Stimulatory Effect Of Amino Acids Research Articles

Induction of Pancreatic Growth and Proteases by Feeding a High Amino Acid Diet Does Not Depend on Cholecystokinin in Rats

We examined differences in pancreatic growth, enzyme content and enzyme concentration between rats fed diets containing normal (2.49 g nitrogen/kg diet) or high (7.46 g nitrogen/kg diet) levels of an amino acid mixture and those in rats fed diets containing normal and high levels of casein for 11 d. Rats fed these diets were injected with a potent cholecystokinin antagonist, MK-329 (2.5 mg/kg body wt·d) or with vehicle only. Pancreatic contents (units in pancreas per 100 g body wt) of protein, trypsinogen and chymotrypsinogen were greater in rats fed a high amino acid diet compared with those in rats fed a normal amino acid diet. Proportionate increases in protein and the serine proteases in pancreata of the high amino acid group relative to those of the normal amino acid group were comparable to those of the high casein group relative to the normal casein group. The pancreatic protease concentrations (units/g pancreas) of rats fed the high casein diet and treated with MK-329 were lower than in rats fed high casein but not treated with MK-329. This difference was not observed in rats fed the high amino acid diet. These results demonstrate that pancreatic growth and proteases are induced by dietary amino acids in rats, and the stimulatory effects of amino acids on exocrine pancreas do not depend on cholecystokinin.

Stimulatory effects of amino acids on \u03b3-polyglutamic acid production by Bacillus subtilis

This paper is about study to increase the γ-PGA yield by developing new methods. The effect of various amino acids on production of γ-PGA by Bacillus subtilis Z15 was investigated. The γ-PGA yield was increased 23.18%, 12.15% and 31.46%, respectively, with 3 g/L aspartic acid (0 h), 1.5 g/L phenylalanine (0 h) and 7 g/L glutamic acid (24 h). Additonally, crude extract of glutamic acid after isoelectric crystallization (CEGA)could be a replacement for glutamate for γ-PGA production. Then, response surface methodology (RSM) was used for further optimization. The final media ingredient of amino acids were obtained as follow: CEGA 9 g/L, aspartic acid 4 g/L, phenylalanine 1.55 g/L. By applying this receipt in 5-L bioreactor, the γ-PGA yield reached 42.92 ± 0.23 g/L after 44 h, which is 63.1% higher than the control without amino acids for production. In addition, amino acids could shorten the lag phase and the average fermentation time (44 h versus 48 h). Fermentation with amino acids addition can be an positive option for γ-PGA production.

Muscle Wasting and Resistance of Muscle Anabolism: The “Anabolic Threshold Concept” for Adapted Nutritional Strategies during Sarcopenia

Skeletal muscle loss is observed in several physiopathological situations. Strategies to prevent, slow down, or increase recovery of muscle have already been tested. Besides exercise, nutrition, and more particularly protein nutrition based on increased amino acid, leucine or the quality of protein intake has generated positive acute postprandial effect on muscle protein anabolism. However, on the long term, these nutritional strategies have often failed in improving muscle mass even if given for long periods of time in both humans and rodent models. Muscle mass loss situations have been often correlated to a resistance of muscle protein anabolism to food intake which may be explained by an increase of the anabolic threshold toward the stimulatory effect of amino acids. In this paper, we will emphasize how this anabolic resistance may affect the intensity and the duration of the muscle anabolic response at the postprandial state and how it may explain the negative results obtained on the long term in the prevention of muscle mass. Sarcopenia, the muscle mass loss observed during aging, has been chosen to illustrate this concept but it may be kept in mind that it could be extended to any other catabolic states or recovery situations.

Regulation of TORC1 by Rag GTPases in nutrient response

TORC1 (Target of rapamycin complex 1) has a critical role in the regulation of cell growth and cell size. A wide range of signals, including amino acids, is known to activate TORC1. Here, we report the identification of Rag GTPases as novel activators of TORC1 in response to amino acid signals. Knockdown of Rag gene expression suppressed the stimulatory effect of amino acids on TORC1 in Drosophila S2 cells. Expression of constitutively active (GTP-bound) Rag in mammalian cells enhances TORC1 in the absence of amino acids while expression of dominant negative Rag blocks the stimulatory effects of amino acids on TORC1. Drosophila genetic studies also show that the Rag GTPases regulate cell growth, autophagy, and animal viability under starvation. Together, our studies establish a novel function of Rag GTPases in TORC1 activation in response to amino acid signals.

Leucine in age-associated sarcopenia

A progressive loss of muscle mass has been well described in both humans and rodents during ageing. This loss of proteins results from an imbalance between protein synthesis and degradation rates. Although some authors have shown a decrease of myofibrillar protein synthesis rates in human volunteers, this imbalance is not clearly apparent when basal rates of protein turnover are measured. A decrease in muscle protein synthesis stimulation has nevertheless been detected in ageing rats during the postprandial period, suggesting that the ‘ meal signal’ is altered during ageing. Many results now suggest that aged muscle is less sensitive to the stimulatory effect of amino acids at physiological concentrations, but is still able to respond if the increase in aminoacidaemia is sufficiently large. Indeed, amino acids play an important role in regulating muscle protein turnover both in vitro and in vivo. Of amino acids, leucine seems to play the key role in regulating the metabolic function. It inhibits proteolysis and stimulates muscle protein synthesis independently of insulin. Leucine has been shown to act as a mediator, by modulating specifically the activities of intracellular kinases linked to the translation of proteins such as phosphatidylinositol 3_ kinase and mammalian target of rapamycin – 70 kDa ribosomal protein S6 (p70S6K) kinases. We recently demonstrated in vitro that protein synthesis in ageing rat muscles becomes resistant to the stimulatory effect of leucine in its physiological concentration range. Protein synthesis was however stimulated normally when the leucine concentration was increased well above its postprandial level. We also studied the effect of meal leucine supplementation on in vivo protein synthesis in adult and ageing rats. Leucine supplementation had no additional effect on muscle protein synthesis in adults but totally restored its stimulation in ageing rats. Whether chronic oral leucine supplementation would be beneficial for maintaining muscle protein mass in elderly humans remains to be studied.

A role for leucine in rejuvenating the anabolic effects of food in old rats

Although the old saw tells us that old age does not come alone, it is discomforting to see our physical capacity dwindling: the prospect of struggling to rise from a chair (or, heaven forbid, the toilet!) is not a pleasant one. Age-related muscle wasting may start as early as the fourth decade and occurs at 0.5–2% per year. The underlying mechanisms are a mystery, although we all know individuals who remain preternaturally young: as well as genetic predisposition, environmental influences, probably including imprinting, are probably very important. Among these, of course, is eating. Since it was realized (Volpi et al. 2001) that initially reported decrements in the basal rate of muscle protein synthesis in elderly people were unfeasibly large, the focus of many of us interested in age-related sarcopenia has turned to the ability of old muscle to respond appropriately to food. In human beings, a meal roughly doubles protein synthesis (Rennie et al. 1982) and most of this effect is due to the stimulatory effects of amino acids (Bennet et al. 1989), particularly branched chain amino acids and particularly leucine (Smith et al. 1992). Oddly, insulin has a negligible effect in modulating muscle protein synthesis in the presence of amino acids (Greenhaff et al. 2005) although there may be a requirement for a small, permissive amount of insulin for the amino acids to signal appropriately. However, the stimulation of protein synthesis is not the entire story and muscle protein breakdown falls as a result of a meal and here previous evidence suggests that the major player is not leucine but insulin, which markedly inhibits muscle protein breakdown. By and large the amplitude of the meal-related increase in protein synthesis is greater than the meal-related depression of protein breakdown but the two combine to replace protein which is lost in the post-absorptive period, during the diurnal cycle. It is now apparent that both in rats and human beings ageing is associated not so much with decrements and basal rates of protein turnover but in an inability to mount appropriate anabolic responses to feeding – what we have termed anabolic resistance (Cuthbertson et al. 2005). This is shown by a decreased sensitivity and responsiveness of protein synthesis in the muscle of old rats and men aged 60–70 and this is itself apparently due to decreased sensing and signalling activity in aged muscle (Guillet et al. 2004; Cuthbertson et al. 2005). Some time ago the late Bernard Beaufrere and co-workers demonstrated that whole body proteolysis appeared to be more resistant to the effects of insulin in older individuals (Boirie et al. 2001), which is obviously pertinent to the question of anabolic resistance of protein breakdown after food. Now Didier Attaix and his group (Combaret et al. 2005) have attempted to unravel the mechanisms of the postprandial blunting of the normal inhibition of protein synthesis using old (22 month) and young (8 month) rats. They confirm, in a paper published in this issue of The Journal of Physiology that by a number of indices the normal feeding-related fall of muscle protein breakdown is less in the old rats. Remarkably, what they have shown is that when old rats are fed a diet which is supplemented with leucine, there is what amounts to a rejuvenation of the normal postprandial inhibition of muscle protein breakdown. This is exciting because it strengthens the idea of a co-ordinated linkage between the meal-related stimulation of protein synthesis (via a pathway involving PKB/mTOR/p70S6 kinase) and the inhibition of the MAFbx component of the ubiquitin-dependent proteasome proteolytic pathway. Although the authors did not test the possibility, it may be that leucine also does something to sharpen up the sensitivity of muscle proteolysis to insulin – something which obviously needs to be tested. What does this tell us about preserving our shrinking muscles? As the lean body mass declines older people need less dietary energy – and cutting down on dietary fat and high glycaemic-index carbohydrates is obviously a good idea for health reasons. However, in addition, although we probably do not need to eat more protein it makes sense that what we do eat is of high quality, and that probably means animal protein, which contains lots of leucine.

Leucine: a key amino acid in ageing-associated sarcopenia?

During ageing, a progressive loss of muscle mass has been well described in both man and rodents. This loss of proteins results from an imbalance between protein synthesis and degradation rates. Although some authors have shown a decrease of myofibrillar protein synthesis rates in human volunteers, this imbalance is not clearly apparent when basal rates of protein turnover are measured. A decrease in muscle protein synthesis stimulation was detected nevertheless in ageing rats during the postprandial period, suggesting that the 'meal signal' was altered during ageing. Many results now suggest that aged muscle is less sensitive to the stimulatory effect of amino acids at physiological concentrations but is still able to respond if the increase in aminoacidaemia is sufficiently large. Indeed amino acids play an important role in regulating muscle protein turnover both in vitro and in vivo. At the molecular level, amino acids modulate gene expression. Amino acid response elements have been characterised in the promoter of transcriptional factor CCAAT-enhancer binding protein homologous protein and asparagine synthetase genes. Among amino acids, leucine seems to play the major role in regulating the metabolic function. It inhibits proteolysis and stimulates muscle protein synthesis independently of insulin. Leucine has been shown to act as a real mediator by modulating specifically the activities of intracellular kinases linked to the translation of proteins such as phosphatidylinosinol 3' kinase and mammalian target of rapamycin-70 kDa ribosomal protein S6 (p70S6K) kinases. We recently demonstrated in vitro that protein synthesis of ageing rat muscles becomes resistant to the stimulatory effect of leucine in its physiological concentration range. However, when leucine concentration was increased greatly above its postprandial level, protein synthesis was stimulated normally. Moreover, we studied the effect of meal leucine supplementation on in vivo protein synthesis in adult and ageing rats. Leucine supplementation had no additional effect on muscle protein synthesis in adults but totally restored its stimulation in ageing rats. Whether chronic oral leucine supplementation would be beneficial for maintaining muscle protein mass in elderly men and women remains to be studied.

Latency, Duration and Dose Response Relationships of Amino Acid Effects on Human Muscle Protein Synthesis

The components of the stimulatory effect of food on net deposition of protein are beginning to be identified and separated. One of the most important of these appears to be the effect of amino acids per se in stimulating muscle anabolism. Amino acids appear to have a linear stimulatory effect within the range of normal diurnal plasma concentrations from postabsorptive to postprandial. Within this range, muscle protein synthesis (measured by incorporation of stable isotope tracers of amino acids into biopsied muscle protein) appears to be stimulated approximately twofold; however, little further increase occurs when very high concentrations of amino acids (>2.5 times the normal postabsorptive plasma concentration) are made available. Amino acids provided in surfeit of the ability of the system to synthesize protein are disposed of by oxidation, ureagenesis and gluconeogenesis. The stimulatory effect of amino acids appears to be time dependent; a square wave increase in the availability of amino acids causes muscle protein synthesis to be stimulated and to fall back to basal values, despite continued amino acid availability. The relationship between muscle protein synthesis and insulin availability suggests that most of the stimulatory effects occur at low insulin concentrations, with large increases having no effect. These findings may have implications for our understanding of the body's requirements for protein. The maximal capacity for storage of amino acids as muscle protein probably sets an upper value on the extent to which amino acids can be stored after a single meal.

Protein supplements and exercise

Active persons ingest protein supplements primarily to promote muscle strength, function, and possibly size. Currently, it is not possible to form a consensus position regarding the benefit of protein or amino acid supplements in exercise training. Determination of whether supplements are beneficial has been hampered by the failure to select appropriate endpoints for evaluation of a positive effect. Furthermore, studies focused at a more basic level have failed to agree on the response of protein metabolism to exercise. An additional complication of dietary studies that is not often taken into account is amount of energy intake. Because of these and other complications, studies at the whole body level have not yielded a clear picture of the need for, or response to, dietary protein or amino acid supplements. Consequently, it is necessary to examine this issue at the tissue level. In untrained subjects, both muscle protein breakdown and synthesis are increased in response to exercise. Amino acid intake further stimulates muscle protein synthesis after exercise as a consequence of stimulating amino acid transport into the intramuscular compartment. The stimulatory effect of amino acids after exercise is greater than the effect of amino acids on muscle protein synthesis when given at rest. These data suggest that not only may the exact composition and amount of an amino acid supplement be important, but the timing of ingestion of the supplement in relation to the exercise must be considered in designing future studies to evaluate the efficacy of amino acid supplements.

Stimulatory effects of amino acids and oleic acid on poly(3-hydroxybutyric acid) synthesis by recombinant Escherichia coli

Addition of cysteine, isoleucine, methionine, or proline promoted poly(3-hydroxybutyric acid) [PHB] synthesis by recombinant Escherichia coli more than two-fold. Oleic acid also enhanced PHB synthesis more than three-fold. A PHB concentration of 70 g/l could be obtained by fed-batch culture of recombinant E. coli in a defined medium supplemented with small amounts of isoleucine, methionine, and proline. The stimulatory effects of amino acids and oleic acid on PHB synthesis seems to be due to the availability of more acetyl-CoA and/or NADPH.

Renal reserve filtration capacity in growth hormone deficient subjects.

In normal subjects, the glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) acutely increase in response to infusion of amino acids and to low doses of dopamine. It is uncertain whether circulatory growth hormone (GH) is a permissive factor for these stimulatory effects. GFR and ERPF (constant infusion technique using 125I-iothalamate and 131I-hippuran, respectively) were measured before and during the infusion of dopamine and amino acids in 8 GH deficient subjects. The clearance study was repeated during concomitant administration of octreotide to investigate whether this somatostatin analogue would modify the amino acid and dopamine-induced renal haemodynamic changes. Dopamine increased baseline GFR from 89 +/- 3 (mean +/- SEM, n = 8) to 102 +/- 4 ml min-1 1.73 m-2 and ERPF from 352 +/- 19 to 476 +/- 26 ml min-1 1.73 m-2, P less than 0.001 for both. During amino acid infusion GFR and ERPF increased to 108 +/- 3 and 415 +/- 23 ml min-1 1.73 m-2, respectively, P less than 0.001 for both. Octreotide did not significantly decrease baseline and dopamine-stimulated renal haemodynamics but lowered the amino acid-stimulated GFR (98 +/- 4 ml min-1 1.73 m-2, P less than 0.05) and ERPF (381 +/- 18 ml min-1 1.73 m-2, P less than 0.05). Basal plasma glucagon concentrations were not suppressed by octreotide, whereas the amino acid-induced increments in plasma glucagon were partially inhibited. It is concluded that GH is not a necessary factor for the stimulatory effects of amino acids and dopamine on renal haemodynamics.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of glycogen metabolism by gluconeogenic and ketogenic substrates in isolated hepatocytes from fed rats

1. 1. This study was conducted to examine the effects of gluconeogenic and ketogenic substrates on the activities of the glycogen-metabolizing enzymes and on glycogenolysis in isolated hepatocytes from fed rats. 2. 2. Gluconeogenic substrates like fructose, dihydroxyacetone or lactate turned out to stimulate the glucose-induced activation of glycogen synthase and this effect may be linked, to some extent, to the increase of the cellular glucose 6-phosphate concentration. 3. 3. The effect of fructose was accompanied by the onset of glycogen synthesis. 4. 4. Energetic substrates like fatty acids were also potent activators of glycogen synthase, especially in the presence of glucose. 5. 5. When fatty acids were added alone or together with a physiological concentration of glucose, they induced or potentiated the inhibition of glycogen phosphorylase-a. 6. 6. This inhibitory effect was mediated by a decrease of lactate release. 7. 7. The stimulatory effect of amino acids on glycogen synthase seemed to be direct, non mediated by an inhibition of the phosphorylase-a activity although hepatic glycogenolysis markedly decreased. 8. 8. Moreover, the amino acid action could be linked to their capacities to induce cell swelling and/or to limit proteolysis.

Improvement of Lactic Acid Production in Ultrafiltered Milk by the Addition of Nutrients

In 4h at 32°C, mesophilic lactic acid bacteria produced .74% acidity in 3.3:1 retentate of ultrafiltration diluted to 2:1 with permeate. When diluted with distilled water instead of permeate, .56% acidity was produced. In 2.4:1 retentate containing .5% yeast extract, .95% acidity was produced compared with .84% in the control (retentate without yeast extract). The B vitamins, lactose, and minerals (NaCl, MgSO4, FeSO4, and MnCl2) had little or no effect. With another lactic starter, 22 amino acids together at .04 mg/g each in 2:1 retentate improved acid production (.72% compared with .61% for the control). Nonessential amino acids as a group were more stimulatory than essential amino acids. Cysteine and cystine at .04 mg/g each were as stimulatory as all amino acids together and all amino acids minus cysteine and cystine. Reducing agent dithiothreitol at .05 mg/g was as stimulatory as cysteine and cystine. Sulfur-containing compounds (methionine and Na2SO4) had no effect, suggesting that the stimulatory effect of amino acids is via the reducing power of cysteine and cystine as well as nutritive value of other amino acids.

Potassium movements associated with amino acid and sugar transport in enterocytes isolated from rabbit jejunum.

Active transport of amino acids and the sugar, alpha-methyl-D-glucoside (alpha-MG) caused an increase in the rate of K efflux from isolated rabbit enterocytes. These effects were inhibited by apamin (5 X 10(-7) M), quinidine (10(-3) M), Ba (5 X 10(-3) M) and trifluoperazine (5 X 10(-5) M) but not by the loop diuretic furosemide (10(-4) M). None of these drugs affected the basal rate of K efflux. The stimulatory effects of amino acids or alpha-MG on K efflux are too great to be explained in terms of an increase in the electrical driving force across the plasma membrane of these cells and a change in membrane permeability is envisaged. An apparent Ca-dependent K permeability in isolated enterocytes can be demonstrated using the Ca ionophore A23187. The effect of the ionophore on K efflux is abolished by apamin or Ba. It is proposed that Ca-dependent K channels mediate the sugar and amino acid induced increases of K efflux. Under control conditions there is a decrease in intracellular K concentration during accumulation of alanine or alpha-MG. Ba by itself does not alter K concentration but it did produce a marked increase when used in conjunction with alanine or alpha-MG. The accumulation of alpha-MG was inhibited in the presence of Ba. This is consistent with an interference with the driving force for sugar accumulation. It is suggested that the increase in K permeability described has a role in both maintaining ion homoeostasis during Na-coupled transport and contributing to the driving force for sugar and amino acid absorption.

A new quantitative analysis of the feeding response inHydra japonica: Stimulatory effects of amino acids in addition to reduced glutathione

In the feeding response ofHydra japonica evoked by reduced glutathione or S-methyl-gluta-thione, tentacles writhed, and were shrunk and intertwined resulting in the state termed tentacle ball formation (TBF).

Effect of topical application of amino acids on gastric pepsin secretion in the rat. Part III: effect of L- and D-isomers of amino acids on gastric secretion in reperfusion system.

This study was undertaken to compare the potency of each L- and D-isomers of 5 amino acids in stimulating gastric acid and pepsin secretion by the intragastric reperfusion preparation in rats. Gastric basal and glycine-stimulated secretions in this preparation were lower than those Ghosh-Lai preparation, but not to a statistically significant extent. All D- and L-isomers of amino acids tested were found to markedly stimulate pepsin secretion, but to only slightly stimulate acid secretion. Pepsin stimulatory response to each D and L-isomer of amino acid was similar. The stimulatory effects of amino acids thus seem to be unrelated to the optical stereoisomeric configuration.

Influence of amino acid availability on protein turnover in perfused skeletal muscle

The effects of amino acids on protein turnover in skeletal muscle were determined in the perfused rat hemicorpus preparation. Perfusion of preparations from fasted young rats (81±2 g) with medium containing either a complete mixture of amino acids at five times (5×) their normal plasma levels, a mixture of leucine, isoleucine, and valine at 5× or 10× levels, or leucine alone (10×) resulted in a 25–50% increase in muscle protein synthesis and a 30% decrease in protein degradation compared to fasted controls perfused in the absence of exogenously added amino acids. When the branched-chain amino acids were omitted from the complete mixture, the remaining amino acids (5×) had no effect on protein turnover. The complete mixture at 1× levels was also ineffective. Comparison of the effects of amino acids with those of glucose and palmitate indicated that amino acids were not acting by providing substrates for energy metabolism. The stimulatory effect of amino acids on protein synthesis was associated with a facilitated rate of peptide-chain initiation as evidenced by a relative decrease in the level of ribosomal subunits. This response was not as great as that produced by insulin, and the amino acids did not augment the effect of insulin. Although protein synthesis in preparations from fed young rats (130±3 g) was stimulated by the addition of a mixture of the branched-chain amino acids (5×) to about the same extent as that observed in the fasted young rats, protein degradation was not affected. Furthermore, neither synthesis nor degradation were affected in preparations from fasted older rats (203±9 g) suggesting that the age and or nitritional state of the animal may influence the response of skeletal muscle to altered amino acid levels.

Regulation of formation of proteases in Bacillus megaterium. IV. The formation of the enzyme in the sporogenous strain KM.

Protease was formed only at the end of the growth phase and its synthesis continued in the stationary phase during the growth of the sporulatingBacillus megaterium KM Sp+ in complex media with amino acids or peptone and glucose. The enzyme was also not formed during the growth phase in the glucose containing mineral medium and was detected only later during the stationary phase, smaller quantities being observed than those formed in the complex medium. The addition of glucose at the beginning of the synthesis of protease inhibited the production of the enzyme. On the other hand, the addition of the mixture of amino acids under the same conditions stimulated its formation several fold. Cysteine blocked the synthesis of the enzyme unlike other amino acids. Within a certain range the stimulatory effect of amino acids was related to their concentration, being manifested only after a lag period of several hours. The ability to form protease dissappeared after the formation of refractile spores in sporangial cells.