The role of insulin in the regulation of protein metabolism: a review

Quantitative measurement of endogenous amino acid absorption in unanaesthetized pigs

The present experiment was carried out with 11 pigs (mean body weight: 53.9 +/- 1.3 kg) fitted with permanent catheters in the portal vein and carotid artery and with an electromagnetic flow probe around the portal vein. They were each subjected to 2 or 3 trials at 3 to 4-day intervals. During each trial the animals received after a previous fasting of 20 h a given amount of a protein-free diet (200 to 1200 g). The blood was collected either continuously for a quantitative determination of amino nitrogen, reducing sugars, urea and ammonia (number of meals 12, mean intake: 727 +/- 60 g) or discontinuously every 30 min between 0 and 8 h after the meal for amino acid analysis (number of meals 8; mean intake 709 +/- 105 g). A rather constant appearance (2 g/h) of amino acids in the portal blood was observed throughout the postprandial period. The intestinal absorption of each amino acid was however variable and represented between 10 and 50% of the daily requirements of the animal during the measuring period (8 h). Glutamine and to a less extent glutamic acid were exceptions as they were taken up by the gut wall from the arterial blood. There was also a marked synthesis of ornithine and citrulline by the latter. Because of the low blood level of urea, there were no apparent exchanges of urea between the blood and the intestine; in contrast, the ammonia absorption represented about 70% of that observed after ingestion of normal protein diets. Most amino acids are largely taken up by the liver and peripheral tissues, but in the case of alanine the syntheses exceed the uptake.

Microdetermination of 2-ketoglutaric acid in plasma and cerebrospinal fluid by capillary gas chromatography mass spectrometry; application to pediatrics

Quantification of 2-ketoglutaric acid in plasma and cerebrospinal fluid as its O-trimethylsilyl++-quinoxalinol derivative by gas chromatography chemical ionization mass spectrometry is described with benzoylformic acid as internal standard. This technique, with ammonia as reactant gas, only detects the protonated molecular ions. The recovery of 2-ketoglutarate from perchloric-deproteinized plasma is 99.7 +/- 1.2%. The normal value of 2-ketoglutarate in children is 8.6 +/- 2.6 mumol l-1 (mean +/- standard deviation) in plasma (n = 25) and 4.8 +/- 1.4 mumol l-1 in cerebrospinal fluid (n = 20). The plasma level of 2-ketoglutarate is correlated with urea concentration (r = 0.96; p less than 0.001) in healthy subjects and in patients with chronic renal insufficiency. Increased values are found in one case of pyruvate carboxylase deficiency, and inconstantly in diabetes; physiological variations are described during fasting and after an oral glucose load.

The protein-sparing effect of carbohydrate. 2. The role of insulin

1. In five experiments with growing female pigs of 38-63 kg, insulin (2 mU/kg) per min) and glucose (9-17 mg/kg per min) were infused continuously for 3-7 d. In three further experiments, glucose (9 mg/kg per min) was infused alone for 5 d. 2. In response to the combined infusion, plasma insulin increased 2-7-fold, plasma glucose decreased, on average, by 50% and plasma urea concentration was reduced by 40%. Urinary excretion of urea and nitrogen decreased after the first day of infusion to values averaging 70% of control levels. 3. The infusion of glucose alone provoked only a small increase in plasma insulin. The reduction of urinary urea and of N excretion were approximately 25% of those observed with the combined infusions.

Changes of free amino acids in plasma of healthy subjects induced by physical exercise

In eight healthy men a 20-min load of 1.5 W/kg body weight on a bicycle ergometer led to a significant increase of alanine and decline of leucine. Exhausting exercise caused in the same subjects a highly significant increase of alanine and decline of isoleucine, threonine, ornithine, leucine, serine, glycine, and asparasine and glutamine. The methionine and citrulline level declines also significantly. The total amino acids practically did not change. Physical exercise led furthermore to a marked increase of serum ammonia and uric acid. Urea nitrogen changed only little and on average had rather a declining tendency. The rise of alanine suggests the existence of a glucose-alanine cycle, the drop of ornithine and citrulline is most probably associated with the inhibition of ureogenesis in the liver. The reduction of leucine and isoleucine is probably the result of the entry of these amino acids into muscle and their deamination.

Evaluation of the rate-limiting steps in the pathway of glucose metabolism in kidney cortex of normal, diabetic, cortisone-treated and growth hormone-treated rats

1. The activities of gluconeogenic and glycolytic enzymes and the concentrations of citrate, ammonia, amino acids, glycogen, glucose 6-phosphate, acetyl-CoA, lactate and pyruvate were measured in kidney cortex of normal, diabetic, cortisone-treated and growth hormone-treated rats. 2. In kidney cortex of diabetic, cortisone-treated and growth hormone-treated rats the activities of glucose 6-phosphatase (EC 3.1.3.9), fructose 1,6-diphosphatase (EC 3.1.3.11) and phosphopyruvate carboxylase (EC 4.1.1.32) were increased. 3. The activities of glutamate dehydrogenase (EC 1.4.1.3), alanine aminotransferase (EC 2.6.1.2), aspartate aminotransferase (EC 2.6.1.10) and pyruvate carboxylase (EC 6.4.1.1) were increased in diabetic and cortisone-treated rats. In growth hormone-treated rats the activity of aspartate aminotransferase was depressed but those of the other three enzymes were unchanged. 4. The activity of hexokinase (EC 2.7.1.1) was not altered in any of these conditions. Phosphofructokinase (EC 2.7.1.11) activity was depressed only in growth hormone-treated rats. Pyruvate kinase (EC 2.7.1.40) activity was depressed in cortisone-treated and growth hormone-treated rats but unchanged in diabetic rats. 5. Amino acids, acetyl-CoA and glucose 6-phosphate contents were increased in rat kidneys in all these three conditions. Ammonia content was increased in diabetic and cortisone-treated rats but was markedly diminished in growth hormone-treated rats. 6. The [lactate]/[pyruvate] ratio was elevated in diabetic and cortisone-treated rats but unchanged in growth hormone-treated rats. Citrate content was increased in the kidney cortex of diabetic and growth hormone-treated rats but was unchanged in cortisone-treated rats. The activity of ATP citrate lyase (EC 4.1.3.8) was depressed in diabetic and growth hormone-treated rats but was increased in cortisone-treated rats. 7. Glycogen content was moderately elevated in growth hormone-treated rats and markedly elevated in diabetic rats, whereas no change in glycogen content was observed in cortisone-treated rats. Glycogen synthetase (EC 2.4.1.11) activity was unchanged in all these three conditions. Phosphorylase (EC 2.4.1.1) activity was not affected in cortisone-treated rats but was depressed in diabetic and growth hormone-treated rats.

The effect of an oral glucose load on serum free amino acid concentrations in children before and after treatment for kwashiorkor

1. The mechanisms by which an oral dose of glucose can stimulate the uptake of essential amino acids from serum was found to be operative even in children with severe, untreated kwashiorkor.

2. Although the percentage fall in the concentration of the essential amino acids in response to glucose was the same before and after treatment, the absolute fall was greater on discharge. The results indicated that the magnitude of this fall might depend on the initial fasting concentration of each individual amino acid.

3. These results have been discussed on the basis of possible long-term pathological effects of a distorted serum amino acid pattern typified by low concentrations of valine, leucine and threonine. The protein-sparing action of carbohydrate, achieved by the preferential synthesis of muscle protein in response to insulin secretion, might be less effective because of substrate limitation.

4. Of the non-essential amino acids, the concentrations of alanine and glycine also fell markedly before treatment, but this did not occur on discharge. The significance of this difference is also discussed.

5. The results also indicate that slightly high, spurious amino acid ratios can be obtained if a child is given a drink containing sugar before a blood sample is taken.

Protein malnutrition in the rat induced by protein-calorie imbalance

1. A study has been made of the effect of feeding growing rats for a long time on a low-protein, high-carbohydrate diet given at two different levels of energy. 2. When the proteins of the diet provided o or 5% of the calories the body-weight fell rapidly and fat accumulated in the liver. Addition of carbohydrate (glucose) to the diets increased the amount of fat in the liver. 3. It is suggested that a deficiency of protein retards the synthesis in the liver of lipoproteins required for removal of triglyceride from the liver. Additional carbohydrate diverts amino acids from the amino acid pool to tissues such as muscles, increasing the liver damage.