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We have examined insulin action on glucose metabolism in six hypothyroid patients before and after regular thyroid hormone treatment, and in six healthy volunteers before and after transient induction of moderate hyperthyroidism. Insulin was infused under euglycaemic and eukalaemic clamps. An appropriate amino acid infusion was used to blunt insulin-induced decreases in amino acid levels. Glucose kinetics were assessed using a primed continuous infusion of [6,6-(2)H(2)]glucose. The results showed that basal plasma insulin and glucose levels (i.e. before infusion) were similar in each case. Despite similar insulin infusion rates, the plateau value of insulin was lower after thyroid treatment in both hypothyroid patients and healthy volunteers. The rate of exogenous glucose needed to maintain plasma glucose at a steady-state level was increased by thyroid hormone in hypothyroid patients (P <0.05), but not in healthy volunteers. Thyroid treatment resulted in a significant increase in basal glucose disposal in both groups (P <0.05). Insulin, in conjunction with glucose and amino acids, significantly stimulated glucose disposal (P <0.05) under all conditions. The incremental increase in glucose disposal after infusion tended to be higher following thyroid hormone treatment, but this was not statistically significant. However, the ratio of the incremental increase in glucose disposal to the increase in plasma insulin was significantly improved after thyroid hormone treatment in hypothyroid patients (P <0.05). It was also increased in healthy volunteers, but not significantly. We conclude that thyroid hormones improve the ability of insulin to stimulate glucose disposal related to insulinaemia. This phenomenon may be highly sensitive, because it was only apparent at low thyroid hormone levels.

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Plasma albumin is well known to decrease in response to inflammation. The rate of albumin synthesis from both liver and plasma was measured in vivo by use of a large dose of L-[(2)H(3)-(14)C]valine in rats injected intravenously with live Escherichia coli and in pair-fed control rats during the acute-phase period (2 days postinfection). The plasma albumin concentration was reduced by 50% in infected rats compared with pair-fed animals. Infection induced a fall in both liver albumin mRNA levels and albumin synthesis relative to total liver protein synthesis. However, absolute liver albumin synthesis rate (ASR) was not affected by infection. In plasma, albumin fractional synthesis rate was increased by 50% in infected animals compared with pair-fed animals. The albumin ASR estimated in the plasma was similar in the two groups. These results suggest that hypoalbuminemia is not due to reduced albumin synthesis during sepsis. Moreover, liver and plasma albumin ASR were similar. Therefore, albumin synthesis measured in the plasma is a good indicator of liver albumin synthesis.

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Glutathione metabolism during infection has been poorly documented. Glutathione concentrations and synthesis rates were studied in infected rats (2 d after infection) and in pair-fed controls. Glutathione synthesis rates were determined in liver, spleen, lung, small and large intestine, skeletal muscle, heart and blood by a 4-h or 6-h (15)N cysteine infusion. The activities of four hepatic enzymes involved in glutathione metabolism were also determined. Glutathione synthesis rates were significantly greater in liver (+465%), spleen (+388%), large intestine (+109%), lung (+100%), muscle (+91%) and heart (+80%) of infected rats compared with pair-fed controls. Glutathione concentrations were also greater in these tissues but were unaffected in small intestine and lower in blood. In keeping with the stimulation of liver glutathione synthesis, the activities of liver gamma-glutamyl-cysteine synthetase and glutathione reductase were significantly greater in liver of infected rats than of pair-fed rats. From the present study, we estimate that glutathione synthesis accounts for at least 40% of the enhanced cysteine utilization during infection. This increased utilization may be the primary cause of an enhanced cysteine requirement in infection.

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We have investigated the effect of hypothyroidism and insulin on protein metabolism in humans. Six hypothyroid patients were studied in a postabsorptive state before and after 5 months of regular treatment for hypothyroidism (153 +/- 17 microg/day of L-T4). The effect of insulin was assessed under hyperinsulinemic euglycemic and eukalemic conditions. Insulin was infused for 140 min at 0.0063 +/- 0.0002 nmol/kg x min. An amino acid infusion was used to blunt insulin-induced hypoaminoacidemia. Whole body protein turnover was measured using L-[1-13C] leucine. When compared to L-T4-induced subclinical thyrotoxic state, hypothyroidism induced a significant decrease (P < 0.05) in leucine endogenous appearance rate (a reflection of proteolysis; 0.89 +/- 0.09 vs. 1.33 +/- 0.05 micromol/kg x min), oxidation (0.19 +/- 0.02 vs. 0.25 +/- 0.03 micromol/kg x min), and nonoxidative disposal (a reflection of protein synthesis; 0.87 +/- 0.11 vs. 1.30 +/- 0.05 micromol/ kg x min). Insulin lowered proteolysis during both the subclinical thyrotoxic and hypothyroid states. Hypothyroidism impaired the antiproteolytic effects of insulin. Thyroid hormones are, therefore, essential for the normal antiproteolytic action of insulin.

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A gas chromatographic-mass spectrometric (GC-MS) procedure for the determination of stable isotope labelled glutathione has been applied to animal and human samples. The method, based on preparation of the N,S-ethoxycarbonyl methyl ester derivative of the intact peptide, is rapid and requires little or minor tissue treatment. The same method was applied to cysteine. The method was found to be reliable in terms of within-day and between-day precision, accuracy and linearity. The procedure was applied in humans and animals to determine in vivo the glutathione fractional synthesis rate using labelled cysteine infusion. The glutathione fractional synthesis rate was found to be 22.5%/day in blood from a healthy volunteer and 337+/-29%/day in rat liver.

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This study was carried out to analyse glucocorticoid-induced muscle wasting and subsequent recovery in adult (6-8 months) and old (18-24 months) rats because the increased incidence of various disease states results in hypersecretion of glucocorticoids in ageing. Adult and old rats received dexamethasone in their drinking water for 5 or 6 d and were then allowed to recover for 3 or 7 d. As dexamethasone decreased food intake, all groups were pair-fed to dexamethasone-treated old rats (i.e. the group that had the lowest food intake). At the end of the treatment, adult and old rats showed significant increases in blood glucose and plasma insulin concentrations. This increase disappeared during the recovery period. Protein synthesis of different muscles was assessed in vivo by a flooding dose of [13C]valine injected subcutaneously 50 min before slaughter. Dexamethasone induced a significant decrease in protein synthesis in fast-twitch glycolytic and oxidative glycolytic muscles (gastrocnemius, tibialis anterior, extensor digitorum longus). The treatment affected mostly ribosomal efficiency. Adult dexamethasone-treated rats showed an increase in protein synthesis compared with their pair-fed controls during the recovery period whereas old rats did not. Dexamethasone also significantly decreased protein synthesis in the predominantly oxidative soleus muscle but only in old rats, and increased protein synthesis in the heart of adult but not of old rats. Thus, in skeletal muscle, the catabolic effect of dexamethasone is maintained or amplified during ageing whereas the anabolic effect in heart is depressed. These results are consistent with muscle atrophy occurring with ageing.

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