Tissue and whole-body protein synthesis in immature Zucker rats and their relationship to protein deposition

Muscle plasticity is influenced by renal function and caloric intake through the FGF23-vitamin D axis

Skeletal muscle, the main metabolic engine in the body of vertebrates, is endowed with great plasticity. The association between skeletal muscle plasticity and two highly prevalent health problems: renal dysfunction and obesity, which share etiologic links as well as many comorbidities, is a subject of great relevance. It is important to know how these alterations impact on the structure and function of skeletal muscle because the changes in muscle phenotype have a major influence on the quality of life of the patients. This literature review aims to discuss the influence of a nontraditional axis involving kidney, bone, and muscle on skeletal muscle plasticity. In this axis, the kidneys play a role as the main site for vitamin D activation. Renal disease leads to a direct decrease in 1,25(OH)₂-vitamin D, secondary to reduction in renal functional mass, and has an indirect effect, through phosphate retention, that contributes to stimulate fibroblast growth factor 23 (FGF23) secretion by bone cells. FGF23 downregulates the renal synthesis of 1,25(OH)₂-vitamin D and upregulates its metabolism. Skeletal production of FGF23 is also regulated by caloric intake: it is increased in obesity and decreased by caloric restriction, and these changes impact on 1,25(OH)₂-vitamin D concentrations, which are decreased in obesity and increased after caloric restriction. Thus, both phosphate retention, that develops secondary to renal failure, and caloric intake influence 1,25(OH)₂-vitamin D that in turn plays a key role in muscle anabolism.

Increased Expression of Syncytin-1 in Skeletal Muscle of Humans With Increased Body Mass Index

Obesity negatively impacts skeletal muscle protein metabolism, and also impairs skeletal muscle maintenance and regeneration. We analyzed muscle biopsy samples from humans with increased body mass index (BMI) (i.e. > 30 kg/m²) and controls (i.e., BMI < 25 kg/m²) for expression of syncytin-1, a fusogenic protein regulating skeletal muscle regeneration. When compared to controls, humans with increased BMI and concomitant reduction in muscle protein synthesis had higher expression of syncytin-1 in skeletal muscle (p < 0.05). Across human subjects, muscle protein synthesis correlated inversely (r = −0.51; p = 0.03) with syncytin-1 expression in muscle. Using a C2C12 cell line we found that expression of syncytin-A (i.e, corresponding protein in murine tissue) is increased by insulin, and that this response is impaired in the presence of fatty acids, whose metabolism is altered within the metabolic environment induced by increased BMI. In C2C12 cells, the response of the protein 4E-BP1, which signals increase in protein synthesis in muscle, resembled that of syncytin-A. These findings provide novel insights into the expression of syncytin-1 in skeletal muscle of humans with increased BMI, as well as its basic regulation by insulin and fatty acids in muscle. The findings signify the need for further research into the regulation of syncytin-1 in skeletal muscle of humans with increased BMI, as well as its biological implications for altering muscle protein metabolism and regeneration.

Obesity-induced discrepancy between contractile and metabolic phenotypes in slow- and fast-twitch skeletal muscles of female obese Zucker rats

A clear picture of skeletal muscle adaptations to obesity and related comorbidities remains elusive. This study describes fiber-type characteristics (size, proportions, and oxidative enzyme activity) in two typical hindlimb muscles with opposite structure and function in an animal model of genetic obesity. Lesser fiber diameter, fiber-type composition, and histochemical succinic dehydrogenase activity (an oxidative marker) of muscle fiber types were assessed in slow (soleus)- and fast (tibialis cranialis)-twitch muscles of obese Zucker rats and compared with age (16 wk)- and sex (females)-matched lean Zucker rats (n = 16/group). Muscle mass and lesser fiber diameter were lower in both muscle types of obese compared with lean animals even though body weights were increased in the obese cohort. A faster fiber-type phenotype also occurred in slow- and fast-twitch muscles of obese rats compared with lean rats. These adaptations were accompanied by a significant increment in histochemical succinic dehydrogenase activity of slow-twitch fibers in the soleus muscle and fast-twitch fiber types in the tibialis cranialis muscle. Obesity significantly increased plasma levels of proinflammatory cytokines but did not significantly affect protein levels of peroxisome proliferator-activated receptors PPARγ or PGC1α in either muscle. These data demonstrate that, in female Zucker rats, obesity induces a reduction of muscle mass in which skeletal muscles show a diminished fiber size and a faster and more oxidative phenotype. It was noteworthy that this discrepancy in muscle's contractile and metabolic features was of comparable nature and extent in muscles with different fiber-type composition and antagonist functions.NEW & NOTEWORTHY This study demonstrates a discrepancy between morphological (reduced muscle mass), contractile (shift toward a faster phenotype), and metabolic (increased mitochondrial oxidative enzyme activity) characteristics in skeletal muscles of female Zucker fatty rats. It is noteworthy that this inconsistency was comparable (in nature and extent) in muscles with different structure and function.

Time-course changes of muscle protein synthesis associated with obesity-induced lipotoxicity

The object of the study was to investigate the sequential changes of protein synthesis in skeletal muscle during establishment of obesity, considering muscle typology. Adult Wistar rats were fed a standard diet for 16 weeks (C; n = 14), or a high-fat, high-sucrose diet for 16 (HF16; n = 14) or 24 weeks (HF24; n = 15). Body composition was measured using a dual-energy X-ray absorptiometry scanner. The fractional synthesis rates (FSRs) of muscle protein fractions were calculated in tibialis anterior (TA) and soleus muscles by incorporation of l-13C-valine in muscle protein. Muscle lipid and mitochondria contents were determined using histochemical analysis. Obesity occurred in an initial phase, from 1 to 16 weeks, with an increase in weight (P < 0.05), fat mass (P < 0.001), muscle mass (P < 0.001) and FSR in TA (actin: 5.3 ± 0.2 vs. 8.8 ± 0.5% day−1, C vs. HF16, P < 0.001) compared with standard diet. The second phase, from 16 to 24 weeks, was associated with a weight stabilization, a decrease in muscle mass (P < 0.05) and a decrease in FSR in TA (mitochondrial: 5.6 ± 0.2 vs. 4.2 ± 0.4% day−1, HF16 vs. HF24, P < 0.01) compared with HF16 group. Muscle lipid content was increased in TA in the second phase of obesity development (P < 0.001). Muscle mass, lipid infiltration and muscle protein synthesis were differently affected, depending on the stage of obesity development and muscle typology. Chronic lipid infiltration in glycolytic muscle is concomitant with a reduction of muscle protein synthesis, suggesting that muscle lipid infiltration in response to a high-fat diet is deleterious for the incorporation of amino acid in skeletal muscle proteins.

Insulin resistance syndrome blunts the mitochondrial anabolic response following resistance exercise

Metabolic risk factors associated with insulin resistance syndrome may attenuate augmentations in skeletal muscle protein anabolism following contractile activity. The purpose of this study was to investigate whether or not the anabolic response, as defined by an increase in cumulative fractional protein synthesis rates (24-h FSR) following resistance exercise (RE), is blunted in skeletal muscle of a well-established rodent model of insulin resistance syndrome. Four-month-old lean (Fa/?) and obese (fa/fa) Zucker rats engaged in four lower body RE sessions over 8 days, with the last bout occurring 16 h prior to muscle harvest. A priming dose of deuterium oxide ((2)H(2)O) and (2)H(2)O-enriched drinking water were administered 24 h prior to euthanization for assessment of cumulative FSR. Fractional synthesis rates of mixed (-5%), mitochondrial (-1%), and cytosolic (+15%), but not myofibrillar, proteins (-16%, P = 0.012) were normal or elevated in gastrocnemius muscle of unexercised obese rats. No statistical differences were found in the anabolic response of cytosolic and myofibrillar subfractions between phenotypes, but obese rats were not able to augment 24-h FSR of mitochondria to the same extent as lean rats following RE (+14% vs. +28%, respectively). We conclude that the mature obese Zucker rat exhibits a mild, myofibrillar-specific suppression in basal FSR and a blunted mitochondrial response to contractile activity in mixed gastrocnemius muscle. These findings underscore the importance of assessing synthesis rates of specific myocellular subfractions to fully elucidate perturbations in basal protein turnover rates and differential adaptations to exercise stimuli in metabolic disease.

Disruption of BCATm in mice leads to increased energy expenditure associated with the activation of a futile protein turnover cycle

Leucine is recognized as a nutrient signal; however, the long-term in vivo consequences of leucine signaling and the role of branched-chain amino acid (BCAA) metabolism in this signaling remain unclear. To investigate these questions, we disrupted the BCATm gene, which encodes the enzyme catalyzing the first step in peripheral BCAA metabolism. BCATm(-/-) mice exhibited elevated plasma BCAAs and decreased adiposity and body weight, despite eating more food, along with increased energy expenditure, remarkable improvements in glucose and insulin tolerance, and protection from diet-induced obesity. The increased energy expenditure did not seem to be due to altered locomotor activity, uncoupling proteins, sympathetic activity, or thyroid hormones but was strongly associated with food consumption and an active futile cycle of increased protein degradation and synthesis. These observations suggest that elevated BCAAs and/or loss of BCAA catabolism in peripheral tissues play an important role in regulating insulin sensitivity and energy expenditure.

Mechanism for the increased skeletal muscle protein degradation in the obese Zucker rat

Obese (fa/fa) Zucker rats showed a decreased protein content in skeletal muscle compared with their lean counterparts. This was associated with both a decrease in the fractional rate of protein synthesis and an enhanced fractional rate of protein degradation in skeletal muscle, as studied by pre-loading body proteins with (14)C-bicarbonate. The increased protein degradation could be the result of the clearly enhanced expression for several transcripts of the ubiquitin genes in skeletal muscle. The results suggest that the increased protein degradation in the skeletal muscle of the obese animals may be due to increased activity of the ubiquitin-dependent proteolytic system.

A developmental switch affecting growth of fatty rats

Fatty (fa/fa) rats accumulate more adipose mass than their littermates soon after birth, but they first appear obese during the fourth week of life. We analyzed the effects of fa genotype on growth of pups housed with their dams through 4 wk of age. The fa genotype effects on daily gain were undetectable from 7 to 22 days of age but became highly significant (P = 10(-18)) at 23 days of age. When litters were reduced to 4 pups, fa genotype effects on daily gain also became detectable at 23 days of age. The fa genotype effects on daily gain, stomach contents weight, liver weight, and plasma insulin of rats killed from 20 to 24 days of age displayed a marked genotype by age interaction, becoming highly significant at 23 days of age. These changes occur without the environmental changes induced by separating pups from their dams. These observations suggest that a developmental switch triggers hyperphagia and rapidly increases growth rate of fatty rats after 22 days of age.

Whole body protein turnover of growing rats in response to different dietary proteins--soy protein or casein

Whole body protein turnover was studied in growing rats fed restrictively on isoenergetic (GE 17.6 MJ/kg DM) and isonitrogenous (104 g CP/kg DM) diets based on soy protein isolate or casein supplemented with D,L-methionine. During each of the three separate experiments six male Fischer rats per group were housed individually in metabolic cages at 24 degrees C. Prefeeding of both dietary groups up to similar body weights at the start of the main experimental periods (105-134 g) lasted up to 16 d for casein-fed rats and up to 30 d for the soy protein-fed rats. Following the energy and nitrogen balance periods whole-body protein synthesis was estimated by the end-product method using a single tracer dose of a mixture of 15N-labelled amino acids. Fractional protein accretion rate [% of the protein pool accreted per day] was significantly lower in soy protein-rats than in casein-fed rats in all three experiments whereas fractional synthesis rate was not significantly lower. Therefore, protein breakdown subsequently calculated as the difference between synthesis and accretion showed a tendency towards higher values in this group. In soy protein-fed rats also a tendency towards higher excretion of 3-methylhistidine as a marker of myofibrillar protein breakdown was observed. It is concluded that increase in lean tissue growth resulting from improved protein quality is brought about by changes of both rates, by small increase of protein synthesis and by reduced rate of body protein breakdown.

Aminoacyl-tRNA and tissue free amino acid pools are equilibrated after a flooding dose of phenylalanine

The flooding dose method, which is used to measure tissue protein synthesis, assumes equilibration of the isotopic labeling between the aminoacyl-tRNA pool and the tissue and blood free amino acid pools. However, this has not been verified for a phenylalanine tracer in an in vivo study. We determined the specific radioactivity of [(3)H]phenylalanine in the aminoacyl-tRNA and the tissue and blood free amino acid pools of skeletal muscle and liver 30 min after administration of a flooding dose of phenylalanine along with [(3)H]phenylalanine. Studies were performed in neonatal pigs in the fasted and refed states and during hyperinsulinemic-euglycemic-amino acid clamps. The results showed that, 30 min after the administration of a flooding dose of phenylalanine, there was equilibration of the specific radioactivity of phenylalanine among the blood, tissue, and tRNA precursor pools. Equilibration of the specific radioactivity of the three precursor pools for protein synthesis occurred in both skeletal muscle and liver. Neither feeding nor insulin status affected the aminoacyl-tRNA specific radioactivity relative to the tissue free amino acid specific radioactivity. The results support the assumption that the tissue free amino acid pool specific radioactivity is a valid measure of the precursor pool specific radioactivity and thus can be used to calculate protein synthesis rates in skeletal muscle and liver when a flooding dose of phenylalanine is administered.

A model of whole-body protein turnover based on leucine kinetics in rodents

The measurement of fractional synthesis rate is based on the following assumptions: amino acids for protein synthesis are supplied by an intracellular pool; amino acids from protein degradation are not recycled preferentially to protein synthesis; and proteins turn over at a homogeneous rate. To test these assumptions, a mechanistic, theoretical model of protein turnover for a nongrowing 26-g mouse was developed on the basis of data from the literature. The model consisted of three protein pools turning over at fast (102 micromol Leu, t1/2= 11.5 h), medium (212 micromol Leu, t1/2 = 16.6 h) or slow (536 micromol Leu, t1/2 = 71.5 h) rates and extracellular (1.69 micromol Leu), leucyl-tRNA (0.0226 micromol Leu) and intracellular (5.72 micromol Leu) amino acid pools that exchanged amino acids. The flow of amino acids from the protein pools to the leucyl-tRNA pool determined the amount of recycling. The flow of amino acids from the extracellular pool to aminoacyl tRNA determined the amount of channeling. Two flooding dose data sets were used to evaluate specific radioactivity changes predicted by the model. Predictions of specific radioactivities using flooding dose, pulse dose or continuous infusion methods indicated that the model can be a useful tool in estimating the rates of channeling and recycling. However, it was found that use of data from flooding dose experiments might cause inaccurate predictions of certain fluxes.

Insulin-like growth factor 1 has beneficial effects, whereas growth hormone has limited effects on postoperative protein metabolism, gut integrity, and splenic weight in rats with chronic mild liver injury

BACKGROUND

Both growth hormone (GH) and insulin-like growth factor 1 (IGF-1) improve protein metabolism after surgical insult in subjects without liver disease. However, these effects in chronic liver injury, in which the GH-IGF-1 axis is impaired, have not been investigated. We examined the anabolic effects of GH and IGF-1 after gastrectomy in rats with chronic mild liver injury.

METHODS

Rats with chronic mild liver injury induced by thioacetamide were used. After gastrectomy, the rats were randomized into vehicle control, GH, and IGF-1 groups. In the latter two groups, 0.8 IU/kg/d of GH or 4 mg/kg/d of IGF-1 was infused for 72 hours. Anabolic effects were assessed by body weight change, 3-methylhistidine (3-MH) excretion, nitrogen excretion, and whole-body protein turnover. Organ weights, plasma levels of glucose, insulin, and IGF-1, tissue IGF-1 levels, hepatic messenger RNA (mRNA) content, and intestinal structure were also determined.

RESULTS

Both GH and IGF-1 decreased nitrogen excretion. IGF-1, but not GH, increased postoperative body weight, whole-body protein turnover, and splenic weight. IGF-1 reduced atrophy of the intestinal mucosa. GH treatment increased hepatic IGF-1-mRNA and the plasma IGF-1 level, whereas IGF-1 treatment increased the plasma IGF-1 level with no change in the hepatic IGF-1-mRNA content. There were no significant differences in plasma glucose or insulin levels among the three groups. Neither GH nor IGF-1 affected the gastrocnemius muscle IGF-1 level.

CONCLUSIONS

IGF-1 has beneficial effects, whereas GH has only limited effects on post-operative protein metabolism, gut integrity, and splenic weight in chronic mild liver injury.

Both maternal over- and undernutrition during gestation increase the adiposity of young adult progeny in rats

We examined the influence of maternal diet during gestation on the growth and body composition of the progeny. On day 1 of gestation, rat dams were assigned to one of four feeding regimens: free access to standard rodent chow throughout gestation (AL); 20 g feed/day (prebreeding intake) throughout gestation (PB); 10 g feed/day from day 1 to day 14, then ad libitum from day 15 to parturition (RAL); 10 g feed/day from day 1 to 14, then 20 g/day to parturition (RPB). Progeny were fed ad libitum on standard chow diet from 3 to 12 weeks of age; food intake and weight gain were measured over this time. Body composition was measured at 12 weeks. The PB regimen restricted maternal food intake during the third trimester only; the RAL regimen restricted intake by 50% for two trimesters and produced hyperphagia in the third; the RPB regimen restricted intake by 50% for two trimesters, then intake (per unit body weight) was similar to that of AL dams during the third trimester. Litter size and progeny birth, weaning, and 12-week body weights were similar among the four groups. At 12 weeks of age, PB progeny had the highest body fat (per kg fat-free mass), despite similar feed intake during the 9-week postweaning period. The increased fat was proportionally distributed among intra-abdominal and subcutaneous depots. Progeny of RAL, AL, and RPB dams had similar amounts of body fat, but in RAL progeny more fat was present in intra-abdominal depots. The weights of fat-free mass, gastrointestinal tract and hindlimb skeletal muscles were unaffected by maternal diet. Restriction of maternal feed intake during the third week of gestation had subtle effects on the body composition of young adult progeny that could not be explained on the basis of differences in postweaning voluntary feed intake.

Gastrointestinal protein turnover and alcohol misuse

Acute and chronic ethanol ingestion causes a variety of pathological changes in the gastrointestinal tract, including gross morphological lesions and functional changes. We review whether these alterations also include changes in protein turnover, to explain the frequently observed villus atrophy and smooth muscle myopathy. The possibility that different regions of the gastrointestinal tract express diverse sensitivities is explored. Acute ethanol dosage profoundly reduced the synthesis of proteins in proximal regions of the rat gastrointestinal tract, but distal regions were less affected. In response to chronic ethanol exposure, similar regional sensitivities of the intestine were observed. In chronic studies the small intestine effects were characterised by selective losses of RNA, principally from the stomach and jejunum. We speculate whether the effects on protein synthesis were primarily due to ethanol or the consequence of acetaldehyde formation. We also determined whether changes in protein synthesis occurred secondary to alterations in nucleotide composition. The possible mediation by free-radical formation or impaired antioxidant status are also discussed. The overall results indicate that both acetaldehyde and ethanol are potent protein synthetic inhibitors and may contribute to the genesis of intestinal myopathy, possibly contributing towards motility disturbances and secondary malnutrition via malabsorption.

Amino acid metabolism in several tissues of the obese Zucker rat as indicated by the tissue accumulation of alpha-amino[1-14C]isobutyrate

Measurements of the tissue accumulation of alpha-amino[1-14C]isobutyrate [1-14C]AIB in lean (+/?) and obese (fa/fa) Zucker rats showed an augmented tissue/plasma ratio in the liver of the obese animals. In contrast, brown adipose tissue AIB accumulation was lower in the fa/fa animals. In response to a 24 h starvation period AIB accumulation was significantly elevated in the liver and plasma of the lean animals and was unchanged in the liver of the fa/fa animals. The circulating concentration of alanine and branched-chain amino acids was elevated in the fa/fa animals as compared to their lean counterparts. These observations suggest that amino acid uptake is not involved in the impaired muscle development observed in the obese Zucker rat and that the ability of brown adipose tissue for amino acid utilization is decreased in the obese animals suggesting that this may partially explain the impaired thermoregulatory capacity observed in brown adipose tissue of obese Zucker rats.

Mechanism of maturational decline of rat intestinal lactase-phlorizin hydrolase

The maturational decline in lactase-phlorizin hydrolase (LPH) activity was studied in groups of young rats ranging from suckling to early post-weaned states. Associated maturational increases in sucrase-isomaltase (SI) and maltase-glucoamylase (MG) activities were also examined as a comparison. Over this time period changes in cellular concentrations of the three enzymes were observed, reflecting corresponding changes in enzyme activities. Synthesis patterns accompanying these maturational changes in concentration were examined using labelled leucine as a marker. Synthesis of LPH was found to be maintained at constant rates independent of the maturation-associated decline in its concentration, whereas the increases in cellular concentrations of SI and MG were due to accelerated synthesis of the enzyme. Turnover of LPH, based on both the fractional synthesis rate and the disappearance rate of labelled leucine from prelabelled LPH pools, was increased in a quantitatively similar way to the decline in LPH concentration. These findings are consistent with our earlier proposal that the maturational decline of LPH occurs because of accelerated turnover, without a decrease in its rate of synthesis.

The effects of ovine growth hormone on protein turnover in rainbow trout

Ovine growth hormone (oGH) was administered to rainbow trout via an intraperitoneal cholesterol implant. After 21 days, plasma oGH levels were recorded as control group, less than 2 ng ml-1, i.e., not detectable, and oGH group, 19.2 +/- 2.8 ng ml-1. oGH-treated fish exhibited significantly increased whole-body growth rates, whole-body protein accretion rates, stimulated tissue protein synthesis, and tissue protein accretion rates. A dramatic decrease in white muscle protein concentration was also observed after oGH treatment. In some tissues (liver and stomach), elevated protein synthesis rates were the result of higher RNA/protein ratios. However, in other tissues (gill and ventricle), increased RNA activity accounted for the differences in rates of protein synthesis. The growth promoting effects of oGH on both whole-body and tissue protein turnover were generally accompanied with no change in the efficiency of deposition of newly synthesized protein. For the same ration size, the oGH group showed higher retentions of ingested nitrogen. It is concluded that oGH significantly enhances whole-body growth rates as a result of the stimulatory effect on protein synthesis rates with little effect on protein degradation.

Regulation of polyunsaturated fatty acid metabolism in tissue phospholipids of obese (fa/fa) and lean (Fa/-) Zucker rats. 1. Effect of dietary lipids on cardiac tissue

The present investigation addressed three questions: (i) Does the obese syndrome alter the fatty acid composition of cardiac tissue and membrane phospholipids in obese (fa/fa) rats? (ii) Are changes, if they occur, similar to those reported for tissues of the genetically obese (ob/ob) mouse? (iii) Can cardiac tissue phospholipids and their component fatty acids be modified by dietary lipids and if so does this occur to the same extent in both fa/fa and lean (Fa/-) rats? Proportions of polyunsaturated fatty acids (PUFA) in cardiac total phospholipids of fa/fa rats differed significantly from those of Fa/- rats and from those reported for ob/ob mice. Increased 18:2n-6 and decreased 20:4n-6 and 22:6n-3 in fa/fa rats indicated impaired PUFA metabolism, possibly reduced delta 6 and/or delta 5 desaturase activity, compared with Fa/- rats. No differences in hepatic delta 6 and delta 5 desaturase activity between fa/fa and Fa/- were found but enhanced activity of delta 9 desaturase activity in fa/fa as compared to Fa/- was evident. Inclusion of sunflower oil (SO) or triolein (TO) at 5% and 20% by weight in the diet elicited marked changes in the fatty acyl composition of cardiac phospholipids in both fa/fa and Fa/- rats when compared with animals fed the control Oxoid diet alone. Supplementation with triolein was most effective, reducing 18:2n-6 and increasing 20:4n-6 proportions in fa/fa rats so that they resembled those in Fa/- rats fed the control Oxoid diet. The type of fat rather than the amount of its dietary intake appears to be the main determinant of the observed changes in phospholipid composition.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Lipid and lipoprotein synthesis in isolated and cultured hepatocytes from lean and obese Zucker rats

Hepatocytes were isolated by EDTA perfusion of livers from lean (Fa/-) and obese (fa/fa) Zucker rats. Triacylglycerol (TG) and sn-glycerol 3-phosphate were increased in fa/fa hepatocytes, but free fatty acids, cholesterol and phospholipid concentrations were similar in both groups. In spite of an identical fatty acid uptake rate, glycerolipid synthesis was higher in obese compared to lean rat hepatocytes, and this difference remained for at least 2-3 days of culture. Triacylglycerol mass secretion was 2-fold higher in obese than in lean rat hepatocytes. This was confirmed by the higher incorporation of labeled glycerol and oleic acid into the medium TG fraction floating at density 1.006 g/ml. Density gradient ultracentrifugation of [14C]oleate-labeled lipoproteins showed that fa/fa hepatocytes secreted more TG-rich lipoproteins, and that 87% of the label was in the VLDL fraction compared with 67% in the medium of Fa/- hepatocytes. Decreased utilisation of leucine for protein synthesis in obese rat compared to lean rat hepatocytes was associated with enhanced leucine oxidation to CO2. [35S]Methionine incorporation showed an identical cell protein synthesis rate. Autoradiography after PAGE separation of secreted apolipoproteins (apoBh, Bl, apoA-VI, apoE, apoA-I, apoC) showed an identical pattern in both cell types.

Respective influences of age and weaning on skeletal and visceral muscle protein synthesis in the lamb

1. The influences of age and weaning on muscle protein synthesis were studied in vivo, by injecting a large dose of [3H]valine into 1-, 5- and 8-week-old suckling or 8-week-old weaned lambs. 2. The fractional rates of protein synthesis, in red- and white-fibre-type skeletal muscles or striated and smooth visceral muscles, were in 8-week-old suckling animals 24-37% of their values at 1 week of age. This developmental decline was related to decreased capacities for protein synthesis, i.e. RNA/protein ratios. 3. At 8 weeks of age, suckling and weaned lambs had similar fractional synthesis rates, capacities for protein synthesis and efficiencies of protein synthesis (i.e. rates of protein synthesis relative to RNA) in skeletal muscles. 4. In contrast, visceral-muscle fractional synthesis rates were lower in 8-week-old suckling lambs than in weaned animals, owing to decreased efficiencies of protein synthesis. It was concluded that developmental factors and the change to a solid diet, or weaning in itself, or both, affect differently skeletal and visceral muscle protein synthesis in the immature lamb.

Contribution of liver, skin and skeletal muscle to whole-body protein synthesis in the young lamb

1. Protein fractional synthesis rate (FSR) was measured in some major tissues and in the whole body of six 1-week-old sucking lambs by a large injection of L-[3H]valine. 2. Upper estimates of tissue protein FSR (%/d), assuming that the tissue-homogenate free-valine specific radioactivity defined that of valyl tRNA, were 115.0 in liver, 24.1 in skin, 22.9 in the white M. tensor fasciae latae, 21.6 in the red M. diaphragma and 19.6 in the remainder (exsanguinated whole body without liver and gastrointestinal tract) of lambs. 3. Absolute synthesis rates (ASR) of tissue protein were 17, 19 and 42 g/d in the liver, skin and skeletal muscle respectively, and 112 g/d in the remainder. The ASR of whole-body protein, derived from the tissue values, was 146 g/d, i.e. 33 g/d per kg body-weight. The calculated whole-body protein FSR was 23.9%/d. 4. The relative percentage contribution of liver, skin and skeletal muscle to whole-body protein synthesis was 11.7, 13.1, and 29.0. 5. We concluded that tissue protein FSR in lambs were in exactly the same decreasing order, from visceral tissues to skeletal muscles, as observed in rats. The ovine FSR estimates and the partitioning of protein synthesis between tissues were in the same range as values recently obtained by flooding-dose experiments in immature rats, piglets, and even in chicks. These findings suggest that inter-species differences are rather limited.

Effect of different total parenteral nutrition fuel mixes on small intestinal growth and differentiation in the infant miniature pig

Insulin has been proposed as an important factor in the regulation of growth and differentiation of the small intestine. In the newborn miniature pig, we induced significant physiologic increases in serum insulin and the insulin/glucagon ratio without altering serum glucose, beta-hydroxybutyrate, glucagon, cortisol, T3, and T4 using glucose-based total parenteral nutrition (TPN) in one group (group G) compared with a combination of glucose and fat in another group (group G/F). Control animals were sham-operated and fed a pelleted diet (group OC). Duodenal villus surface area and mucosal height were significantly greater in group G/F compared with group G. No other differences between the TPN groups were found in small intestinal growth, mucosal protein, deoxyribonucleic acid and ribonucleic acid content, and disaccharidase activities. As anticipated, group OC demonstrated increased intestinal length, weight, and villous surface area compared with the TPN groups. Ileal sucrase and jejunal and ileal maltase activities were greater in the TPN groups compared with those in group OC. Physiologic changes in serum insulin and the insulin/glucagon ratio induced by the TPN fuel mix do not appear to have altered small intestinal growth, composition, and differentiation in the healthy small intestine.

Lipoprotein secretion in lean and obese Zucker female rats in vivo and in a single-pass-perfused liver preparation

The plasma lipoprotein composition as well as lipoprotein synthesis and secretion were studied in vivo and in a single-pass-perfused liver preparation in lean and obese Zucker rats. Compared with their lean littermates the levels in the plasma of very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL) + low density lipoprotein (LDL) and high density lipoprotein (HDL) were increased 4-, 2- and 2.5 fold, respectively, in obese rats. In these rats both VLDL and IDL + LDL were enriched in triglycerides, while the HDL were enriched in cholesterol. Although the VLDL and IDL + LDL protein concentrations were the same in lean and obese rats, the HDL protein concentration was 3-fold greater in the obese rats. Both the lean and obese rats incorporated similar amounts of [14C]leucine into total liver protein. However, obese rats incorporated 2.5-fold and 6-fold more [14C]leucine into VLDL and HDL in vivo, 2.7-fold and 1.7 fold more [35S]methionine in VLDL and HDL present in the perfusate, than did lean rats. The perfusate [35S]S-labelled apoproteins (apo-B100, B48; apo-E, apo-AI, apo-AIV and apo-C) were separated by gel electrophoresis and identified by autoradiography. Incorporation of [3H]glycerol into liver, VLDL, IDL + LDL and HDL triglycerides was 2-, 48-, 13- and 1.5-fold higher in obese than in lean rats, respectively. The [3H]-labelled triglycerides in VLDL and IDL + LDL present in the perfusate was 5.4-fold and 4.4-fold more in obese rat. There was no difference in the incorporation of [3H]glycerol into triglycerides of perfusate HDL between the two genotypes of rats. Thus, the hypertriglyceridaemia observed in obese Zucker rats results from very high synthetic rates of both the lipid and protein moieties of plasma lipoproteins. Before this study, no report of the simultaneous triglycerides and protein synthesis in vivo and in a single-pass-perfused liver preparations had been reported.

Protein synthesis and growth in the gastrointestinal tract of the young preruminant lamb

1. In Expt 1, fractional synthesis rates (FSR) of tissue protein were measured along the gastrointestinal tract (GIT) of six 1-week-old, milk-fed lambs by using a large amount of L-[3,4(n)-3H]valine. 2. In Expt 2, eighteen lambs were used to determine the fractional growth rate (FGR) of gastrointestinal tissue protein. 3. FSRMinimum (Min) and FSRMaximum (Max) were calculated assuming plasma or tissue homogenate free valine specific radioactivity was representative of the valine precursor pool for protein synthesis. There were no significant differences between FSRMin and FSRMax in any gastrointestinal tissue of lambs used in Expt 1 (P greater than 0.05). FSR gradually and significantly (P less than 0.05) increased from the oesophagus (FSRMax 26.5%/d), reticulo-rumen (30.1%/d), omasum (41.0%/d) and abomasum (56.1%/d) to small intestine (87.5%/d), and then declined significantly (P less than 0.05) towards the caecum (45.2%/d) and the colon (38.4%/d). No significant differences were observed between FSR in the duodenum, jejunum or ileum (P greater than 0.05). 4. FGR ranged from 2.6%/d in the oesophagus to 8.7%/d in the omasum. The ratio, FGR:FSR, which reflected the efficiency of protein deposition, was at a maximum in the stomachs and caecum and at a minimum in the small intestine. 5. The relative contribution of the oesophagus, stomachs, small intestine and large intestine to GIT protein synthesis was 1, 13, 76 and 10% respectively. The GIT accounted for approximately 11.5% of whole-body protein synthesis.

Assessment of in vivo protein synthesis in lamb tissues with [3H]valine flooding doses

Week-old lambs received an intravenous injection of 4.3, 8.5, 12.8 or 17.1 mmol [3H]valine/5 kg body weight, i.e., 3.6-14.4-times the whole-body free valine content. To ensure that protein synthesis measurements in lambs are reliable within a 30-min period, these large amounts of valine must account for at least around 11-times the total free pool of valine. This amounted to 12.8 mmol valine/5 kg body weight. There were no significant variations in plasma insulin and plasma glucagon levels 5, 13 and 30 min after the injection of so much valine. The fractional rates of protein synthesis were determined in tissues of animals receiving either 12.8 or 17.1 mmol valine/5 kg body weight. The rates of protein synthesis in the jejunum (87.5%/day), liver (106.6%/day) and tensor fasciae latae muscle (18.8%/day) of lambs injected with the 12.8 mmol [3H]valine flooding dose, were in the range of data obtained in immature rats. Increasing the flooding amount of valine up to 17.1 mmol/5 kg body weight did not significantly alter protein synthesis rates in the jejunum, liver or skeletal muscle. This suggested that both the flooding-dose method in itself and valine had no effect on in vivo protein synthesis.

Effects of adrenalectomy before weaning in the genetically obese Zucker rat (fa/fa)

1. Lean (Fa/?) and obese (fa/fa) Zucker rats were adrenalectomized or sham-operated at 19 d of age (3 d before weaning). Injection of corticosterone for 3 d after weaning (1.0 mg/d) was necessary to ensure survival of adrenalectomized fa/fa but not Fa/? rats. Intact and adrenalectomized fa/fa rats had a lower rectal temperature than Fa/? animals before and 3 d after adrenalectomy. The post-weaning survival of adrenalectomized fa/fa rats was enhanced by maintenance at an ambient temperature of 30 degrees rather than 22 degrees. 2. Adrenalectomized and sham-operated rats were therefore kept at 30 degrees, fed ad-lib. and killed at 34 d. Adrenalectomy had only small effects on the growth, body composition and appetite of Fa/? rats. The hyperphagia, greater lipid content, reduced protein content and hyperinsulinaemia of fa/fa rats were completely abolished by adrenalectomy. 3. Intact fa/fa rats had higher liver glycogen contents and higher activities of the hepatic enzymes tyrosine aminotransferase (EC 2.6.1.5) and acetyl CoA carboxylase (EC 6.4.1.2) than intact Fa/? animals. Adrenalectomy abolished these phenotypic differences. 4. Injection of adrenalectomized rats with 1.0 mg corticosterone-21-acetate daily from weaning to 34 d restored the abnormal body composition, hyperphagia, hyperinsulinaemia, higher hepatic glycogen and enzyme activities of fa/fa rats. 5. In a second experiment adrenalectomized rats were injected with 1.0 mg corticosterone-21-acetate daily from weaning to 34 d and kept at 22 degrees. fa/fa rats adrenalectomized and injected with corticosterone had a reduced body lipid content compared with intact fa/fa rats but still contained more lipid than intact or similarly treated Fa/? animals. 6. In both experiments adrenalectomized Fa/? and fa/fa rats injected daily with corticosterone had the same plasma concentrations of this hormone when killed 3 h after the last injection at 34 d. It is concluded that corticosterone is required for expression of the abnormal appetite, hyperinsulinaemia and body composition of the fa/fa rat.

The fate of 14C derived from radioactively labelled dietary precursors in young rats of the Zucker strain (Fa/- and fa/fa)

The metabolic fate of 14C derived from radioactively labelled dietary precursors was determined in immature (18- and 25-day-old) lean and obese Zucker rats. This included measurement of 14C incorporated into body lipid, non-essential amino acids and expired CO2. Before weaning (18 days) there was no phenotypic difference between the fates of [14C]palmitate and [14C]-glucose. However, after weaning (25 days) all the precursors studied exhibited an increase in the fraction incorporated into lipid in the obese rat as compared with the lean animal. This was reflected in the fate of acetyl-CoA in the tricarboxylic acid cycle. There was little phenotypic difference in the fraction of leucine or valine catabolized. The results presented here suggest that the high rate of lipogenesis found in the obese rat is supported by carbon from all the dietary precursors studied. It is also argued that the decreased protein deposition found in the obese rat is not caused by the high rate of lipogenesis removing precursors for protein synthesis, as has been suggested elsewhere [Cleary, Vasselli & Greenwood (1980) Am. J. Physiol. 238, E284-E292].

Protein synthesis in skeletal muscle measured at different times during a 24 hour period

Six groups of 5 male rats (starting body weight 109 g) were allowed free access to a conventional rat diet. At 4 hourly intervals, starting at 10.00 h muscle protein synthesis was measured. By relating the weights of the gastrocnemius and soleus muscles to the initial body weights of the animals (i.e., at 09.30, day 1), a linear increase in muscle weight throughout the day was demonstrated. The fractional rate of muscle protein synthesis varied from 16.8% per day to 20.3% per day in gastrocnemius muscle and from 17.9% per day and 22.1% per day in the soleus. It was calculated that the maximum error incurred in estimating daily muscle protein synthesis by extrapolation of the value at any one time was 6% in gastrocnemius and 9% in soleus. It is concluded that calculations of the average rate of muscle protein degradation based on the difference between the rates of synthesis and deposition are generally valid in rats allowed free access to an adequate diet.

Effects on growth and endocrine status of maintaining obese and lean Zucker rats at 22 degrees C and 30 degrees C from weaning

Lean and genetically obese Zucker rats were maintained from 21 to 34 days of age at an ambient temperature of 22 degrees C or 30 degrees C. Maintenance at 30 degrees C normalised the lower rectal temperature of obese rats. At 30 degrees C obese rats reduced their food intake compared with obese animals at 22 degrees C but they were still hyperphagic compared with non-obese animals at 30 degrees C. At 30 degrees C obese rats accumulated as much lipid in their carcase, but deposited less protein than obese animals at 22 degrees C. Obese rats were hyperinsulinemic compared with non-obese animals at either temperature. Plasma T3 and T4 levels did not differ between phenotypes and T4 levels were lower in both phenotypes at 30 degrees C. Plasma corticosterone levels were higher in obese rats at both temperatures and this phenotypic difference was larger at 30 degrees C. In a second experiment rats were maintained at 22 degrees C from 21-34 days of age and obese rats pair-fed to the ad lib intake of non-obese animals. Obese rats still became more obese than non-obese animals and deposited less protein than either non-obese or ad lib fed obese animals. Pair-fed obese rats were hyperinsulinemic and had higher plasma corticosterone concentrations than non-obese animals. These findings suggest that the raised plasma corticosterone concentrations of obese rats kept at 30 degrees C, or not allowed to express their greater appetite, may cause reduced protein deposition.

Digestion of polysaccharides, protein and lipids by adult cockerels fed on diets containing a pectic cell-wall material from white lupin (Lupinus albus L.) cotyledon

1. The cell-wall material of white lupin (Lupinus albus L.) cotyledon is characterized by low contents of cellulose (47 g/kg) and lignin (17 g/kg) and a high content of pectic substances (710 g/kg). The digestion of lupin cell-wall material by adult cockerels was estimated using gas-liquid chromatographic analyses of alditol acetates derived from polysaccharide sugars. The analyses were performed in the destarched water-insoluble fractions of feed and excreta. Digestibility measurements were carried out using a 3 d balance period including a 2 d feeding period and a 24 h final starvation period. 2. In the first experiment, six animals were given a diet containing 510 g white lupin cotyledon flour/kg which was the only source of protein and cell walls in the diet. The apparent digestibility of cell-wall components was near zero. 3. In the second experiment, three diets were prepared by diluting a fibre-free basal diet (diet A) by a semi-purified cell-wall preparation introduced at two different levels: 100 g/kg (diet B) and 200 g/kg (diet C). The semi-purified cell walls were prepared from the white lupin cotyledon flour used in the first experiment. The true digestibilities of polysaccharides measured in birds given diets B and C were near zero. It is suggested that the measurement of the neutral-detergent fibre (NDF) content according to Van Soest & Wine (1967) is not a suitable procedure for estimating the undigestible fibre content in poultry nutrition as the cell-wall pectic substances are not included in the NDF measurement. 4. Addition of the semi-purified cell-wall preparation (Expt 2) resulted in a slight decrease in the apparent protein digestibility. This decrease might be explained by the addition of undigestible cell-wall protein. 5. Addition of the semi-purified cell-wall preparation (Expt 2) had no effect on the apparent lipid digestibility. 6. The metabolizable energy values of the basal diet fraction of diets B and C were calculated assuming that the added plant cell-wall fraction was of no energy value. These calculated values were similar to the measured metabolizable energy value of diet A (basal). Thus, the tested pectic plant cell walls seemed to act as a diluent. It is suggested that, on the point of the digestion yields, all types of plant cell walls would act as diluents, in poultry.

Intestinal cellular proliferation and protein synthesis in zinc-deficient rats

Immature male Wistar rats were given a low-zinc semi-synthetic diet (2 mg Zn/kg) for 28 d. Control groups received a similar diet supplemented with 58 mg Zn/kg either ad lib. or in amounts matched to the consumption of the Zn-deficient group. Rates of growth, food consumption and small intestinal length were significantly reduced in the Zn-depleted rats. Zn deficiency in the rat was associated with a reduction in the ratio, crypt: villus and a lower rate of crypt cell division in the jejunum. This resulted in a substantial decrease in the net influx of new cells into the villi of the Zn-deficient animals compared with controls. The fractional rates of protein synthesis in jejunal mucosa were measured by a technique based on the determination of L-[4-3H]phenylalanine incorporation. There was no evidence of a decline in the protein synthetic rate in total mucosa from Zn-deficient rats. It is suggested that a reduction in cell influx into the villi may be responsible for the morphological and functional changes observed in the small intestine of rats fed on a low-Zn diet.

Differences in intestinal protein synthesis and cellular proliferation in well-nourished rats consuming conventional laboratory diets

Male Wistar rats (100 g) were given a commercial pellet feed or a semi-synthetic diet ad lib. Although the pellet-fed group grew slightly faster than the other group during the early part of the feeding period, there was no significantly difference between the final weights of the groups. The fractional rates of protein synthesis in jejunum, proximal ileum and liver were measured by a technique based on the determination of L-[4-3H]phenylalanine incorporation over a short time period. Protein synthesis was higher in both jejunum and ileum of the pellet-fed rats compared with those eating the semi-synthetic diet, but there was no difference between the rates of protein synthesis measured in the livers of the groups. The rate of mucosal cell division was significantly faster in the ileal mucosa of the pellet-fed group compared with the other group, and there were significant differences in some aspects of mucosal morphology. The maintenance of higher rates of cell turnover and protein synthesis in animals given a commercial pellet feed is unexplained, but it may be related to the presence of non-absorbable polysaccharides or other complex plant materials in the pellet feed.

Changes in prostaglandin release associated with inhibition of muscle protein synthesis by dexamethasone

Forelimb digit extensor muscles from fed rabbits were incubated in the absence or presence of dexamethasone (100 nM). The presence of dexamethasone decreased the rates of protein synthesis, prostaglandin F2 alpha and prostaglandin E2 release after a time lag of 2.5-3 h. Although intermittent stretching stimulated both protein synthesis and prostaglandin release in the presence of dexamethasone, the absolute activities of both processes were lower in the presence of the steroid than in its absence. It is suggested that the inhibitory action of dexamethasone on muscle protein synthesis in vitro results from its effect on the activity of plasma-membrane phospholipase A2.

Regulation of hepatic tyrosine aminotransferase in genetically obese rats

The activities of hepatic tyrosine aminotransferase, tryptophan oxygenase and serine dehydratase were increased in obese rats shortly after weaning. Immunotitration experiments showed that the increase in tyrosine aminotransferase activity resulted from an increase in enzyme protein in obese rats. No increase in hepatic tyrosine aminotransferase was observed in suckling pre-obese rats. The post-weaning increase in hepatic tyrosine aminotransferase of obese rats was only observed during the light phase of the diurnal cycle, but was prevented by pair-feeding and by starvation. Tryptophan increased hepatic tyrosine aminotransferase of lean rats to obese levels but had no effect in obese rats until tyrosine aminotransferase levels were reduced by starvation or adrenalectomy. Adrenalectomy abolished the increase in hepatic tyrosine aminotransferase activity in obese rats although serum corticosterone was normal in these animals. Hepatic and brain tyrosine concentrations were decreased in obese rats but normalized after adrenalectomy. The results suggest that the corticosteroid-dependent increase in food and tryptophan intake may be the primary cause of the increased hepatic amino acid catabolism of obese rats.