Insulin and incorporation of amino acids into protein of muscle. Cellular amino acid levels and aminoisobutyric acid uptake

The role of apoptosis and autophagy in the insulin-enhancing activity of oxovanadium(IV) bipyridine complex in streptozotocin-induced diabetic mice

Metal-based therapies (e.g. Vanadium) possess an attractive proposition in medicinal treatment of diabetes mellitus. Defective insulin secretion can result from impaired β-cell function which is mediated by many process including apoptosis and autophagy. In this study. diabetes was induced by administration of streptozotocin then treatment was performed by vanadyl sulfate and [VO(bpy)₂ Cl] Cl.H₂O complex. Blood glucose level, AST, ALT, BUN, CR, TCHO, TG and total protein were determined in serum. MDA, NO, erythrocyte GSH and SOD were estimated. LC3 and Caspase 3 levels in pancreatic cells were assessed by flow cytometer. Histopathological investigation of pancreatic tissue was performed. Results of Diabetic group showed a significant increase in transaminases activities, TCHO, TG, MDA, NO and Caspase 3 levels and significant decrease in TP, GSH, SOD and LC3 levels. Oral administration of vanadium complex resulted in normoglycemia, significant increase in blood GSH, SOD, TP and LC3 levels, significant decrease in ALT, AST, BUN, TCHO, TG, MDA and NO and Caspase 3 levels. In addition, proliferative effect of complex prevents islet atrophy. From previous results, the insulin-enhancing effect induced by this complex indicated that this new complex can be a valuable candidate as insulin-enhancing and antioxidant compound than inorganic vanadyl sulfate.

Accumulation of amino acids in muscle of perfused rat heart. Effect of insulin

1. Rat heart perfused with Krebs-Henseleit bicarbonate buffer released material containing ninhydrin-positive nitrogen, but the amount was less than that reported to be released by diaphragm; glucose, but not insulin, decreased the release of ninhydrin-positive nitrogen and increased the concentration of the same material in the intracellular water of heart. 2. When heart was perfused with a mixture of amino acids and glucose, there was actually a net uptake, and an increase in intracellular concentration, of ninhydrin-positive nitrogen. Changes in the concentration of ninhydrin-positive nitrogen did not accurately reflect changes in concentration of amino acids. 3. The effect of insulin on the actual concentration of individual amino acids in heart muscle was examined by perfusing the heart with a mixture of amino acids and other ninhydrin-positive substances in the same concentration as they are found in plasma. 4. The effect of insulin on the concentrations of amino acids in the medium and in the intracellular water of the heart was determined after perfusion for different periods of time. No clear or meaningful effect of insulin was observed, despite the fact that insulin significantly increased the accumulation, in each of the same hearts, of radioactivity from amino[(14)C]isobutyric acid.

FURTHER OBSERVATIONS ON THE PRODUCTION OF AMINO ACIDS BY RAT-LIVER MITOCHONDRIA AND OTHER SUBCELLULAR FRACTIONS

1. Rat-liver mitochondria showed a decrease in amino acid production after preparation in 0.25m-sucrose containing EDTA (1mm), but an increase in water content. When EDTA was replaced by Mn(2+) (1mm) or succinate (1mm), both amino acid production and water content were lowered, whereas preparation in 0.9% potassium chloride caused an increase in both. 2. Amino acid production by rat-liver homogenates prepared in 0.9% potassium chloride or 0.25m-sucrose was similar (q(amino acid) 0.047 and 0.042 respectively aerobically). After freezing-and-thawing q(amino acid) values were approximately doubled, and approached that of a homogenate prepared in water. 3. All cations tested inhibited amino acid production by mitochondria, Hg(2+) and Zn(2+) being the most effective in tris-hydrochloric acid buffer. In phosphate buffer Mg(2+) and Mn(2+) had no effect. Of the anions tested only pyrophosphate and arsenate had any inhibitory effect at final concn. 1mm. 4. Iodosobenzoate (1mm) and p-chloromercuribenzenesulphonate (1mm) inhibited mitochondrial amino acid production by 70-80%, whereas soya-bean trypsin inhibitor, EDTA and di-isopropyl phosphorofluoridate inhibited by a maximum of 30%. Respiratory inhibitors had no effect. 5. Rat-liver homogenate and subcellular fractions each showed an individual pattern of inhibition when a series of inhibitors was tested. 6. Amino acid production by mitochondria was decreased by up to 50% in the presence of oxidizable substrate, apart from alpha-glycerophosphate and palmitate, which had no effect. CoA stimulated amino acid production in tris-hydrochloric acid but not in phosphate buffer, alpha-oxoglutarate abolishing the stimulation. 7. Cysteine and glutathione stimulated amino acid production by whole mitochondria by 30%, but only reduced glutathione stimulated production in broken mitochondria. 8. Adrenocorticotrophic hormone and growth hormone stimulated mitochondrial amino acid production by 21-24%, whereas insulin inhibited production by 25%. 9. Coupled oxidative phosphorylation increased amino acid production by up to 154% at 25 degrees and 40 degrees . The increase was abolished by 2,4-dinitrophenol. 10. Amino acid incorporation in mitochondria was accompanied by an increase in amino acid production, both being decreased by chloramphenicol. 11. Mitochondrial production of ninhydrin-positive material was increased in the presence of albumin. The biggest increase was noted for the soluble fraction of broken mitochondria. No increase was found in the presence of (14)C-labelled algal protein or denatured mitochondrial protein.

Labile protein reserves and protein turnover

The concept of a labile protein reserve is based on the relatively slow establishment of a new equilibrium in the rate of nitrogen excretion after an abrupt change in dietary supply. The evidence reviewed shows that a majority of this nitrogen is derived from or deposited in skeletal muscle proteins. The rates of synthesis and degradation of total body protein are rapid in large animals (man and swine) and are correlated with heat production. The rate of protein synthesis in skeletal muscle greatly exceeds the rate of growth and is sensitive to nutritional status. The rate exceeds the rate of degradation in response to the ingestion of an adequate diet so that tissue proteins are accumulated, but it decreases below the rate of degradation under conditions of deprivation. In this latter state, proteins of skeletal muscle supply amino acids for energy or for the synthesis of other more essential proteins, e.g., milk proteins during lactation. Thus, we conclude that the labile protein reserve is a product of the normal, dynamic metabolism of protein.

The effect of insulin on free amino acid pools and protein synthesis in rat skeletal muscle in vitro

1. The effects of insulin in vitro on tissue pools and incorporation into protein of glycine and leucine in the extensor digitorum longus muscle of the rat are reported. 2. It was found that insulin decreased the lag period before the establishment of a linear rate of incorporation of radioactive glycine into protein. 3. The hormone increased the size of the free intracellular glycine pool. No such effect was found for leucine. The accumulation of radioactive glycine in the intracellular fluid compartment was increased. The content of radioactive leucine in the intracellular compartment was decreased. 4. Insulin decreased the specific radioactivity of both glycine and leucine in the extracellular fluid. 5. The hormone also decreased protein catabolism. 6. The effect on protein synthesis was not caused by an increase in the specific radioactivity of the extracellular pool but was possibly related to increased amino acid concentrations in this pool, which could in turn have affected the aggregation of ribosomes.

Action of insulin and growth hormone on protein synthesis in muscle from non-hypophysectomized rabbits

The separate effects of insulin and growth hormone on the uptake and incorporation of five amino acids into diaphragm muscle from non-hypophysectomized rabbits has been examined. Both growth hormone and insulin, when present in the medium separately, stimulated the incorporation into protein of the amino acids, leucine, arginine, valine, lysine and histidine. Insulin also stimulated amino acid uptake, but growth hormone did not. When insulin and growth hormone were present in the incubation medium together, the uptake and incorporation of valine, the only amino acid studied under these conditions, tended to be greater than the sum of the separate effects of the two hormones.

The control by insulin of amino acid accumulation in muscle

1. The capacity of insulin to enhance the accumulation by muscle of several amino acids was studied. Reports that threonine uptake is enhanced by insulin were not confirmed, despite its enhanced incorporation into protein. Uptake of beta-alanine and gamma-aminobutyric acid also did not respond to the hormone. A stimulation of accumulation of alanine, histidine and ethionine was observed. 2. The capacity of inhibitors of protein synthesis to reveal a stimulation by insulin of accumulation of several amino acids, hitherto considered unresponsive to insulin in the absence of inhibitor, was confirmed. Cycloheximide was as effective as puromycin. However, two of these amino acids, alanine and histidine, here showed response to insulin in the absence of inhibitor. The presence of cycloheximide was found to enhance uptake of cycloleucine, ethionine and threonine; in its presence insulin enhanced uptake of beta-alanine and alpha-methyltyrosine. 3. It is concluded that the influence of inhibitors of protein synthesis on amino acid accumulation and the response of amino acid accumulation to insulin are not adequately explained on the basis of inhibition of protein synthesis allowing amino acids to accumulate more readily. 4. The technical problems of whether linear rates of incorporation of amino acids into protein really indicate more than one cell pool are discussed and safeguards suggested. That initial rates of incorporation of label into protein are likely to be non-linear is shown.

Amino acid balance across tissues of the forearm in postabsorptive man. Effects of insulin at two dose levels

Amino acid balance across skeletal muscle and across subcutaneous adipose tissue plus skin of the forearm has been quantified in postabsorptive man before and after insulin infusion into the brachial artery. Skeletal muscle released significant amounts of alpha amino nitrogen after an overnight fast. Most individual amino acids were released. Alanine output was by far the greatest. The pattern of release probably reflects both the composition of muscle protein undergoing degradation and de novo synthesis of alanine by transamination. A significant correlation was observed between the extent of release of each amino acid and its ambient arterial concentration. Elevation of forearm insulin in eight subjects from postabsorptive (12 muU/ml) to high physiologic levels (157 muU/ml) in addition to stimulating muscle glucose uptake blocked muscle alpha amino nitrogen release by 74%. Consistent declines in output were seen for leucine, isoleucine, tyrosine, phenylalanine, threonine, glycine, and alpha-aminobutyric acid. Alanine output was insignificantly affected. Doubling forearm insulin levels (from 10 to 20 muU/ml) in eight subjects increased muscle glucose uptake in three and blocked alpha amino nitrogen output in two although both effects were seen concurrently in only one subject. Changes in net amino acid balance after insulin could be accounted for by increased transport of amino acids into muscle cells or retardation of their exit. It is likely that ambient arterial amino acid concentrations are established and maintained primarily by the extent of muscle amino acid release. The individual amino acids whose outputs from forearm muscle decline after forearm insulinization correspond well with those whose levels fall systematically after systemic insulinization. This suggests that declines in amino acid levels after systemic insulinization are due to inhibition of muscle release. Doubling basal insulin approaches the threshold both for blockade of muscle amino acid output and stimulation of glucose uptake, effects which appear to occur independently.

Relationship between intracellular amino acids and protein synthesis in the extensor digitorum longus muscle of rats

1. The incorporation into protein, and the accumulation into the free amino acid pools, of radioactive l-leucine and glycine was studied in rat extensor digitorum longus muscle. 2. The tissue was incubated first with (14)C-labelled and then with (3)H-labelled amino acid. 3. The experimental results were consistent with a model based on the premise that the amino acids in protein were incorporated directly from the extracellular pool.

Effect of denervation on the synthesis of ribonucleic acid and deoxyribonucleic acid in rat diaphragm muscle

1. The effect of unilateral denervation of rat diaphragm muscle on its content of nucleic acids and their incorporation of precursors was investigated. 2. After denervation the paralysed hemidiaphragm hypertrophies and within 3 days its content of RNA increases considerably. The concentration of DNA/unit mass remains fairly constant. 3. During this period there is some increase in the rate of incorporation of [(14)C]adenine into RNA, whereas there is some diminution in the rate of incorporation of [(14)C]orotic acid. 4. Incorporation of [(14)C]adenine and [(3)H]thymidine into DNA is much increased in the paralysed tissue, reaching its maximum by about the third day, but returning to normal by the tenth. 5. The significance of these results in relation to the hypertrophy after denervation is discussed.

The effects of potassium ions and denervation on protein synthesis and the transport of amino acids in muscle

1. The effects of varying concentrations of K(+) during incubation, of denervation and of various drugs on the accumulation of (14)C-labelled amino acids, their incorporation into protein and the stimulation of these processes by insulin in rat diaphragm preparations were studied. 2. The accumulation of glycine and aminoisobutyrate and incorporation of glycine into protein was less in tissue incubated in K(+)-free buffer or 20mm-K(+) than with 5-10mm-K(+). Incorporation of leucine was unaffected. 3. Incorporation into protein of amino acids by diaphragm that had been denervated 3 days previously was elevated. Accumulation of both glycine and aminoisobutyrate was also raised but that of phenylalanine was unaffected. 4. Accumulation of glycine by diaphragm and extensor digitorum longus muscle was decreased by a number of agents including cocaine and mepyramine. 5. The stimulation of incorporation by insulin was unaffected by changes in K(+) or in the presence of cocaine and mepyramine. Denervated tissue was markedly less responsive to insulin than its control. 6. The results are discussed in the context of the relation of amino acid accumulation to operation of the Na(+) pump and the influence of insulin thereon.

Some factors affecting the response of muscle to insulin

1. The effect of various changes in the composition of the supporting medium on the capacity of isolated rat diaphragm to incorporate amino acids into its protein has been studied. 2. Replacement of most of the normal ionic constituents by sucrose is inhibitory towards protein synthesis, as is also substitution of choline or K(+) for Na(+). 3. The capacity of the tissue to respond to a stimulatory effect of insulin is impaired in the sucrose media and under certain conditions in the absence of Na(+), particularly when Na(+) is replaced by K(+) and the (14)C-labelled amino acid is presented at a relatively high concentration. 4. Cutting of the tissue before incubation also decreases incorporating capacity and markedly decreases responsiveness to insulin. 5. In abnormal media the cellular content of ATP falls sharply. 6. The ATP content of the tissue also declines in the presence of 2-deoxyglucose. This change is prevented by the addition of glucose but not of pyruvate and succinate. 7. Although affecting the rate of amino acid incorporation the ATP content is not thought generally to limit sensitivity to insulin.