Rates Of Protein Synthesis In Liver Research Articles

The Effects of Age and Adiposity on Casein Digestibility and Tissue Protein Synthesis in Rats

ObjectivesAge and adiposity can impact the digestibility of dietary proteins and the metabolic response to their ingestion. The objective was to evaluate the effects of age and adiposity on casein digestibility and protein synthesis in tissues and organs. MethodsWistar rats of 1 month (n = 15) and 10 months (n = 15) at their arrival were fed ad libitum with a standard diet or High Fat diet to obtain rats of normal and high adiposity levels. Four groups were constituted (n = 7/8): 2 months/normal adiposity, 2 months/high adiposity, 11 months/normal adiposity and 11 months/high adiposity. At the end of the dietary intervention, they were fed the standard diet for 1 week before the test meal. The rats consumed a 4g meal containing 15N-labeled casein (Prodiet® 85B). Six hours after meal ingestion, the rats were euthanized. Intravenous injection of a massive dose of 13C-valine prior to euthanasia was used to determine protein synthesis rate in liver, kidneys, skin and muscle. Body composition was evaluated and digestive contents were collected to measure casein digestibility. ResultsNo weight difference between rats of the same age was observed. However, a significant difference in adiposity was noted, with a surge in body fat of 3% in young rats and 7% in older rats. Digestibility increased with a higher adiposity level (P = 0.04). In young rats, it was 94.1 ± 1.1% in lean rats and 95.2 ± 1.7% in fat rats. In older rats, it was 94.5 ± 2.2% and 95.8 ± 0.7%, in lean and fat rats respectively. Significant effects of age (P < 0.01) and adiposity (P < 0.01) were observed in the muscle fractional synthesis rate (FSR), with age decreasing it and adiposity increasing it. In young rats, FSR was 10.1 ± 2.1%/day and 12.0 ± 3.0%/day in lean and fat rats, respectively, these values being 6.2 ± 1.5%/day and 10.6 ± 2.0%/day in older rats. In the skin, younger rats exhibited a higher FSR (P < 0.01) as it was 11.1 ± 2.6%/day and 12.6 ± 3.7%/day in lean and fat rats respectively, and 8.3 ± 2.3%/day and 8.2 ± 2.7%/day in older rats. No differences were found for the liver and kidneys. ConclusionsProtein synthesis in muscle decreased with age while adiposity increased it. This is consistent with an improvement in ribosomal activity at an intermediate state of obesity. The surge in casein digestibility with higher adiposity, although moderate, could have contributed to the improvement in muscle anabolism response. Funding SourcesIngredia.

Tumour-specific and organ-specific protein synthesis rates in patients with pancreatic cancer.

BackgroundLiving tissues maintain a fine balance between protein synthesis and protein breakdown rates. Animal studies indicate that protein synthesis rates are higher in organs when compared with skeletal muscle tissue. As such, organ and tumour protein synthesis could have major effects on whole‐body protein metabolism in wasting disorders such as cancer cachexia. We aimed to assess protein synthesis rates in pancreatic tumour tissue and healthy pancreas, liver, and skeletal muscle tissue in vivo in humans.MethodsIn eight patients with pancreatic cancer undergoing pancreaticoduodenectomy, primed continuous infusions with L‐[ring‐13C6]phenylalanine and L‐[3,5‐2H2]tyrosine were started prior to surgery and continued throughout the surgical procedures. During surgery, plasma samples and biopsies from the pancreas, pancreatic tumour, liver, and vastus lateralis muscle were taken. Post‐absorptive fractional protein synthesis rates were determined by measuring incorporation of labelled L‐[ring‐13C6]phenylalanine in tissue protein using the weighed plasma L‐[ring‐13C6]phenylalanine enrichments as the precursor pool.ResultsFive male patients and three female patients with a mean age of 67 ± 2 years were included into this study. Plasma L‐[ring‐13C6]phenylalanine enrichments (6–9 mole per cent excess) did not change during surgery (P = 0.60). Pancreatic tumour protein synthesis rates were 2.6‐fold lower than surrounding pancreatic tissue protein synthesis rates (0.268 ± 0.053 vs. 0.694 ± 0.228%/h, respectively; P = 0.028) and 1.7‐fold lower than liver protein synthesis rates (0.268 ± 0.053 vs. 0.448 ± 0.043%/h, respectively; P = 0.046). Among healthy organ samples, protein synthesis rates were 20‐fold and 13‐fold higher in pancreas and liver, respectively, compared with skeletal muscle tissue (0.694 ± 0.228 and 0.448 ± 0.043 vs. 0.035 ± 0.005%/h, respectively; P < 0.05).ConclusionsLiver and pancreas tissue protein synthesis rates are higher when compared with pancreatic tumour and skeletal muscle tissue protein synthesis rates and can, therefore, strongly impact whole‐body protein metabolism in vivo in humans.

Effects of mustard oil cake on liver proteins of Channa punctatus (Bloch).

Mustard oil cake is a biofertilizer widely used in agriculture and fish cultivation almost in all South East Asian Countries including India. The study was carried out to observe the effects of this biofertilizer on the liver proteins of Channa punctatus. At sublethal concentration (0.42 g/L ), fishes were exposed for a prolonged period of 35 days and amount of total liver protein (TLP) was measured. The investigation showed a low rate of liver protein synthesis in treated fish after 4 days of exposure. An increase in the amount of protein was observed between the 7th and 35th day. But such increment was below the amount of TLP of control fish, indicating physiological stress in the treated fish.

Effect of high chronic intake of sucrose on liver metabolism in aging rats. Modulation by rutin and micronutrients.

High-sugar intake and senescence share common deleterious effects, in particular in liver, but combination of these two factors was little studied. Our aims were to examine the effect of a high-sucrose diet in liver of old rats and also the potential benefices of a polyphenol/micronutrient supplementation. Four groups of 22-month-old male rats fed during 5months with a diet containing either 13 or 62% sucrose, supplemented or not with rutin, vitamin E, A, D, selenium, and zinc were compared. We measured liver macronutrient composition, glycation/oxidative stress, enzyme activities (lipogenesis, β-oxidation, fructokinase), gene expression (enzymes and transcription factors), in vivo protein synthesis rates and plasma parameters. Sucrose induced an increase in plasma and liver lipid content, and a stimulation of liver protein synthesis rates. Gene expression was little changed by sucrose, with lower levels for LXR-α and LXR-β. Polyphenol/micronutrient supplementation tended to limit liver triglyceride infiltration through variations in fatty acid synthase, acyl coA oxidase, and possibly ATP-citrate lyase activities. In conclusion, despite differences in enzymatic regulations, and blunted responses of gene expression, high-sucrose diet was still able to induce a marked increase in liver lipid content in old animals. However, it probably attenuated the positive impact of polyphenol/micronutrients.

Maslinic acid and ration size enhanced hepatic protein-turnover rates of gilthead sea bream (Sparus aurata)

Maslinic acid (MA) is a natural triterpene that can be used as an additive in the diet of trout. We investigated the effects of feeding with MA and a fixed ration (10 g kg−1 body weight) with respect to growth, protein-turnover rates and nucleic-acid concentration in the liver of gilthead sea bream grown under fish-farm conditions. Five groups of 12 g of a mean body mass were fed for 210 days with diets containing 0 (control), 0.05 and 0.1 g of MA per kg of diet. Two groups were fed ad libitum (control AL and MA100AL), and three with a fixed ration (control R, MA50R and MA100R). At the end of the experiment, higher body weights, liver weight, feed-efficiency and PER were found in MA100AL and MA100R fish. Fractional and absolute protein-synthesis rates in liver of MA100R fish were higher than in the control, resulting in a higher absolute protein-accumulation rate and tissue growth. Total DNA content in MA100AL and MA100R was higher than in control. Studies of light and electron microscopy corroborated these results. These findings indicate that MA added to the diet can stimulate growth, hepatic protein-turnover rates and tissue hyperplasia in gilthead sea bream.

Probiotics Stimulate Liver and Plasma Protein Synthesis in Piglets with Dextran Sulfate-Induced Colitis and Macronutrient Restriction

Adequate nutrition and probiotics have each been shown to reduce the severity of colitis, but their impact on hepatic and gastrointestinal protein metabolism has not been studied. Our objective was to determine whether maintaining adequate nutrition compared with administering probiotics affected protein synthesis, colon histopathology, and oxidative stress in our macronutrient-restricted piglet model of colitis. Piglets (n = 8/group) receiving dextran sulfate to induce colitis were randomized to 3 treatment groups: macronutrient restricted (MR); macronutrient restricted with VSL #3 probiotics (MRP), or well nourished (WNC). An additional 8 piglets served as healthy references for comparative purposes given the unique nature of the experimental model. A primed, constant infusion of the tracer L-[ring-2H5]phenylalanine was performed in colitis piglets after 14 d to determine the fractional synthesis rates of proteins in small intestinal mucosa, colon, and liver and of plasma proteins (total protein, fibrinogen, albumin). Colon histopathology and oxidative stress were also assessed. Compared with MR piglets, both WNC and MRP piglets had higher protein synthesis rates in liver and plasma protein pools. However, only adequate nutrition increased protein synthesis in the colon and decreased colitis severity. Whereas probiotics did not stimulate gastrointestinal protein synthesis or reduce colitis severity, a still-unidentified signaling mechanism between the gut and liver seems to be responsible for the probiotic-induced increase in liver protein and plasma protein synthesis. These data underscore the importance of maintaining nutrient intake in pediatric patients with gastrointestinal disease. A strategy for correcting compromised nutrition seems to be more beneficial for reducing damage during colitis than using probiotics only.

Synthesis rates of total liver protein and albumin are both increased in patients with an acute inflammatory response

The general perception that catabolism and inflammation are associated with a high synthesis rate of total liver protein and a low albumin synthesis rate has been challenged in recent years by several studies in man, indicating that the synthesis rate of albumin in response to a catabolic insult is increased rather than decreased. Thus changes in liver protein synthesis rates in conjunction with catabolism and acute inflammation in man need to be characterized better. The aim of the present study was to measure protein synthesis rates of total liver protein and albumin during a state of acute inflammation. Patients (n = 10) undergoing acute laparoscopic cholecystectomy due to acute cholecystitis were investigated. FSRs (fractional synthesis rates) of total liver protein (liver biopsy specimens) and albumin (plasma samples) were investigated as early as possible during the surgical procedure, using a flooding dose of L-[2H5]phenylalanine. The results were compared with a reference group of patients without cholecystitis undergoing elective laparoscopic cholecystectomy (n = 17). FSR of total liver protein was 60% higher (P < 0.001) and the FSR of albumin was 45% higher (P < 0.01) in the cholecystitis patients compared with the control group. In conclusion, the synthesis rates of total liver protein and albumin are both increased in patients with an acute general inflammatory reaction undergoing laparoscopic cholecystectomy.

Response of protein synthesis to hypercapnia in rats: independent effects of acidosis and hypothermia

Acute metabolic acidosis has been shown to inhibit muscle protein synthesis, although little is known on the effect of acidosis of respiratory origin. The aim of this study was to investigate the effect of acute respiratory acidosis on tissue protein synthesis. Rats (n = 8) were made acidotic by increasing the CO 2 content of inspired air to 12% for 1 hour. Similar rats breathing normal air served as controls (n = 8). Muscle and liver protein synthesis rates were then measured with l-[ 2H 5]phenylalanine (150 μmol per 100 g body weight, 40 mol%). The results show that protein synthesis is severely depressed in skeletal muscle (−44% in gastrocnemius, −39% in plantaris, and −24% in soleus muscles, P < .01) and liver (−20%, P < .001) in acidotic animals. However, because breathing CO 2-enriched air was found to lower body temperature by approximately 2°C, in a second experiment (n = 10), the difference in body temperature between treated and control animals was minimized by gently wrapping rats breathing CO 2-enriched air in porous cloths. This second experiment confirmed that respiratory acidosis depresses protein synthesis in muscle (−22% in gastrocnemius, P < .001; −19% in plantaris, P < .01; and −4% in soleus, P = NS). However, no effect on liver protein synthesis could be detected, suggesting that liver protein synthesis may be sensitive to changes in body temperature but is not affected by acute respiratory acidosis for 1 hour. The results show that respiratory acidosis inhibits protein synthesis in skeletal muscle and indicates that acidosis, whether of metabolic or respiratory origin, may contribute to loss of muscle protein in patients with compromised renal or respiratory function.

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.

Deficiency of dietary EAA preferentially inhibits mRNA translation of ribosomal proteins in liver of meal-fed rats.

The goal of these studies was to investigate the mechanisms by which amino acid supply regulates global rates of protein synthesis as well as the translation of ribosomal protein (rp) mRNAs in liver. In the experiments conducted, male weanling rats were trained over a 2-wk period to consume their daily food intake within 3 h. On day 14, rats were fed the control diet or an isocaloric, isonitrogenous diet lacking glycine, tryptophan, leucine, or the branched-chain amino acids (BCAA) for 1 h. Feeding Trp-, Leu-, or BCAA-deficient diets resulted in significant reductions in serum insulin, hepatic protein synthesis, eukaryotic initiation factor 2B (eIF2B) activity, and phosphorylation of eIF4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase (S6K1). Phosphorylation of eIF2alpha was inversely related to eIF2B activity under all conditions. Alterations in the hepatic synthesis of rp were assessed by changes in the distribution of rp (S4, S8, L26) mRNAs across sucrose density gradients and compared with non-rp (beta-actin, albumin) mRNAs. In all dietary treatments, non-rp mRNAs were mostly polysome associated. Conversely, the proportion of rp mRNAs residing in polysomes was two- to fivefold less in rats fed diets lacking tryptophan, leucine, or BCAA compared with rats fed the control diet. Total hepatic abundance of all mRNAs examined did not differ among treatment groups. For all parameters examined, there were no differences between rats fed the glycine-deficient diet and rats fed the control diet. The data suggest that essential amino acid (EAA) deficiency inhibits global rates of liver protein synthesis via a block in translation initiation. Additionally, the translation of rp mRNAs is preferentially repressed in association with decreased S6K1 phosphorylation.

Responses in the absorptive phase in muscle and liver protein synthesis rates of growing rats

The effect of time after beginning of a meal (30, 60, 90 and 120 min) on liver and gastrocnemius muscle protein synthesis was tested in growing male rats using the large dose technique, based on a 10 min exposure to [15N]phenylalanine. The fractional synthesis rate was estimated from the ratio between the atom percent excess of tissue protein‐bound and free labelled phenylalanine. The latter was measured by gas chromatography mass spectrometry using the tertiary‐butyldimethylsilyl amino acid derivatives. The protein‐bound phenylalanine of gastrocnemius muscle was separated from the other amino acids using preparative amino acid chromatography and then oxidised to N2 in an automated carbon‐nitrogen Roboprep (CN) combustion module attached to a continuous flow isotope ratio mass spectrometer (IRMS), with m/z ions 28 and 29 monitored. The protein‐bound phenylalanine from liver was separated by a gas Chromatograph attached to a sample preparation module and an isotope ratio mass spectrometer (GC‐C‐IRMS), with again m/z ions of 28 and 29 monitored. The following results were obtained: the daily fractional protein synthesis rates (ks) in gastrocnemius muscle and liver were 13.9% and 65.6% respectively, in 12h fasted 145 g rats. These ks increased within 30 min after ingestion of meal to 14.9% and 91.8% for muscle and liver, respectively, and remained at these values for the next 90 min (14.6% and 87.4% at 60 min, and 14.3% and 88.6% at 120 min after the beginning of feeding). It was concluded that measurement of protein synthesis rates characteristic for the absorptive phase can be undertaken in a period from thirty minutes to two hours after start of a meal, without significant changes in the ks values.

Repartitioning of maternal muscle protein towards the foetus induced by a polyclonal antiserum to rat GH.

Growth and protein accretion were studied in maternal muscle and liver and in foetuses of rats on day 20 of pregnancy. In young rats, weighing 120 g at mating, muscle mass and protein content of three hind-limb muscles, soleus, plantaris and gastrocnemius, increased on average by 7% compared with non-pregnant controls although the rate of muscle protein synthesis was decreased. In mature rats, rates of muscle protein synthesis were also reduced on day 20 of pregnancy but no change in muscle mass was observed. Rates of liver protein synthesis and accretion were increased in the pregnant animals; the effect was larger in the young pregnant rat. Administration of an antibody to rat GH (anti-rGH) for 10 days to young pregnant rats reversed the effect on the same three maternal muscles and resulted in a 9-11% lower muscle mass and protein content, compared with control pregnant animals. In both young and mature dams serum IGF-I concentrations were halved on day 20 of pregnancy, a further small reduction was observed in response to anti-rGH. No significant change in serum insulin or corticosterone levels was observed. Anti-rGH treatment also reduced food intake but foetal weight at 20 days was significantly increased (14%). The effects on maternal muscle were not the result of loss of appetite associated with anti-GH administration as, in rats pair-fed to the intake of the anti-rGH group, maternal muscle and foetal weights were the same as in animals with food available ad libitum. The data suggest that the GH/IGF axis is involved in the partitioning of nutrients between the dam and the foetus.

Relationship of Protein Synthesis to mRNA Levels in the Liver of Chicks under Various Nutritional Conditions

The rate of liver protein synthesis in vivo and hepatic messenger ribonucleic acid (mRNA) levels were measured to examine the mechanisms regulating liver protein synthesis in chicks under various nutritional conditions. Fractional synthesis rate (FSR) of liver protein was measured by using a large dose injection of L-[4-3H]phenylalanine. Poly(A)+RNA was extracted as mRNA from total RNA using poly(U)-Sepharose 4B affinity chromatography. The influence of varying dietary protein levels (0–60%) on protein synthesis and mRNA concentration in the liver of chicks was examined. Both FSR (%/d) and absolute synthesis rate (ASR, mg/d) of liver protein increased with increasing dietary protein levels from 0 to 20% (the dietary protein requirement), whereas when dietary protein increased from 20 to 40%, both rates decreased significantly. This change was accounted for by changes in RNA concentration (RNA:protein ratio) and hepatic mRNA content. Second, the time course of changes in liver protein synthesis and hepatic mRNA content were measured in force-fed chicks that had been deprived of food for 2 d. Liver protein synthesis (FSR) was significantly greater 30 min after refeeding compared with that of unfed chicks. Liver protein synthesis (FSR) correlated with the activity of RNA to synthesize protein (r = 0.61, P < 0.001), and ASR was correlated with hepatic mRNA content (r = 0.37, P = 0.030). Third, experimental diets containing individual nutrients were force-fed to chicks after 2 d of food deprivation. When chicks were refed carbohydrate and protein diets, liver protein synthesis (FSR) tended to be greater by 24% (P = 0.063) and 22% (P = 0.075) at 30 min after refeeding compared with that of unfed chicks, respectively. These findings indicate that changes in the rate of liver protein synthesis in vivo due to the alteration in nutritional conditions are, at least partially, accounted for by not only RNA concentration and the activity of RNA synthesizing protein but also hepatic mRNA levels.

Prolonged Changes in Protein and Amino Acid Metabolism after Zymosan Treatment in Rats

1. Intraperitoneal injections of zymosan were given to rats, according to a modified procedure, in order to create a pattern of illness with an acute critical phase for 36 h followed by a prolonged recovery phase lasting for at least 10 days. Changes in amino acid and protein metabolism were studied in both phases. 2. Differences between this modified and the original zymosan model are a lower mortality (16%), which is limited to the first 36 critical hours, and the absence of signs of severe illness during the prolonged recovery phase. 3. Wasting of muscle protein and decreased protein synthesis rates in muscle were observed during the acute phase of illness. Liver size and liver protein synthesis rates were increased during the same period. The decrease in the total amount of muscle protein and the increase in liver weight were still present 12 days after zymosan treatment, despite a normalization of protein synthesis rates. Large decreases were observed in the concentrations of the conditionally essential amino acids glutamine and arginine in muscle over 6 days. Decreases in plasma glutamine and arginine on day 12 after zymosan indicated that the rats were still not fully recovered on this day. 4. We conclude that injection of a single dose of zymosan in rats leads to metabolic derangements both during the acute phase of critical illness and during the prolonged recovery phase. The model seems suited for investigating the biochemical mechanisms behind these metabolic derangements and for studying therapeutic and nutritional interventions during recovery from critical illness.

Responses of tissue protein synthesis to nutrient intake in rats exposed to interleukin-1 beta or turpentine.

1. The influence of an acute-phase reaction on the ability of protein synthesis rates in liver and three different muscles (gastrocnemius, soleus and heart) to respond to a short intravenous infusion of nutrients (glucose plus amino acids) was investigated during experimental inflammation induced by injection of human recombinant interleukin-1 beta or turpentine in young male rats. 2. Interleukin-1 beta induced a consistent increase of 3 degrees C in body temperature between 3 and 5 h after injection, whereas turpentine induced a delayed fever, peaking by 13 h. 3. Interleukin-1 beta and turpentine stimulated fractional rates of protein synthesis in liver. The synthesis rate was inhibited by interleukin-1 beta in gastrocnemius and soleus muscle, but an elevation was seen in heart muscle. In this study there was no significant response of muscle to turpentine injection. 4. Two hours of parenteral nutrition increased fractional synthesis rates in all tissues when compared with Ringer's lactate. Somewhat larger responses to feeding were observed as a result of either interleukin-1 beta or turpentine injection in all tissues, but these improvements were not significant. 5. We conclude that the response of protein synthesis rates in liver and skeletal muscle to parenteral nutrition is not inhibited, and may be somewhat enhanced, during acute inflammatory conditions in the growing rat.

Recombinant Porcine Somatotropin and Dietary Protein Enhance Protein Synthesis in Growing Pigs

The effects of a daily porcine somatotropin injection on protein synthesis rate in muscle (longissimus), liver and intestine, as influenced by dietary protein, were investigated in 17 pigs. The measurements were made at wk 3 of treatment following 1 wk for adaptation to the diet and 1 wk for determination of nitrogen balance. The fractional rates of protein synthesis in the muscle, liver and intestine were measured using a flooding dose of L-[1-13C]valine. Positive responses of weight gain and nitrogen balance were observed, primarily at higher dietary protein intake, after porcine somatotropin treatment. As expected, porcine somatotropin-treated pigs had a higher proportion of muscle and less fat. Fractional protein synthesis rate was 16% higher in the liver of porcine somatotropin-treated pigs (P < 0.05). In the longissimus muscle fractional protein synthesis rate increased with porcine somatotropin dose from 3.2 to 3.7%/d and from 4.1 to 5.1%/d at low and high protein intake, respectively (P < 0.05). The effect of dietary protein on fractional protein synthesis rate in longissimus was significant, but there was no porcine somatotropin x protein interaction. Ribonucleic acid concentration followed the same pattern as fractional protein synthesis rate in liver and longissimus. In the duodenal tissue, porcine somatotropin treatment depressed fractional protein synthesis rate (P < 0.05) without an effect of dietary protein and RNA concentration did not change. In porcine somatotropin compared with placebo-treated pigs, plasma glucose, insulin and insulin-like growth factor-I concentrations were elevated whereas plasma thyroxine was depressed and plasma triiodothyronine remained constant. There was no clear effect of dietary protein on plasma hormones. We concluded that, in pigs fed an adequate level of protein, porcine somatotropin stimulates protein synthesis in the liver and the muscle, primarily through increased ribosomal capacity.

Porcine Colostrum and Milk Stimulate Visceral Organ and Skeletal Muscle Protein Synthesis in Neonatal Piglets

Our objective was to determine the relative contributions of protein synthesis and protein absorption in the rapid accretion of gastrointestinal protein in suckling piglets during the early neonatal period. We measured the rates of tissue protein synthesis using a flooding dose of L-[4-3H]phenylalanine in various visceral and peripheral tissues of neonatal piglets fed water, mature milk or colostrum for 6 h. The jejunal and ileal protein synthesis rates in piglets fed either colostrum or milk were three- to fourfold higher than in piglets fed water. The increased jejunal and ileal protein synthesis could not, however, account for the differences in protein mass between the colostrum-fed and water-fed groups. The relative abundance of IgG, a major porcine colostral protein, in jejunal tissue was markedly higher in piglets fed colostrum than in piglets fed either milk or water. The fractional protein synthesis rates in liver, kidney, spleen and skeletal muscle and the absolute protein synthesis rates in liver and spleen were also greater in piglets fed colostrum than in those fed milk or water. Increased endogenous protein synthesis made only a minor contribution to the increased intestinal protein accretion in neonatal piglets fed colostrum. A much larger proportion of this increase seemed to be a result of absorption and retention of ingested immunoglobulins.

Effect of dietary tryptophan on muscle, liver and whole-body protein synthesis in weaned piglets: relationship to plasma insulin

Two experiments were carried out with piglets, 3-5 kg live weight, to evaluate the effects of feeding a tryptophan (TRP)-deficient diet for 2 weeks on protein synthesis rates measured in vivo 2 h after a meal. In the first experiment on twenty piglets fed on 250 g protein/kg diets, TRP deficiency (0.77 g/16 g nitrogen) as compared with adequacy (1.17 g/16 g N) significantly decreased feed intake, growth performance and fractional protein synthesis rates (FSR), without variation of RNA in longissimus dorsi (LD) and with parallel increases in RNA in semitendinosus (ST) muscle and liver. In the second experiment thirty-two piglets were tube-fed deficient and adequate diets at the two feeding levels (LF) previously achieved. Both TRP and LF significantly increased growth performance and FSR, but not RNA, in LD and ST muscle, with a trend to a synergy between the two factors (TRP x LF interaction). In another muscle, trapezius (TR), the same interaction was only apparent in RNA content. Among the three muscles it was in LD that FSR was the most responsive to dietary TRP (significant muscle x TRP interaction). In the liver the TRP x LF interaction on FSR and not RNA was the major significant effect, indicating that higher TRP and higher LF were both required to get the maximum protein synthesis rate. At 30 min after a meal the same significant interaction effect was shown on plasma glucose, whilst the higher LF increased plasma insulin with both diets. After a further 30 min the appearance of a similar significant effect of the TRP x LF interaction on plasma insulin resulted from its abatement when the deficient diet had been fed at high LF. These results suggest that dietary TRP deficiency decreased muscle and liver protein synthesis rates in relation to a decrease in the post-prandial release of insulin following a decreased rate of nutrient absorption.

Relationship Between Protein Intake and Hepatic Protein Synthesis in Rats

Protein synthesis has been measured in vitro in postmitochondrial extracts from livers of rats fed levels of casein ranging from 0 to 40% by weight. The maximal capacity for protein synthesis per milligram of RNA, measured with each amino acid added at 250 mumol/L, was 40-60% higher in rats fed a protein-free diet than in those fed 6 or 15% casein. Our results suggest that the livers of rats fed a protein-free diet are primed for the synthesis of tissue proteins and, given an adequate supply of amino acids, the rate of protein synthesis would be as high as or higher than the rate in protein-replete animals. When amino acids were added to the in vitro system at concentrations found in plasma of rats fed 0, 6, 15 or 40% casein the rate of protein synthesis increased by three- to fourfold over this range, with the highest rate observed for the 15% dietary casein level. We conclude that when protein intake is below the requirement level, the rate of liver protein synthesis may be limited by amino acid supply, by the capacity of the system for protein synthesis or by both.

Influence of feeding on protein metabolism in Atlantic salmon ( Salmo salar)

Two experiments were conducted to study the post-prandial changes in protein synthesis rate of different tissues in juvenile Atlantic salmon after an i.v. injection of a large dose (1.5 μmole/g) of L-[2,3- 3H]-leucine. In the first experiment, the changes in protein synthesis rate of the muscle and scales were followed over a period of 18 h after a meal, in salmon (mean body weight 43.4 g) starved for 1 week. The rate of protein synthesis in muscle increased until 18 h after feeding. No significant changes were observed in the protein synthesis rate of the scales. In the second experiment, the salmon (mean body weight 36.8 g) were regularly fed on a commercial diet. Immediate analysis of protein metabolism (2 h after the morning meal) as compared with analysis after an overnight fast showed no significant effect of feeding on the protein synthesis rate in liver, gill, digestive tract and muscle.