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

Protein elongation rates in tissues of growing and adult sheep

To identify the relative roles of translation initiation and elongation in the long term control of protein synthesis in ovine tissues, fractional synthesis rates (FSR) and ribosomal transit times (RTT) were measured in vivo in 24 ewe lambs at 3 levels of intake [maintenance (M), 1.5M, and 2M] and 8 mature ewes at 2M intake. After 17 to 25 d on treatment, animals were given an i.v. flooding dose of l-[ring-2,6-(3)H]phenylalanine and tissues were collected for analysis of radioactivity in free protein, total protein, and nascent ribosome-associated proteins. Ribosome transit time (the inverse of elongation rate) averaged 83, 393, 183, 241, 85, and 113 s for liver, duodenum, skin, rumen, semimembranosus, and LM, respectively. In response to an increased level of intake, protein FSR increased (P < 0.01) in all tissues except rumen and was attributed to greater translational efficiency. There was no effect (P > 0.50) of intake on RTT in these tissues, and the estimated proportion of ribosomes attached to and actively translating mRNA was increased (P < 0.07), indicating that an upregulation of initiation was responsible for the greater FSR. Mature ewes exhibited lower (P < 0.10) protein FSR in all tissues compared with lambs, which was related to a decline in the RNA:protein ratio in all tissues except for liver and duodenum. In all tissues but liver and semimembranosus, RTT increased (P < 0.10) with age. The lower elongation rate was not considered to have influenced the protein synthetic rate, but it caused an increase in the proportion of ribosomes actively translating mRNA. It is anticipated that this work will provide direction to future studies of the molecular mechanisms of chronic FSR control.

Perspectives on ruminant nutrition and metabolism. II. Metabolism in ruminant tissues

The discovery of the dominance of short-chain fatty acids as energy sources in the 1940s and 1950s, as discussed in part I of this review (Annison & Bryden, 1998) led to uncertainties concerning the interrelationships of glucose and acetate in ruminant metabolism. These were resolved in the following decade largely by use of14C-labelled substrates. Although only small amounts of glucose are absorbed in most dietary situations, glucose availability to ruminant tissues as measured by isotope dilution was shown to be substantial, indicating that gluconeogenesis is a major metabolic activity in both fed and fasted states. Studies with14C-labelled glucose and acetate revealed that in contrast to non-ruminants, acetate and not glucose is the major precursor of long-chain fatty acids in ruminant tissues. Interest in the measurement of energy metabolism in livestock grew rapidly from the 1950s. Most laboratories adopted indirect calorimetry and precise measurements of the energy expenditure of ruminants contributed to the development of new feeding systems. More recently, alternative approaches to the measurement of energy expenditure have included the use of NMR spectroscopy, isotope dilution and the application of the Fick principle to measure O2consumption in the whole animal and in defined tissues. The refinement of the classical arterio-venous difference procedure in the study of mammary gland metabolism in the 1960s, particularly when combined with isotope dilution, encouraged the use of these methods to generate quantitative data on the metabolism of a range of defined tissues. The recent introduction of new methods for the continuous monitoring of both blood flow and blood O2content has greatly increased the precision and scope of arterio-venous difference measurements. The impact of data produced by these and other quantitative procedures on current knowledge of the metabolism of glucose, short-chain fatty acids and lipids, and on N metabolism, is outlined. The role of the portal-drained viscera and liver in N metabolism is discussed in relation to data obtained by the use of multi-catheterized animals. Protein turnover, and the impact of stress (physical, social and disease related) on protein metabolism have been reviewed. The growth of knowledge of mammary gland metabolism and milk synthesis since the first quantitative studies in the 1960s has been charted. Recent findings on the regulation of amino acid uptake and utilization by the mammary gland, and on the control of milk secretion, are of particular interest and importance.

Luminal amino acids acutely decrease intestinal mucosal protein synthesis and protease mRNA in piglets

Because parenteral feeding is associated with negative N balance and reduced rates of protein synthesis in intestinal mucosa, we hypothesized that luminal exposure to specific amino acids or energy fuels would stimulate intestinal protein synthesis. We studied the acute effects of luminal nutrients on mucosal protein synthesis in the absence of systemic influences. Multiple jejunal segments constructed in piglets deprived of food overnight (n = 6) were randomly assigned to luminal perfusion with saline, 30 mmol/L amino acid mixture with or without 50 mmol/L glucose, or 30 mmol/L glutamine for 90 min. Protein synthesis was then measured by luminal perfusion with L-[2,6-(3)H]-phenylalanine. Energy substrates (glucose, short-chain fatty acids or beta-hydroxybutyrate) had no effect on mucosal protein synthesis. Relative to saline, a 30 mmol/L amino acid mixture or 30 mmol/L glutamine suppressed mucosal protein synthesis by 20-25% (P < 0.05). On the basis of these surprising results, we speculated that a coordinate reduction of proteolytic processes would be required to maintain positive intestinal N balance. Although intestinal protein catabolism cannot be assessed directly, the 30 mmol/L amino acid mixture acutely suppressed mucosal levels of mRNA encoding ubiquitin, 14-kDa ubiquitin conjugating enzyme and the C9 subunit of the proteasome by 20-30% (P < 0.05), demonstrating the sensitivity of components of the ATP-ubiquitin proteolytic pathway to acute regulation by nutrients. The suppression of protein synthesis by luminal amino acids in the absorptive state might lower intestinal utilization of amino acids to ensure efficient allocation of absorbed nutrients to nonintestinal tissues.

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.

The effect of a cold environment on protein and energy metabolism in calves

Eleven Holstein bull calves 35 d of age were assigned to one of three treatment groups: (1) W72, warm environment (20 degrees), 72 g feed/kg body weight (BW)0.75 per d, (2) C72, cold environment (-5 degrees), 72 g feed/kg BW0.75 per d, or (3) C90, cold environment (-5 degrees), 90 g feed/kg BW0.75 per d. Fractional synthesis rates (FSR) of protein in the rumen wall, rumen papillae, omasum, duodenum, kidney, liver, heart, longissimus dorsi, biceps femoris and skin were determined following a continuous infusion of [3H]phenylalanine. Phenylalanine flux was elevated in both groups of cold-adapted calves. FSR of protein in the two muscles and skin were reduced along with N retention in the calves in the C72 group compared with the other two groups. Muscle protein degradation, estimated from urinary N tau-methylhistidine excretion, tended to be elevated in both groups of cold-adapted calves. Reduced protein synthesis and increased protein degradation in the C72 group contributed to reduced muscle protein gain. It appears that when feed intake is limited in cold-adapted animals, muscle and skin have a lower priority for nutrients than other organs and tissues, resulting in reduced protein synthesis. It seems unlikely that thermogenesis due to enhanced protein synthesis contributed to the increased heat production in the cold.

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.

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.