Absolute Rates Of Protein Synthesis Research Articles

Hepatic protein synthesis in suckling rats: effects of stage of development and fasting.

We studied the developmental changes in hepatic protein synthesis in suckling rats between postpartum d 1 and 28 and investigated the effect of fasting for 10 or 18 h on hepatic protein turnover at postpartum d 5, 10, 16, and 28. Fractional protein synthesis rates (KS, %/d) were measured in vivo using a flooding dose of L-[4-3H]phenylalanine. Although hepatic KS and translation efficiency (protein synthesis/unit RNA) were significantly higher at postpartum d 28 than d 1, the pattern of change was biphasic: KS and translational efficiency were higher at d 10 and 28 than at d 5 and 16. The largest increase in KS and translational efficiency occurred during the period normally associated with weaning (between postpartum d 16 and 28). At all stages of development, the KS and translational efficiency in fasted rats were significantly lower than those in control (fed) rats, although the relative decline in both measurements was largest at postpartum d 10. The absolute rates of hepatic protein synthesis declined to similar levels on d 5, 10, and 16 after 10 h of fasting and changed little after 18 h of fasting; this level was significantly higher at postpartum d 28. Our results suggest that postnatal development in suckling rats was marked by a biphasic pattern in the rates of hepatic protein synthesis, which increased during the neonatal and weaning periods. The relative changes in the synthesis and loss of hepatic protein in response to fasting were greater during the neonatal than during the late suckling and weaning periods.

EFFECTS OF CHRONIC ETHANOL CONSUMPTION AND PAIR FEEDING ON RATES OF PROTEIN SYNTHESIS AND NUCLEIC ACID COMPOSITION IN RAT TIBIA

An investigation was made into the chronic effects of ethanol feeding on bone (represented by the tibia). Treated rats were fed a liquid diet containing ethanol as 36% of total calories, and controls were pair-fed identical amounts of the same diet in which ethanol was substituted by isocaloric glucose. Bone DNA and RNA contents in ethanol-fed rats were not significantly different from glucose-fed controls at days 3, 7, 14, 28 and 42 of treatment. Fractional rates of bone protein synthesis were measured with [43H]-phenylalanine. At 3, 7, 14, 28 and 42 days, ethanol feeding had no effect on free and protein-bound specific radioactivities, nor on fractional or absolute rates of protein synthesis. Synthesis rates relative to RNA (RNA activities) and DNA (cellular efficiencies) were also not significantly altered by ethanol feeding at these time points. Comparisons were made between rats fed a standard solid laboratory diet ad libitum (i.e. normal rats), and those fed restricted amounts of glucose-containing liquid diet (i.e. dietary-restricted rats) for 42 days. In normal rats, there was an increase in tibial mass and accretion of total collagen content, but in dietary-restricted rats, this accretion was markedly impaired. Furthermore, whilst RNA and DNA contents were increased in tibia of normal rats, the contents of these nucleic acids were reduced in bones of dietary restricted rats. Fractional rates of bone protein synthesis in normal rats were unaltered after 42 days, but reduced by feeding the control liquid diet in restricted amounts.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue protein synthesis in lactating and dry goats

Intravenous infusion of L-[3H]phenylalanine (Phe) was carried out for 8 h in dry, non-pregnant and lactating dairy goats. Nitrogen balance was positive in the dry group and negative in the lactating group. Whole-body Phe flux was 50% greater in lactating goats (P less than 0.01). Fractional synthesis rates (Ks) of tissue proteins were estimated from plasma- (Ksp) and tissue- (Ksh) specific radioactivities of Phe. In lactating goats, Ksp for mammary gland, duodenum and diaphragm was increased (P less than 0.05). Ksp also tended to increase in liver, kidney and rumen (P less than 0.08) of lactating goats, but was not different in uterus, spleen, caecum or heart. Values of Ksh were higher than Ksp; however, these measures agreed qualitatively. When absolute rates of protein synthesis were calculated, an increased contribution of mammary and visceral organs was seen in lactating goats. Ks and absolute rates of protein synthesis of hind-limb skin were less in lactating goats (P less than 0.05). A decreased proportion of skeletal muscle (P less than 0.05). Decreased rates of muscle and skin protein synthesis would appear to participate in alterations of protein metabolism, permitting lactation to occur at the expense of body reserves.

Brief fasting decreases protein synthesis in the brain of adult rats

The influence of starvation on protein synthesis in the adult rat brain was studied in vivo by an intravenous injection of a flooding dose of unlabeled valine including a tracer dose of L-[3,4(n)-3H]valine. Brief starvation (24 hours) induced a 20% decline in fractional and absolute rates of brain protein synthesis. This decline resulted from a 20% decrease in the efficiency of protein synthesis (microgram protein synthesized per day per microgram RNA) whereas the capacity for protein synthesis (microgram RNA per mg protein) was maintained. Prolonged starvation (5 days) was marked by no further significant changes in the fractional rate, absolute rate and efficiency of protein synthesis, whereas the capacity for protein synthesis decreased slightly. The relative contribution of brain to whole-body protein synthesis increased during fasting, and neither the protein nor the RNA brain content did change during the experiment. These results clearly indicate that brain proteins are spared in response to brief and prolonged food deprivation, and that brain protein synthesis is very sensitive to short-term fasting.

Control of rRNA synthesis in Escherichia coli at increased rrn gene dosage. Role of guanosine tetraphosphate and ribosome feedback

The effects of extra, plasmid-borne rRNA genes on the synthesis rate of rRNA in Escherichia coli were examined by measuring the fraction of total RNA synthesis that is rRNA and tRNA (rs/rt), the cytoplasmic concentration of guanosine tetraphosphate (ppGpp), and the absolute rates of RNA and protein synthesis. Experiments were carried out in different growth media and with two different strains of E. coli, B/r and K-12. The results indicated: 1) increased rrn gene dosage from either intact or defective rrn genes reduced bacterial growth rates and ribosome activity (protein synthesis rate/average ribosome), and increased rs/rt. 2) Extra intact, but not extra defective, plasmid-borne rrn genes caused the level of ppGpp to be increased in comparison to the pBR322-carrying control strain. 3) As a function of ppGpp, rs/rt was increased with either intact or defective rrn genes. 4) The rRNA synthesis rate/rrn gene was reduced in the presence of extra rrn genes; this reduction in gene activity was greater with intact than with defective rrn genes. An analysis of these results showed that they are consistent with the ppGpp hypothesis of rRNA control but not with a feedback effector role of translating ribosomes.

CHANGES IN PROTEIN, RNA AND DNA AND RATES OF PROTEIN SYNTHESIS IN MUSCLE-CONTAINING TISSUES OF THE MATURE RAT IN RESPONSE TO ETHANOL FEEDING: A COMPARATIVE STUDY OF HEART, SMALL INTESTINE AND GASTROCNEMIUS MUSCLE

1. The relative sensitivity of heart, small intestine and skeletal muscle to chronic ethanol feeding was investigated in mature Wistar rats fed ethanol as 36% of total energy intake; controls were fed the same diet in which ethanol was substituted by isoenergetic glucose. 2. Chronic ethanol feeding had no apparent effect on the protein, RNA and DNA contents of heart homogenates (atria and ventricles). The ratios of RNA/protein (synthetic capacity), RNA/DNA (synthetic material per nucleus) and protein/DNA (DNA-unit or apparent cell size) were also unaltered in the hearts of alcohol-fed rats. Fractional rates of cardiac protein synthesis (ks), and synthesis relative to RNA (kRNA) and DNA (kDNA) and absolute rates of protein synthesis (Vs) were unaffected by ethanol feeding. The total content of cardiac soluble proteins was unaltered by chronic ethanol feeding, but there were small and statistically significant decreases in the contents of the myofibrillar and stromal protein fractions. There were no differences in ks in any of the cardiac subcellular protein fractions. 3. In the small intestine, ethanol feeding had no statistically significant effect on either protein or RNA contents, but there was an apparent increase in RNA when expressed relative to either protein or DNA, though the DNA-unit was unaltered. There were also substantial decreases in ks, kRNA, kDNA and Vs of approximately 15-35%. 4. In the gastrocnemius, RNA contents were significantly reduced by ethanol feeding but protein and DNA contents were unaffected. Indices of the synthetic capacity and synthetic material per nucleus were also reduced, but the DNA-unit was unaltered. These observations were accompanied by approx. 15-30% reductions in ks, kRNA, kDNA and Vs in response to ethanol feeding. 5. It is concluded that various aspects of protein metabolism in the heart, small intestine and skeletal muscle are adversely affected by chronic ethanol toxicity. The characteristics and magnitude of the responses in each tissue differ. Effects in the heart may be subtle, though haemodynamic indices may ensue. The ethanol-induced alterations in the small intestine and skeletal muscle may be responsible for gastrointestinal disturbances in motility and skeletal muscle weakness, respectively.

Fetal Hyperinsulinemia and Protein Turnover in Fetal Rat Tissues

The effects of fetal hyperinsulinemia on protein turnover in various tissues of fetal rats were determined after transuteral injection of insulin to rat fetuses at day 19 of gestation. Tissue protein content was measured on the subsequent days of gestation (days 20-22), and protein synthesis was determined at day 20 of gestation in fetal tissues after intravenous injection of [3H]phenylalanine into the maternal circulation, followed by measurements of tissue free and protein-bound phenylalanine specific radioactivity in fetal diaphragm, brain, heart, and liver. Rates of protein degradation in these fetal tissues were calculated by subtracting protein accretion rates from rates of protein synthesis. The injection of insulin to rat fetuses at day 19 of gestation resulted in relative macrosomia versus saline-injected controls from the same litter (body wt at day 20 of gestation, 3.26 +/- 0.15 g for saline-injected fetuses and 3.60 +/- 0.25 g for insulin-injected fetuses, P less than 0.001) and increased protein and RNA content of brain, heart, and liver. Although fractional rates of protein synthesis were not significantly elevated in tissues from the hyperinsulinemic fetuses, absolute rates of protein synthesis were increased in brain, heart, and liver of hyperinsulinemic fetuses. Hyperinsulinemia did not reduce calculated rates of protein breakdown in fetal brain, heart, or liver but did in fetal diaphragm. We conclude that the major effect of fetal hyperinsulinemia on protein turnover in rats is to increase protein synthesis in selected tissues without simultaneously affecting protein breakdown.

Protein Synthesis in Bone and Skin of the Rat Are Inhibited by Ethanol: Implications for Whole Body Metabolism

The acute effects of ethanol (75 mmol/kg body weight, intraperitoneal) on rates of protein synthesis in bone tibia) and skin of young (approximately 100 g body weight) laboratory rats was investigated. Plasma ethanol levels were raised to approximately 40 mmol/liter. At 2.5 hr, rates of protein synthesis were measured with a flooding dose of L-[4-3H]phenylalanine. In bone the protein-bound specific radioactivities, fractional synthesis rates, and synthesis rates relative to RNA and DNA were significantly reduced by approximately 30%. In skin these variables similarly decreased in response to ethanol treatment, by approximately 25%. The reduction in absolute rates of protein synthesis in bone (delta, 0.7 g protein/day/kg body weight) and skin (delta, 3.4 g protein/day/kg body weight) were comparable to the reductions in liver and skeletal muscle in response to acute ethanol. As bone and skin contribute to a quarter of whole body protein synthesis, it was concluded that these observations may have important implications for whole body protein homeostasis.

Protein synthesis of muscle fractions from the small intestine in alcohol fed rats.

The effects of chronic ethanol feeding on the amounts and synthesis rates of cytoplasmic, contractile, and stromal protein fractions were investigated in the small intestine of eight pairs of immature and seven pairs of mature rats. Treated rats were fed ethanol as 36% of total energy in a nutritionally adequate liquid diet. Paired controls were fed isovolumetric amounts of the same diet in which ethanol was substituted by isocaloric glucose. After six weeks the total cytoplasmic and contractile protein content in immature rats was reduced by 18% and 31%, respectively (p less than or equal to 0.007). The decline in the stromal protein content (26%) was not statistically significant (p = 0.130). In mature rats the protein contents were also reduced in the cytoplasmic (25%, p = 0.035) and contractile (27%, p = 0.005) protein fractions, though the stromal protein fraction was unaltered (p = 0.913). In immature rats fractional rates of protein synthesis in cytoplasmic and contractile protein fractions of the small intestine were unaltered by chronic ethanol feeding (p less than or equal to 0.853). In mature rats, the synthesis rates of corresponding fractions declined, by 18% and 31%, respectively, but were also not statistically significant (p less than or equal to 0.369). Absolute rates of protein synthesis in immature rats fell by 6% (p = 0.549) in the cytoplasmic and 31% in the contractile protein fraction (p = 0.045). In mature rats, the corresponding reductions were 38% (p = 0.106) and 48% (p = 0.033), respectively. Virtually no radioactivity could be detected in the stromal fraction, signifying very low synthesis rates. Chronic ethanol feeding reduces the amount of protein in the small intestine of the immature and mature rat with the contractile protein fraction showing the greatest decrease. In the absence of statistically significant reductions in fractional synthesis rates a partial adaptation in turnover rates may have occurred.

METABOLISM IN ADIPOSE TISSUE AND MUSCLE OF THE YOUNG PIG

Subcutaneous adipose tissue and intercostal and sartorius muscles from five barrows and five gilts at 20 kg liveweight were used to study lipogenesis, lipolysis, Na+, K+-ATPase-dependent respiration and protein synthesis. Lipogenesis rate measured by 14C-acetate incorporation into lipid was similar between barrows and gilts; and 100 μg insulin per mL enhanced (P < 0.1) subcutaneous adipose tissue lipogenesis by 74%. Lipolysis rate quantitated by glycerol release was similar between barrows and gilts (3546 and 4160 nmol g−1 2 h−1). Adenosine deaminase and norepinephrine together enhanced adipose tissue lipolytic response by 102%. Fractional and absolute rates of protein synthesis were similar between barrows and gilts (3.24 and 3.69% d−1; 6.01 and 6.06 mg g−1 d−1); and between intercostal and sartorius muscles. Barrows had lower Na+, K+-ATPase-dependent respiration than gilts and the maintenance of Na+ and K+ transmembrane ionic gradient in the muscle preparations accounted for 23–26% of total respiration. Key words: Pigs, adipose tissue, skeletal muscle, metabolism

The acute and chronic effects of ethanol on cardiac muscle protein synthesis in the rat in vivo

An investigation was made into the acute and chronic effects of ethanol on rates of protein synthesis in the hearts of young rats (80–100 g body weight). Acute ethanol administration (75 mmol/kg body weight, IP) significantly reduced the fractional rate of protein synthesis by 20% after 2.5 hr, compared with saline-treated controls. Chronic ethanol feeding (36% of total calories) for 6 weeks significantly reduced cardiac wet weight by 11%, when compared to rats fed isovolumetric amounts of the same diet in which ethanol was substituted by isocaloric glucose. Neither the concentration nor the content of mixed cardiac proteins relative to body weight were overtly altered by chronic ethanol feeding, although, the total content of mixed cardiac proteins were significantly decreased. RNA concentrations and RNA relative to body weight increased slightly, but total cardiac DNA decreased. Indices for the capacity or potential of the heart to synthesis protein (indicated by the RNA/protein and RNA/DNA ratios) and the “DNA-unit” (protein/DNA ratio) were increased in response to chronic ethanol treatment. The fractional and absolute rates of mixed protein synthesis in the heart were (relatively) unaltered by chronic ethanol treatment, as was RNA efficiency and synthesis relative to DNA. It was concluded that the heart displays contrasting responses to acute and chronic ethanol exposure.

Protein metabolism in the small intestine of the ethanol‐fed rat

The effects of chronic ethanol feeding on the small intestine were investigated in young rats. Rats were fed a nutritionally-adequate liquid diet, containing 36 per cent of total energy as ethanol (treated, n = 7), or isovolumetric amounts of the same diet in which ethanol was substituted by isocaloric glucose (controls, n = 7). After six weeks the wet weight and total tissue contents of protein, RNA and DNA were significantly reduced by 21 per cent, 23 per cent, 16 per cent and 28 per cent respectively, (p less than 0.014). Rates of protein synthesis were measured with L[4(3H)]phenylalanine and fractional rates (defined as the percentage of constituent tissue protein synthesised each hour, i.e. ks, % h-1) were calculated from the specific radioactivity of free phenylalanine in both tissue homogenates and plasma. Ethanol-feeding reduced ks by approx 10 per cent (p less than 0.181). The amount of protein synthesized unit-1 RNA was also reduced by approx 15 per cent (p less than 0.059) but the amount of protein synthesis unit-1 DNA was unaffected by ethanol-feeding (p less than 1.000). In contrast, the absolute rates of protein synthesis were reduced by approximately 30 per cent (p less than 0.022). It was concluded that, as the small intestine contributes to approx. 20-25 per cent of whole body synthesis these results may have an important effect on whole body nitrogen homeostasis and may have implications for the gastrointestinal effects of ethanol seen during chronic alcoholic abuse.

Synthesis of subcellular protein fractions in the rat heart in vivo in response to chronic ethanol feeding.

An investigation was made into the chronic effects of ethanol on the subcellular protein fractions in the hearts of young rats (80-100 g body weight). Rats were fed a nutritionally adequate liquid diet containing ethanol as 36% of total energy. Controls were fed the same diet in which ethanol was substituted by iso-energetic glucose. At the end of 6 weeks, rats were killed and hearts were fractionated into sarcoplasmic, myofibrillar and stromal protein fractions by differential solubilisation. The total myofibrillar protein content was significantly reduced by chronic ethanol feeding, though the contents of other fractions were relatively unaltered. The fractional and absolute rates of myofibrillar protein synthesis were significantly increased, but the synthesis rates of sarcoplasmic and stromal protein fractions were unaffected by ethanol feeding. This suggests independent regulation of myofibrillar protein content in the ethanol exposed heart. These changes may be responsible for alterations in myocardial function in alcoholic cardiomyopathy.

Influences of Age and Weaning on In Vivo Pancreatic Protein Synthesis in the Lamb

In vivo pancreatic protein synthesis rates were obtained from the uptake of L-[3,4(n)-3H]valine co-injected with a flooding dose of unlabeled valine into 1-, 5-, and 8-wk-old suckling lambs, and 8-wk-old weaned animals. Protein fractional synthesis rate was 184%/d at 1 wk of age and 153%/d in 5-wk-old animals (P greater than 0.05). This lack of developmental change resulted from constant (P greater than 0.05) ribosomal capacities (total RNA/protein ratios) and efficiencies of protein synthesis (synthetic rates relative to RNA). No further alteration for protein fractional synthesis rate (144%/d) occurred in 8-wk-old suckling animals (P greater than 0.05). In contrast, 8-wk-old ruminants exhibited higher protein fractional synthesis rate (244%/d) than 8-wk-old suckling animals, although ribosomal capacity was markedly higher in both 8-wk-old groups than in youngest animals (P less than 0.05). The present findings clearly indicate that in vivo protein synthesis in the developing ovine pancreas depends primarily on age. Potentialities for increased rates of pancreatic protein synthesis, i.e., increases in total RNA content and ribosomal capacity appear between 5 and 8 wk of age in this species. At 8 wk of age, however, when lambs are generally weaned, solid food ingestion resulted in a rise for both fractional and absolute rates of protein synthesis, essentially because ruminants maintained a higher efficiency of protein synthesis than milk-fed animals (P less than 0.005). Finally, there was a relationship between pancreatic protein synthesis and protein intake in only ruminant lambs.

Dietary protein-induced changes in porcine muscle respiration, protein synthesis and adipose tissue metabolism.

Growth rate, physically separable tissues of the ham and loin, heat production, skeletal muscle respiration and protein synthesis, and lipogenesis and lipolysis in s.c. adipose tissue were measured in a single experiment in which pigs were offered a 13 (n = 8), or 21% (n = 6) protein diet from 20 to 100 kg live weight. Pigs that were fed the 13% protein diet gained body weight slower, ate less, converted feed less efficiently and took 31 d longer to reach 100 kg live weight. Fat depth (cm) was greater (P less than .05) and loin eye area (cm2) was less (P less than .01) in pigs fed the 13% protein diet (2.6 vs 2.3 and 29.8 vs 35.3). Pigs that were fed the 13% protein diet had lower (P less than .05) ham and loin separable muscle and greater (P less than .05) ham and loin separable fat. The mean heat production was less (P less than .05) in pigs offered the 13% (22.49) vs 21% (24.63 MJ/d) protein diets. In the intercostal muscle preparation, total and Na+,K+-ATPase-dependent respiration (microliter O2.mg-1.h-1) were lower (P less than .05) in pigs offered the 13% (2.39 and .41) vs the 21% (3.89 and .68) protein diets. The energy used for the support of Na+ transport across membrane accounted for approximately 17% of muscle respiration. Absolute rates of protein synthesis in the muscle preparations were lower (P less than .01) at 13 than at 21% dietary protein. Lipogenesis in s.c. adipose tissue was not affected by dietary protein level. There was no difference in basal and norepinephrine-stimulated lipolysis between the two dietary protein levels.

AN INVESTIGATION INTO THE EFFECTS OF CHRONIC ETHANOL FEEDING ON HEPATIC MIXED PROTEIN SYNTHESIS IN IMMATURE AND MATURE RATS

1. The response of the liver to chronic ethanol feeding was investigated in sexually immature (85 g) and sexually mature (280 g) male Wistar rats. Rats received a nutritionally adequate liquid diet ad libitum, in which ethanol comprised 36% of total calories, for up to 6 weeks. Controls were pair-fed the same liquid diet in which ethanol was substituted by isocaloric glucose. 2. In immature rats, total hepatic protein, RNA and DNA contents were reduced by 12-23%. The amount of RNA, relative to protein or DNA, was also decreased by 11-12%, though the amount of protein relative to DNA was unaltered. In mature rats, no change in total hepatic protein and DNA contents were observed, though total RNA, RNA/protein ratio, RNA/DNA ratio and the amount of protein relative to DNA was reduced by 7-18%. 3. Rates of protein synthesis were measured with a flooding dose of L[4-3H]-phenylalanine without anaesthesia or surgical stress. In both immature and mature rats the fractional and absolute rates of hepatic protein synthesis and protein synthesis relative to DNA were reduced by approx 25%.

The effect of chronic ethanol ingestion on protein metabolism in type-I- and type-II-fibre-rich skeletal muscles of the rat

1. The effects of chronic ethanol feeding on muscles containing a predominance of either Type I (aerobic, slow-twitch) or Type II (anaerobic, fast-twitch) fibres were studied. Male Wistar rats, weighing approx. 90 g or 280 g, were pair-fed on a nutritionally complete liquid diet containing 36% of total energy as ethanol, or isovolumetric amounts of the same diet in which ethanol was replaced by isoenergetic glucose. After 6 weeks feeding, fractional rates of protein synthesis were measured with a flooding dose of L-[4-(3)H]-phenylalanine and muscles were analysed for protein, RNA and DNA. 2. Ethanol feeding decreased muscle weight, protein, RNA and DNA contents in both small and large rats. Type-II-fibre-rich muscles showed greater changes than did Type-I-fibre-rich muscles. Changes in protein paralleled decreases in DNA. 3. The capacity for protein synthesis (RNA/protein), fractional rates of protein synthesis and absolute rates of protein synthesis were decreased by ethanol feeding in both small and large rats. The amounts of protein synthesized relative to RNA and DNA were also decreased. Changes were less marked in Type-I than in Type-II-fibre-rich muscles. Loss of protein, RNA and DNA was greater in small rats, but protein synthesis was more markedly affected in large rats. 4. It was concluded that chronic ethanol feeding adversely affects protein metabolism in skeletal muscle. Fibre composition and animal size are also important factors in determining the pattern of response.

Effect of Diet Type on Enhanced Intestinal Protein Synthesis by the Gut Microflora in the Chick

An experiment was conducted to investigate the effect of diet type on enhanced protein synthesis by the gut microflora in the chick intestine. Both germ-free (GF) and conventional (CV) White Leghorn chicks were fed ad libitum from 4 to 14 d of age either a nonpurified diet or a purified diet having 19% crude protein and 12.1 kJ/g metabolizable energy value. At 14 d of age, protein synthesis was measured in duodenum, jejunoileum and ceca after a large dose of L-[4-3H]phenylalanine injected intravenously via a wing vein. The results showed that wet tissue weights for all intestinal sections were significantly greater in CV birds than in GF counterparts with consistently larger differences between the two environments in chicks fed the nonpurified diet than in those fed the purified diet. Protein synthesis in all intestinal sections was significantly enhanced by the presence of gut microflora in terms of both fractional rate (%/d) and absolute rate [mg/(100 g BW.d)]. The effect of diet type on fractional and absolute rates of protein synthesis was most remarkable in ceca where the significant difference between GF and CV states was found only in chicks fed the nonpurified diet but not in those fed the purified diet.

Enhanced skeletal muscle and liver protein synthesis with structured lipid in enterally fed burned rats

We assessed the effects of total enteral nutrition with long-chain triacylglycerides (LCT), medium-chain triacylglycerides (MCT), or two structured lipids, modified dairy fat (MDF) and modified MCT (Captex 810B, Capital City Products, Columbus, OH), on protein and energy metabolism in hypermetabolic burned rats (25% to 30% body surface area). Male Sprague-Dawley rats (200 ± 10 g) were continuously gastrostomy-fed isovolemic diets that provided 50 kcal/d, 2 g amino acids/d and 40% nonprotein calories as lipid for three days. Changes in body weight, nitrogen balance, serum albumin, indirect calorimetry, whole body leucine kinetics, and rectus muscle and liver protein kinetics were determined. Whole body leucine kinetics and tissue fractional protein synthetic rates (FSR, percent per day) were estimated using a four-hour constant intravenous infusion of l-[1- 14C]leucine on day 3. The group of rats enterally fed MDF lost less body weight than the other groups ( P ≤ .05). MDF and Captex 810B produced a positive and significantly greater ( P ≤ .05) daily and cumulative nitrogen balance than either LCT or MCT. Oxygen consumption ( P ≤ .05) and total energy expenditure ( P ≤ .05) were elevated approximately 22% with MDF as compared with LCT or MCT. Rectus muscle FSR and absolute rate of protein synthesis were increased 19% with MDF ( P ≤ .05) as compared with LCT or MCT. Liver FSR and absolute rate of protein synthesis were increased ( P ≤ .01) 70% and 84%, respectively, with MDF when compared with either LCT, MCT, or Captex 810B. MDF, Captex 810B, and MCT maintained higher serum albumin levels than LCT ( P ≤ .01). These results indicate that the use of structured lipids in postburn enteral nutrition attenuates the net protein catabolic effects of burn trauma by improving nitrogen utilization, at least in part, as a result of higher protein synthetic rates in skeletal muscle and liver.

THE EFFECT OF CHRONIC ETHANOL FEEDING ON BODY AND PLASMA COMPOSITION AND RATES OF SKELETAL MUSCLE PROTEIN TURNOVER IN THE RAT

(1) Sexually immature and mature rats were fed a nutritionally-complete liquid diet or isovolumetric quantities of the same diet in which 36% of the calories as glucose were substituted by isocaloric ethanol. (2) After 6 weeks ethanol feeding, significant reductions in body weight (approx. 15%) occurred in both groups of rats. In immature rats there were significant reductions (7-21%) in bone, gastrocnemius, liver, and skin weights. The total skeletal muscle mass was reduced by 20%. Lung and kidney weights were not significantly altered. In mature rats smaller decreases in organ weights were found, which were only significant for skeletal muscle and skin. The gastrocnemius protein content was significantly reduced in immature but not in mature rats. Plasma protein concentrations were unaltered in both groups. (3) Plasma aspartate aminotransaminase, gamma glutamyl transferase and creatine kinase activities in immature and mature rats were not significantly altered by ethanol feeding, but there were increases in plasma alkaline phosphatase activities in immature, but not in mature, rats. Plasma glucose was slightly raised by ethanol feeding in immature but not mature rats. Plasma triglycerides and insulin were unaltered in both groups of rats. (4) Protein synthesis was measured with a flooding dose of L[4(3)H]-phenylalanine. Rates of protein breakdown were calculated from the difference between synthesis and growth. Fractional and absolute rates of skeletal muscle protein synthesis were reduced by 13-30% by ethanol treatment, in immature and mature rats. Fractional rates of protein breakdown were also reduced by ethanol, by 13 and 19% in immature and mature rats, respectively.