increase in plasma leucine for 60 and 120 min increases translation initiation factor activation in muscle of neonatal pigs. Although muscle protein synthesis is increased by leucine at 60 min, it is not maintained at 120 min, perhaps because of the decrease in plasma amino acids (AA). In the present study, 7- and 26-day-old pigs were fasted overnight and infused with leucine (0 or 400 micromol.kg(-1).h(-1)) for 120 min to raise leucine within the postprandial range. The leucine was infused in the presence or absence of a replacement AA mixture (without leucine) to maintain baseline plasma AA levels. AA administration prevented the leucine-induced reduction in plasma AA in both age groups. At 7 days, leucine infusion alone increased eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1) phosphorylation, decreased inactive 4E-BP1.eIF4E complex abundance, and increased active eIF4G.eIF4E complex formation in skeletal muscle; leucine infusion with replacement AA also stimulated these, as well as 70-kDa ribosomal protein S6 kinase, ribosomal protein S6, and eIF4G phosphorylation. At 26 days, leucine infusion alone increased 4E-BP1 phosphorylation and decreased the inactive 4E-BP1.eIF4E complex only; leucine with AA also stimulated these, as well as 70-kDa ribosomal protein S6 kinase and ribosomal protein S6 phosphorylation. Muscle protein synthesis was increased in 7-day-old (+60%) and 26-day-old (+40%) pigs infused with leucine and replacement AA but not with leucine alone. Thus the ability of leucine to stimulate eIF4F formation and protein synthesis in skeletal muscle is dependent on AA availability and age.

Developmental regulation of the activation of signaling components leading to translation initiation in skeletal muscle of neonatal pigs

The rapid growth of neonates is driven by high rates of skeletal muscle protein synthesis. This high rate of protein synthesis, which is induced by feeding, declines with development. Overnight-fasted 7- and 26-day-old pigs either remained fasted or were refed, and the abundance and phosphorylation of growth factor- and nutrient-induced signaling components that regulate mRNA translation initiation were measured in skeletal muscle and liver. In muscle, but not liver, the activation of inhibitors of protein synthesis, phosphatase and tensin homolog deleted on chromosome 10, protein phosphatase 2A, and tuberous sclerosis complex 1/2 increased with age. Serine/threonine phosphorylation of the insulin receptor and insulin receptor substrate-1, which downregulates insulin signaling, and the activation of AMP-activated protein kinase, an inhibitor of protein synthesis, were unaffected by age and feeding in muscle and liver. Activation of positive regulators of protein synthesis, mammalian target of rapamycin (mTOR), ribosomal protein S6 kinase 1 (S6K1), and eIF4E-binding protein-1 (4E-BP1) decreased with age in muscle but not liver. Feeding enhanced mTOR, S6K1, and 4E-BP1 activation in muscle, and this response decreased with age. In liver, activation of S6K1 and 4E-BP1, but not mTOR, was increased by feeding but was unaffected by age. Raptor abundance and the association between raptor and mTOR were greater in 7- than in 26-day-old pigs. The results suggest that the developmental decline in skeletal muscle protein synthesis is due in part to developmental regulation of the activation of growth factor and nutrient-signaling components.

Regulation of cardiac and skeletal muscle protein synthesis by individual branched-chain amino acids in neonatal pigs

Skeletal muscle grows at a very rapid rate in the neonatal pig, due in part to an enhanced sensitivity of protein synthesis to the postprandial rise in amino acids. An increase in leucine alone stimulates protein synthesis in skeletal muscle of the neonatal pig; however, the effect of isoleucine and valine has not been investigated in this experimental model. The left ventricular wall of the heart grows faster than the right ventricular wall during the first 10 days of postnatal life in the pig. Therefore, the effects of individual BCAA on protein synthesis in individual skeletal muscles and in the left and right ventricular walls were examined. Fasted pigs were infused with 0 or 400 micromol x kg(-1) x h(-1) leucine, isoleucine, or valine to raise individual BCAA to fed levels. Fractional rates of protein synthesis and indexes of translation initiation were measured after 60 min. Infusion of leucine increased (P < 0.05) phosphorylation of eukaryotic initiation factor (eIF)4E-binding protein-1 and increased (P < 0.05) the amount and phosphorylation of eIF4G associated with eIF4E in longissimus dorsi and masseter muscles and in both ventricular walls. Leucine increased (P < 0.05) the phosphorylation of ribosomal protein (rp)S6 kinase and rpS6 in longissimus dorsi and masseter but not in either ventricular wall. Leucine stimulated (P < 0.05) protein synthesis in longissimus dorsi, masseter, and the left ventricular wall. Isoleucine and valine did not increase translation initiation factor activation or protein synthesis rates in skeletal or cardiac muscles. The results suggest that the postprandial rise in leucine, but not isoleucine or valine, acts as a nutrient signal to stimulate protein synthesis in cardiac and skeletal muscles of neonates by increasing eIF4E availability for eIF4F complex assembly.

Physiological rise in plasma leucine stimulates muscle protein synthesis in neonatal pigs by enhancing translation initiation factor activation

Protein synthesis in skeletal muscle of adult rats increases in response to oral gavage of supraphysiological doses of leucine. However, the effect on protein synthesis of a physiological rise in plasma leucine has not been investigated in neonates, an anabolic population highly sensitive to amino acids and insulin. Therefore, in the current study, fasted pigs were infused intra-arterially with leucine (0, 200, or 400 micromol.kg(-1).h(-1)), and protein synthesis was measured after 60 or 120 min. Protein synthesis was increased in muscle, but not in liver, at 60 min. At 120 min, however, protein synthesis returned to baseline levels in muscle but was reduced below baseline values in liver. The increase in protein synthesis in muscle was associated with increased plasma leucine of 1.5- to 3-fold and no change in plasma insulin. Leucine infusion for 120 min reduced plasma essential amino acid levels. Phosphorylation of eukaryotic initiation factor (eIF)-4E-binding protein-1 (4E-BP1), ribosomal protein (rp) S6 kinase, and rpS6 was increased, and the amount of eIF4E associated with its repressor 4E-BP1 was reduced after 60 and 120 min of leucine infusion. No change in these biomarkers of mRNA translation was observed in liver. Thus a physiological increase in plasma leucine stimulates protein synthesis in skeletal muscle of neonatal pigs in association with increased eIF4E availability for eIF4F assembly. This response appears to be insulin independent, substrate dependent, and tissue specific. The results suggest that the branched-chain amino acid leucine can act as a nutrient signal to stimulate protein synthesis in skeletal muscle of neonates.

Regulation of neonatal liver protein synthesis by insulin and amino acids in pigs

The high efficiency of protein deposition during the neonatal period is driven by high rates of protein synthesis, which are maximally stimulated after feeding. Infusion of amino acids, but not insulin, reproduces the feeding-induced stimulation of liver protein synthesis. To determine whether amino acid-stimulated liver protein synthesis is independent of insulin in neonates, and to examine the role of amino acids and insulin in the regulation of translation initiation in neonatal liver, we performed pancreatic glucose-amino acid clamps in overnight-fasted 7-day-old pigs. Pigs (n = 9-12/group) were infused with insulin at 0, 10, 22, and 110 ng.kg(-0.66).min(-1) to achieve 0, 2, 6, and 30 microU/ml insulin, respectively. At each insulin dose, amino acids were maintained at fasting or fed levels or, in conjunction with the highest insulin dose, allowed to fall to below fasting levels. Insulin had no effect on the fractional rate of protein synthesis in liver. Amino acids increased fractional protein synthesis rates in liver at each dose of insulin, including the 0 microU/ml dose. There was a dose-response effect of amino acids on liver protein synthesis. Amino acids and insulin increased protein S6 kinase and 4E-binding protein 1 (4E-BP1) phosphorylation; however, only amino acids decreased formation of the inactive 4E-BPI.eukaryotic initiation factor-4E (eIF4E) complex. The results suggest that amino acids regulate liver protein synthesis in the neonate by modulating the availability of eIF4E for 48S ribosomal complex formation and that this response does not require insulin.

Endotoxin induces differential regulation of mTOR-dependent signaling in skeletal muscle and liver of neonatal pigs

In the present study, differential responses of regulatory proteins involved in translation initiation in skeletal muscle and liver during sepsis were studied in neonatal pigs treated with lipopolysaccharide (LPS). LPS did not alter eukaryotic initiation factor (eIF) 2B activity in either tissue. In contrast, binding of eIF4G to eIF4E to form the active mRNA-binding complex was repressed in muscle and enhanced in liver. Phosphorylation of eIF4E-binding protein, 4E-BP1, and ribosomal protein S6 kinase, S6K1, was reduced in muscle during sepsis but increased in liver. Finally, changes in 4E-BP1 and S6K1 phosphorylation were associated with altered phosphorylation of the protein kinase mammalian target of rapamycin (mTOR). Overall, the results suggest that translation initiation in both skeletal muscle and liver is altered during neonatal sepsis by modulation of the mRNA-binding step through changes in mTOR activation. Moreover, the LPS-induced changes in factors that regulate translation initiation are more profound than previously reported changes in global rates of protein synthesis in the neonate. This finding suggests that the initiator methionyl-tRNA-rather than the mRNA-binding step in translation initiation may play a more critical role in maintaining protein synthesis rates in the neonate during sepsis.

Regulation of translation initiation by insulin and amino acids in skeletal muscle of neonatal pigs

Previous studies have shown that intravenous infusion of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates and that insulin and amino acids act independently to produce this effect. The goal of the present study was to delineate the regulatory roles of insulin and amino acids on muscle protein synthesis in neonates by examining translational control mechanisms, specifically the eukaryotic translation initiation factors (eIFs), which enable coupling of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. Insulin secretion was blocked by somatostatin in fasted 7-day-old pigs (n = 8-12/group), insulin was infused to achieve plasma levels of approximately 0, 2, 6, and 30 microU/ml, and amino acids were clamped at fasting or fed levels or, at the high insulin dose, below fasting. Both insulin and amino acids increased the phosphorylation of ribosomal protein S6 kinase (S6K1) and the eIF4E-binding protein (4E-BP1), decreased the binding of 4E-BP1 to eIF4E, increased eIF4E binding to eIF4G, and increased fractional protein synthesis rates but did not affect eIF2B activity. In the absence of insulin, amino acids had no effect on these translation initiation factors but increased the protein synthesis rates. Raising insulin from below fasting to fasting levels generally did not alter translation initiation factor activity but raised protein synthesis rates. The phosphorylation of S6K1 and 4E-BP1 and the amount of 4E-BP1 bound to eIF4E and eIF4E bound to eIF4G were correlated with insulin level, amino acid level, and protein synthesis rate. Thus insulin and amino acids regulate muscle protein synthesis in skeletal muscle of neonates by modulating the availability of eIF4E for 48S ribosomal complex assembly, although other processes also must be involved.

Modulation of intestinal protein synthesis and protease mRNA by luminal and systemic nutrients

Route of nutrient supply is important in regulation of intestinal protein metabolism, because total parenteral nutrition, compared with enteral feeding, leads to profound atrophy. Participation of the fractional rate of protein synthesis (Ks), their degradation in regulation of gut protein balance, and their possible modulation by specific nutrients are the focus of our work. We developed an in situ experimental system that allows controlled exposure of intestinal mucosa to nutrients systemically, luminally, or both. We examined the effects of systemic glucose and amino acid (AA) infusion in overnight-fasted piglets. Jejunal segments within each piglet were simultaneously, luminally perfused with solutions containing various AAs or glucose. Intravenous infusion of glucose increased mucosal Ks by 16% (P < 0.05), whereas intravenous infusion of AA had no effect on Ks. Systemic glucose infusion had no effect on mRNA levels for components of the ubiquitin-proteasome proteolytic pathway. However, levels of these mRNA were reduced by intravenous or luminal AA supply. This effect was greatest (-50%) when highest tissue concentrations of AAs were achieved by the simultaneous infusion of AA by both routes (P < 0.05). Our findings suggest that not only is the modulation of protein balance in the intestine in response to nutrients in part attributable to anabolic stimulation of protein synthesis initiated by the systemic appearance of glucose, but a fall in protein degradation is also a likely contributor. AAs appear to be a key factor required to reduce expression of genes connected with proteolysis.

Insulin and amino acids independently stimulate skeletal muscle protein synthesis in neonatal pigs

Infusion of physiological levels of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates. To determine whether insulin and amino acids independently stimulate skeletal muscle protein synthesis in neonates, insulin secretion was blocked with somatostatin in fasted 7-day-old pigs (n = 8-12/group) while glucose and glucagon were maintained at fasting levels and insulin was infused to simulate either less than fasting, fasting, intermediate, or fed insulin levels. At each dose of insulin, amino acids were clamped at either the fasting or fed level; at the highest insulin dose, amino acids were also reduced to less than fasting levels. Skeletal muscle protein synthesis was measured using a flooding dose of l-[4-(3)H]phenylalanine. Hyperinsulinemia increased protein synthesis in skeletal muscle during hypoaminoacidemia and euaminoacidemia. Hyperaminoacidemia increased muscle protein synthesis during hypoinsulinemia and euinsulinemia. There was a dose-response effect of both insulin and amino acids on muscle protein synthesis. At each insulin dose, hyperaminoacidemia increased muscle protein synthesis. The effects of insulin and amino acids on muscle protein synthesis were largely additive until maximal rates of protein synthesis were achieved. Amino acids enhanced basal protein synthesis rates but did not enhance the sensitivity or responsiveness of muscle protein synthesis to insulin. The results suggest that insulin and amino acids independently stimulate protein synthesis in skeletal muscle of the neonate.

Endotoxemia reduces skeletal muscle protein synthesis in neonates

Protein synthesis in skeletal muscle is reduced by as much as 50% as early as 4 h after a septic challenge in adults. However, the effect of sepsis on muscle protein synthesis has not been determined in neonates, a highly anabolic population whose muscle protein synthesis rates are elevated and uniquely sensitive to insulin and amino acid stimulation. Neonatal piglets (n = 10/group) were infused for 8 h with endotoxin [lipopolysaccharide (LPS), 0 and 10 microg. kg(-1). h(-1)]. Plasma amino acid and glucose concentrations were kept at the fed level by infusion of dextrose and a balanced amino acid mixture. Fractional protein synthesis rates were determined by use of a flooding dose of [(3)H]phenylalanine. LPS infusion produced a septic-like state, as indicated by an early and sustained elevation in body temperature, heart rate, and plasma tumor necrosis factor-alpha, interleukin-1, cortisol, and lactate concentrations. Plasma levels of insulin increased, whereas glucose and amino acids decreased, suggesting the absence of insulin resistance. LPS significantly reduced protein synthesis in longissimus dorsi muscle by only 11% and in gastrocnemius by only 15%, but it had no significant effect in masseter and cardiac muscles. LPS increased protein synthesis in the liver (22%), spleen (28%), kidney (53%), jejunum (19%), diaphragm (21%), lung (50%), and skin (13%), but not in the stomach, pancreas, or brain. These findings suggest that, when substrate supply is maintained, skeletal muscle protein synthesis in neonates compared with adults is relatively resistant to the catabolic effects of sepsis.

Acute IGF-I infusion stimulates protein synthesis in skeletal muscle and other tissues of neonatal pigs

Studies have shown that protein synthesis in skeletal muscle of neonatal pigs is uniquely sensitive to a physiological rise in both insulin and amino acids. Protein synthesis in cardiac muscle, skin, and spleen is responsive to insulin but not amino acid stimulation, whereas in the liver, protein synthesis responds to amino acids but not insulin. To determine the response of protein synthesis to insulin-like growth factor I (IGF-I) in this model, overnight-fasted 7- and 26-day-old pigs were infused with IGF-I (0, 20, or 50 microg. kg(-1). h(-1)) to achieve levels within the physiological range, while amino acids and glucose were clamped at fasting levels. Because IGF-I infusion lowers circulating insulin levels, an additional group of high-dose IGF-I-infused pigs was also provided replacement insulin (10 ng. kg(-0.66). min(-1)). Tissue protein synthesis was measured using a flooding dose of L-[4-(3)H]phenylalanine. In 7-day-old pigs, low-dose IGF-I increased protein synthesis by 25-60% in various skeletal muscles as well as in cardiac muscle (+38%), skin (+24%), and spleen (+32%). The higher dose of IGF-I elicited no further increase in protein synthesis above that found with the low IGF-I dose. Insulin replacement did not alter the response of protein synthesis to IGF-I in any tissue. The IGF-I-induced increases in tissue protein synthesis decreased with development. IGF-I infusion, with or without insulin replacement, had no effect on protein synthesis in liver, jejunum, pancreas, or kidney. Thus the magnitude, tissue specificity, and developmental change in the response of protein synthesis to acute physiological increases in plasma IGF-I are similar to those previously observed for insulin. This study provides in vivo data indicating that circulating IGF-I and insulin act on the same signaling components to stimulate protein synthesis and that this response is highly sensitive to stimulation in skeletal muscle of the neonate.

Stimulation of protein synthesis by both insulin and amino acids is unique to skeletal muscle in neonatal pigs

In neonatal pigs, the feeding-induced stimulation of protein synthesis in skeletal muscle, but not liver, can be reproduced by insulin infusion when essential amino acids and glucose are maintained at fasting levels. In the present study, 7- and 26-day-old pigs were studied during 1) fasting, 2) hyperinsulinemic-euglycemic-euaminoacidemic clamps, 3) euinsulinemic-euglycemic-hyperaminoacidemic clamps, and 4) hyperinsulinemic-euglycemic-hyperaminoacidemic clamps. Amino acids were clamped using a new amino acid mixture enriched in nonessential amino acids. Tissue protein synthesis was measured using a flooding dose of L-[4-(3)H]phenylalanine. In 7-day-old pigs, insulin infusion alone increased protein synthesis in various skeletal muscles (from +35 to +64%), with equivalent contribution of myofibrillar and sarcoplasmic proteins, as well as cardiac muscle (+50%), skin (+34%), and spleen (+26%). Amino acid infusion alone increased protein synthesis in skeletal muscles (from +28 to +50%), also with equivalent contribution of myofibrillar and sarcoplasmic proteins, as well as liver (+27%), pancreas (+28%), and kidney (+10%). An elevation of both insulin and amino acids did not have an additive effect. Similar qualitative results were obtained in 26-day-old pigs, but the magnitude of the stimulation of protein synthesis by insulin and/or amino acids was lower. The results suggest that, in the neonate, the stimulation of protein synthesis by feeding is mediated by either amino acids or insulin in most tissues; however, the feeding-induced stimulation of protein synthesis in skeletal muscle is uniquely regulated by both insulin and amino acids.

Developmental decline in components of signal transduction pathways regulating protein synthesis in pig muscle

Our previous studies showed that the feeding-induced stimulation of protein synthesis in skeletal muscle of neonatal pigs is accompanied by enhanced phosphorylation of the eukaryotic initiation factor (eIF)4E-binding protein (4E-BP1) and the ribosomal protein S6 kinase (S6K1). These effects of feeding are substantially reduced with development. The goal of the present investigation was to delineate the basis for the reduced responsiveness to feeding observed in the older animals. In these studies, the content and activity of protein kinases located upstream of S6K1 and 4E-BP1 in signal transduction pathways activated by amino acids, insulin, and insulin-like growth factor I were examined in 7- and 26-day-old pigs that were either fasted overnight or fed porcine milk after an overnight fast. Feeding stimulated phosphatidylinositol (PI) 3-kinase activity to the same extent in muscle of 7- and 26-day-old pigs, suggesting that PI 3-kinase is not limiting in muscle of older animals. In contrast, protein kinase B (PKB) activity was significantly less in muscle from 26- vs. 7-day-old pigs, regardless of nutritional status, suggesting that its activity is regulated by mechanisms distinct from PI 3-kinase. In part, the reduced PKB responsiveness can be attributed to a developmental decline in PKB content. Likewise, muscle content of the protein kinase termed mammalian target of rapamycin (mTOR) in 26-day-old pigs was <25% of that in 7-day-old animals. Finally, in agreement with our earlier work showing that S6K1 phosphorylation is reduced in older animals, S6K1 activity was stimulated to a lesser extent in 26- compared with 7-day-old pigs. Overall, the results suggest that the blunted protein synthetic response observed in 26- vs. 7-day-old neonatal pigs is due in part to decreased content and/or activity of signaling components downstream of PI 3-kinase, e.g., PKB, mTOR, and S6K1.

Developmental changes in the feeding-induced activation of the insulin-signaling pathway in neonatal pigs

In neonatal animals, feeding stimulates skeletal muscle protein synthesis, a response that declines with development. Both the magnitude of the feeding response and its developmental decline can be reproduced by insulin infusion, suggesting that an altered responsiveness to insulin is a primary determinant of the developmental decline in the stimulation of protein synthesis by feeding. In this study, 7- and 26-day-old pigs were either fasted overnight or fed porcine milk after an overnight fast. We examined the abundance and degree of tyrosine phosphorylation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), and IRS-2 in skeletal muscle and, for comparison, liver. We also evaluated the association of IRS-1 and IRS-2 with phosphatidylinositol 3-kinase (PI 3-kinase). The abundance of IR protein in muscle was twofold higher at 7 than at 26 days, but IRS-1 and IRS-2 abundances were similar in muscle of 7- and 26-day-old pigs. The feeding-induced phosphorylations were greater at 7 than at 26 days of age for IR (28- vs. 13-fold), IRS-1 (14- vs. 8-fold), and IRS-2 (21- vs. 12-fold) in muscle. The associations of IRS-1 and IRS-2 with PI 3-kinase were also increased by refeeding to a greater extent at 7 than at 26 days (9- vs. 5-fold and 6- vs. 4-fold, respectively). In liver, the abundance of IR, IRS-1, and IRS-2 was similar at 7 and 26 days of age. Feeding increased the activation of IR, IRS-1, IRS-2, and PI 3-kinase in liver only twofold, and these responses were unaffected by age. Thus our findings demonstrate that the feeding-induced activation of IR, IRS-1, IRS-2, and PI 3-kinase in skeletal muscle decreases with development. Further study is needed to ascertain whether the developmental decline in the feeding-induced activation of early insulin-signaling components contributes to the developmental decline in translation initiation in skeletal muscle.

Differential effects of insulin on peripheral and visceral tissue protein synthesis in neonatal pigs

We recently demonstrated in neonatal pigs that, with amino acids and glucose maintained at fasting levels, the stimulation of protein synthesis in longissimus dorsi muscle with feeding can be reproduced by a physiological rise in insulin alone. In the current report, we determine whether the response of protein synthesis to insulin in the neonatal pig is 1) present in muscles of different fiber types, 2) proportional in myofibrillar and sarcoplasmic proteins, 3) associated with increased translational efficiency and ribosome number, and 4) present in other peripheral tissues and in viscera. Hyperinsulinemic-euglycemic-amino acid clamps were performed in 7- and 26-day-old pigs infused with 0, 30, 100, or 1,000 ng. kg(-0.66). min(-1) of insulin to reproduce insulin levels present in fasted, fed, refed, and supraphysiological conditions, respectively. Tissue protein synthesis was measured using a flooding dose of L-[4-(3)H]phenylalanine. Insulin increased protein synthesis in gastrocnemius muscle and, to a lesser degree, masseter muscle. The degree of stimulation of protein synthesis by insulin was similar in myofibrillar and sarcoplasmic proteins. Insulin increased translational efficiency but had no effect on ribosome number in muscle. All of these insulin-induced changes in muscle protein synthesis decreased with age. Insulin also stimulated protein synthesis in cardiac muscle and skin but not in liver, intestine, spleen, pancreas, or kidney. The results support the hypothesis that insulin mediates the feeding-induced stimulation of myofibrillar and sarcoplasmic protein synthesis in muscles of different fiber types in the neonate by increasing the efficiency of translation. However, insulin does not appear to be involved in the feeding-induced stimulation of protein synthesis in visceral tissues. Thus different mechanisms regulate the growth of peripheral and visceral tissues in the neonate.

Developmental changes in the feeding-induced stimulation of translation initiation in muscle of neonatal pigs

The rapid gain in skeletal muscle mass in the neonate is associated with a marked elevation in skeletal muscle protein synthesis in response to feeding. The feeding-induced response decreases with development. To determine whether the response to feeding is regulated at the level of translation initiation, the expression, phosphorylation, and function of a number of eukaryotic initiation factors (eIF) were examined. Pigs at 7 and 26 days of age were either fasted overnight or fed porcine milk after an overnight fast. In muscle of 7-day-old pigs, the hyperphosphorylated form of the eIF4E repressor protein, 4E-binding protein 1 (4E-BP1), was undetectable in the fasting state but rose to 60% of total 4E-BP1 after feeding; eIF4E phosphorylation was unaffected by feeding status. The amount of eIF4E in the inactive 4E-BP1. eIF4E complex was reduced by 80%, and the amount of eIF4E in the active eIF4E. eIF4G complex was increased 14-fold in muscle of 7-day-old pigs after feeding. The amount of 70-kDa ribosomal protein S6 (p70(S6)) kinase in the hyperphosphorylated form rose 2.5-fold in muscle of 7-day-old pigs after feeding. Each of these feeding-induced responses was blunted in muscle of 26-day-old pigs. eIF2B activity in muscle was unaffected by feeding status but decreased with development. Feeding produced similar changes in eIF characteristics in liver and muscle; however, the developmental changes in liver were not as apparent as in skeletal muscle. Thus the results demonstrate that the developmental change in the acute stimulation of skeletal muscle protein synthesis by feeding is regulated by the availability of eIF4E for 48S ribosomal complex formation. The results further suggest that the overall developmental decline in skeletal muscle protein synthesis involves regulation by eIF2B.

Feeding stimulates protein synthesis in muscle and liver of neonatal pigs through an mTOR-dependent process

Protein synthesis is repressed in both skeletal muscle and liver after a short-term fast and is rapidly stimulated in response to feeding. Previous studies in rats and pigs have shown that the feeding-induced stimulation of protein synthesis is associated with activation of the 70-kDa ribosomal protein S6 kinase (S6K1) as well as enhanced binding of eukaryotic initiation factor eIF4E to eIF4G to form the active eIF4F complex. In cells in culture, hormones and nutrients regulate both of these events through a protein kinase termed the mammalian target of rapamycin (mTOR). In the present study, the involvement of mTOR in the feeding-induced stimulation of protein synthesis in skeletal muscle and liver was examined. Pigs at 7 days of age were fasted for 18 h, and then one-half of the animals were fed. In addition, one-half of the animals in each group were administered rapamycin (0.75 mg/kg) 2 h before feeding. The results reveal that treating 18-h fasted pigs with rapamycin, a specific inhibitor of mTOR, before feeding prevented the activation of S6K1 and the changes in eIF4F complex formation observed in skeletal muscle and liver after feeding. Rapamycin also ablated the feeding-induced stimulation of protein synthesis in liver. In contrast, in skeletal muscle, rapamycin attenuated, but did not prevent, the stimulation of protein synthesis in response to feeding. The results suggest that feeding stimulates hepatic protein synthesis through an mTOR-dependent process involving enhanced eIF4F complex formation and activation of S6K1. However, in skeletal muscle, these two processes may account for only part of the stimulation of protein synthesis, and thus additional steps may be involved in the response.

Somatotropin increases protein balance independent of insulin's effects on protein metabolism in growing pigs

Somatotropin (ST) administration enhances protein deposition and elicits profound metabolic responses, including hyperinsulinemia. To determine whether the anabolic effect of ST is due to hyperinsulinemia, pair-fed weight-matched growing swine were treated with porcine ST (150 microg x kg body wt(-1) x day(-1)) or diluent for 7 days (n = 6/group, approximately 20 kg). Then pancreatic glucose-amino acid clamps were performed after an overnight fast. The objective was to reproduce the insulin levels of 1) fasted control and ST pigs (basal insulin, 5 microU/ml), 2) fed control pigs (low insulin, 20 microU/ml), and 3) fed ST pigs (high insulin, 50 microU/ml). Amino acid and glucose disposal rates were determined from the infusion rates necessary to maintain preclamp blood levels of these substrates. Whole body nonoxidative leucine disposal (NOLD), leucine appearance (R(a)), and leucine oxidation were determined with primed, continuous infusions of [(13)C]leucine and [(14)C]bicarbonate. ST treatment was associated with higher NOLD and protein balance and lower leucine oxidation and amino acid and glucose disposals. Insulin lowered R(a) and increased leucine oxidation, protein balance, and amino acid and glucose disposals. These effects of insulin were suppressed by ST treatment; however, the protein balance remained higher in ST pigs. The results show that ST treatment inhibits insulin's effects on protein metabolism and indicate that the stimulation of protein deposition by ST treatment is not mediated by insulin. Comparison of the protein metabolic responses to ST treatment during the basal fasting period with those in the fully fed state from a previous study suggests that the mechanism by which ST treatment enhances protein deposition is influenced by feeding status.

Regulation of myofibrillar protein turnover during maturation in normal and undernourished rat pups

The study tested the hypothesis that a higher rate of myofibrillar than sarcoplasmic protein synthesis is responsible for the rapid postdifferentiation accumulation of myofibrils and that an inadequate nutrient intake will compromise primarily myofibrillar protein synthesis. Myofibrillar (total and individual) and sarcoplasmic protein synthesis, accretion, and degradation rates were measured in vivo in well-nourished (C) rat pups at 6, 15, and 28 days of age and compared at 6 and 15 days of age with pups undernourished (UN) from birth. In 6-day-old C pups, a higher myofibrillar than sarcoplasmic protein synthesis rate accounted for the greater deposition of myofibrillar than sarcoplasmic proteins. The fractional synthesis rates of both protein compartments decreased with age, but to a greater degree for myofibrillar proteins (-54 vs. -42%). These decreases in synthesis rates were partially offset by reductions in degradation rates, and from 15 days, myofibrillar and sarcoplasmic proteins were deposited in constant proportion to one another. Undernutrition reduced both myofibrillar and sarcoplasmic protein synthesis rates, and the effect was greater at 6 (-25%) than 15 days (-15%). Decreases in their respective degradation rates minimized the effect of undernutrition on sarcoplasmic protein accretion from 4 to 8 days and on myofibrillar proteins from 13 to 17 days. Although these adaptations in protein turnover reduced overall growth of muscle mass, they mitigated the effects of undernutrition on the normal maturational changes in myofibrillar protein concentration.

Somatotropin increases protein balance by lowering body protein degradation in fed, growing pigs

Somatotropin (ST) administration enhances protein deposition in well-nourished, growing animals. To determine whether the anabolic effect is due to an increase in protein synthesis or a decrease in proteolysis, pair-fed, weight-matched ( approximately 20 kg) growing swine were treated with porcine ST (150 microg. kg(-1). day(-1), n = 6) or diluent (n = 6) for 7 days. Whole body leucine appearance (R(a)), nonoxidative leucine disposal (NOLD), urea production, and leucine oxidation, as well as tissue protein synthesis (K(s)), were determined in the fed steady state using primed continuous infusions of [(13)C]leucine, [(13)C]bicarbonate, and [(15)N(2)]urea. ST treatment increased the efficiency with which the diet was used for growth. ST treatment also increased plasma insulin-like growth factor I (+100%) and insulin (+125%) concentrations and decreased plasma urea nitrogen concentrations (-53%). ST-treated pigs had lower leucine R(a) (-33%), leucine oxidation (-63%), and urea production (-70%). However, ST treatment altered neither NOLD nor K(s) in the longissimus dorsi, semitendinosus, or gastrocnemius muscles, liver, or jejunum. The results suggest that in the fed state, ST treatment of growing swine increases protein deposition primarily through a suppression of protein degradation and amino acid catabolism rather than a stimulation of protein synthesis.