Effects of high doses of glucocorticoids on free amino acids, ribosomes and protein turnover in human muscle

BACKGROUND

Treatment with glucocorticosteroids causes a negative nitrogen balance, but the kinetic mechanisms responsible for this catabolic effect are controversial. We investigated the effects of 60 mg day(-1) prednisolone on protein synthesis and degradation in human skeletal muscle.

MATERIALS AND METHODS

Healthy adults (n = 9) were studied in the postabsorptive state, before and after 3 days of prednisolone treatment. The L-[ring 2,6(-3)H(5)]-phenylalanine tracer technique, concentration and size distribution of the ribosomes, mRNA content of the ubiquitin-proteasome pathway components in muscle, phenylalanine flux across the leg, and the free amino acid concentrations in skeletal muscle were used to study muscle protein metabolism.

RESULTS

The concentrations of most amino acids in arterial blood increased after prednisolone. There were also increased effluxes of phenylalanine, asparagine, arginine, alanine, methionine and isoleucine from the leg. The rate of protein degradation, as measured by the appearance rate (Ra) of phenylalanine, increased by 67% (P = 0.023) which, together with a doubling of the net release of phenylalanine from the leg (P = 0.007), indicated accelerated protein degradation. The pathway was not identified but there was no significant increase in mRNAs' encoding components of the ubiquitin-proteasome pathway. There was a 6% reduction in polyribosomes (P = 0.007), suggesting a decrease in the capacity for protein synthesis, although there was no measured decrease in the rate of protein synthesis.

CONCLUSIONS

These findings indicate that high doses of prednisolone lead to a sharp increase in net protein catabolism, which depends more on enhanced protein breakdown, and an uncertain effect on protein synthesis. The mechanisms stimulating proteolysis and the pathway stimulated to increase muscle protein degradation should be explored.

Effect of hemodialysis on protein synthesis

BACKGROUND

Earlier studies have shown that hemodialysis (HD) treatment stimulates net protein catabolism. Several factors associated with HD affect protein catabolism, such as an inflammatory effect due to blood-membrane contact and loss of amino acids and glucose into the dialysate.

SUBJECTS, MATERIAL AND METHODS

We have studied protein synthesis in skeletal muscle of healthy volunteers (n = 9) before and after a single heparin-free HD. Protein synthesis (PS) was studied, using 2 independent techniques: the incorporation of labeled 2H5-phenylalanine into muscle protein, which gives a quantitative measure of the fractional synthesis rate of muscle proteins, and the concentration and size distribution of ribosomes, which gives a qualitative estimate of protein synthesis. Furthermore, free amino acid concentrations were determined in muscle and plasma.

RESULTS

The rate of PS, expressed as the fractional synthesis rate, decreased by 13% during HD (p < 0.02). The capacity for PS, as reflected by the total concentration of ribosomes, was reduced by 22% (p < 0.02) and the activity of PS, expressed as the relative proportion of polyribosomes, decreased from 48.4 +/- 0.9% to 44.8 +/- 0.8% after dialysis (p < 0.01). There was a total loss of 5.8 +/- 0.3 g amino acid to the dialysate. Plasma and muscle free amino acid concentrations were determined at four time points; before and after the phenylalanine incorporation period, before dialysis and before and after the second incorporation period after dialysis. Immediately after dialysis, there was a decrease in plasma asparagine, histidine, alanine, taurine, valine and tryptophane. In muscle, no changes occurred except for a slight increase in leucine after dialysis. In blood, the glucose concentration decreased and the total amount of glucose lost to the dialysate was 21 +/- 3.0 g. In summary, one single hemodialysis treatment decreases fractional protein synthesis rate in skeletal muscle.

CONCLUSION

The results demonstrate substantial losses of amino acids and glucose to the dialysate and decreased amino acid concentrations in plasma, but only minimal changes in the intracellular amino acid concentrations in muscle, suggesting that the decreased PS is caused not by lack of amino acid precursors at the site of the synthesis activity, but by other mechanisms.

Correction of acidosis in dialysis patients increases branched-chain and total essential amino acid levels in muscle

Earlier studies have shown increased oxidation of the branched-chain amino acids (BCAA), valine, isoleucine and leucine, in experimental acidosis and low levels of valine in the muscle of acidotic HD patients. Using HPLC, free amino acids in plasma and muscle were studied before and after correction of acidosis in 9 HD patients over 6 months by dialysis with a high bicarbonate solution. The predialysis standard bicarbonate concentration in blood increased from 20.6 +/- 1.3 mmol/l (mean +/- SD) before correction of acidosis to 25.9 +/- 1.8 mmol/l after correction. Correction of acidosis resulted in a significant increase in the i.c. concentrations of valine, isoleucine and leucine by 48%, 28% and 32%, as well as for the sum of BCAA and EAA, from a level lower than controls. The intra- and extracellular gradient increased for several amino acids and for the sum of EAA and BCAA, suggesting an increased influx or reduced efflux of amino acids across the cell membrane. Anthropometric data and the levels of S-albumin and transferrin did not change after correction of acidosis. The increases in the i.c. concentrations of BCAA after correction of acidosis suggest that the catabolism of these amino acids had been reduced. The effects of correction of acidosis on the concentrations of essential amino acids could be beneficial since low concentrations in muscle may reduce protein synthesis.

Ribosome and free amino acid content in muscle during hemodialysis

Patients (N = 8) with chronic renal failure and uremia treated with hospital hemodialysis were in a pilot study investigated before and after a single hemodialysis session. The extracorporeal dialysis circuit was flushed regularly with saline to avoid clotting and the use of heparin. Percutaneous skeletal muscle biopsies were taken before and after the dialysis to determine the content of free amino acids together with the concentration and size distribution of ribosomes before and after dialysis. After dialysis the alanine concentration in muscle decreased by 20% (P less than 0.05), while all other amino acids were unaffected. The total ribosome concentration per mg of DNA decreased by 31% (P less than 0.01) and the relative proportion of polyribosomes by 7% (P less than 0.05) after the dialysis compared to predialytic values. All individual plasma amino acids decreased during the dialysis procedure except for threonine and arginine, which were unaltered, and leucine and isoleucine, which increased. The decline in ribosome and polyribosome content together with the changes in amino acid levels indicate a low capacity for protein synthesis and increased catabolism in muscle of hemodialyzed patients.

Comparison of methionyl human growth hormone and pituitary growth hormone on somatic growth of hypophysectomized rats

In hypophysectomized male rats the biological effects of two batches of methionyl human growth hormone, Somatonorm, (met-hGH), have been compared to those of human pituitary growth hormone, Crescormon (hGH). The rats were treated with doses ranging from 10 mIU per day to 145 mIU per day for 10 days. The parameters studied were total weight gain, longitudinal bone growth, measured by the tetracycline method and indirect cartilage growth, measured by uptake of radioactive sulphate. The results obtained demonstrated that Somatonorm stimulated weight increase in a linear and dose-dependent way, similar to that seen with the native hormone. Longitudinal bone growth, measured by the tetracycline method and the growth of different cartilages, measured as uptake of radioactive sulphate, were also similar between the two hormones.