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Thyroid hormones and muscle protein turnover. The effect of thyroid-hormone deficiency and replacement in thryoidectomized and hypophysectomized rats.

ABSTRACT: We have investigated the effects of thyroidectomy, hypophysectomy and 3,3',5-tri-iodothyronine replacement on protein synthesis and degradation in skeletal muscle in vivo. Thyroidectomy resulted in a decrease in the rate of protein synthesis as a result of a loss of RNA. However, RNA activity, the rate of protein synthesis per unit of RNA, was not decreased. This was the case in both young growing rats and mature nongrowing rats. Tri-iodothyronine treatment of thyroidectomized rats increased protein synthesis by increasing RNA concentration without changes in RNA activity, and this occurred even when food intake was restricted to prevent any increase in growth. The rate of protein degradation was decreased by thyroidectomy and increased by tri-iodo-thyronine replacement in both animals fed ad libitum and food-restricted animals. Hypophysectomy decreased protein synthesis by decreasing both RNA concentration and activity. these changes were reversed by tri-iodothyronine treatment even in the presence of persistent marked hypoinsulinaemia. This indicates that tri-iodothyronine can activate athe translational phase of protein synthesis in muscle in the absence of significant quantities of insulin. However, tri-iodothyronine does not seem to be obligatory for the maintenance of normal RNA activity in muscle, since in the thyroidectomized rat, in which plasma insulin concentrations are normal, RNA activity is maintained. From a consideration of the magnitude of changes in RNA activity observed in these experiments, it would appear that alterations in rates of elongation as well as initiation are involved in the changes in RNA activity.

SUBMITTER: Brown JG

PROVIDER: S-EPMC1162812 | biostudies-other | 1981 Mar

REPOSITORIES: biostudies-other

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Project description:1. The effect of thyroidectomy on turnover rates of liver, kidney and brain mitochondrial proteins was examined. 2. In the euthyroid state, liver and kidney mitochondria show a synchronous turnover with all protein components showing more or less identical half-lives compared with the whole mitochondria. The brain mitochondrial proteins show asynchronous turnover, the soluble proteins having shorter half-lives. 3. Mitochondrial DNA (m-DNA) of liver and kidney has half-lives comparable with that of whole mitochondria from these tissues. 4. Thyroidectomy results in increased half-lives of liver and kidney mitochondria, with no apparent change in the half-life of brain mitochondria. 5. A detailed investigation of the turnover rates of several protein components revealed a significant decrease in the turnover rates of mitochondrial insoluble proteins from the three tissues under study. 6. The turnover rates of m-DNA of liver and kidney show a parallel decrease. 7. Thus it is apparent that thyroid hormone(s) may have a regulatory role in maintaining the synchrony of turnover of liver and kidney mitochondria in the euthyroid state. Turnover of brain mitochondria may perhaps be regulated by some other factor(s) in addition to thyroid hormone(s). 8. It seems likely that during mitochondrial turnover m-DNA and insoluble proteins may constitute a major unit. 9. The mitochondrial protein contents of the three tissues are not affected by thyroidectomy. 10. No correlation was seen between the turnover rate of mitochondria and cathepsin activity in any of the tissues under study in normal or thyroidectomized animals. 11. On the other hand, mitochondrial proteinase activity shows good correlation with the turnover rates of mitochondria in normal animals, and a parallel decrease in activity comparable with the decreased rates of turnover is observed after thyroidectomy. 12. It is concluded that mitochondrial proteinase activity may play a significant role in their protein turnover.

Project description:The effect of corticosterone on protein turnover in skeletal muscle was investigated in growing rats. Protein synthesis was measured in vivo by the constant infusion of [(14)C]tyrosine. The extent to which any effect of corticosterone is modulated by the hyperinsulinaemia induced by steroid treatment was examined by giving the hormone not only to adrenalectomized rats but also to streptozotocin-induced diabetic rats maintained throughout the treatment period on two dosages of insulin by an implanted osmotic minipump. Approximate rates of protein degradation were also estimated in some cases as the difference between synthesis and net change in muscle protein mass. Measurements were also made of free 3-methylhistidine concentration in muscle and plasma. At 10mg of corticosterone/100g body wt. per day, growth stopped and muscle wasting occurred, whereas at 5 mg of corticosterone/100g body wt. per day no net loss of protein occurred. However, this low dose did induce muscle wasting when insulin concentration was regulated by a dose of 1.2 units/day. Protein synthesis was markedly depressed in all treated groups, the depression in the insulin-maintained rats being marginally more than in the hyperinsulinaemic adrenalectomized rats. The oxidative soleus muscle appeared to be less susceptible to the effect of the corticosterone than was the more glycolytic plantaris or gastrocnemius muscle. Any effect of the corticosterone on protein degradation was much less than its effects on protein synthesis. Where increases in the degradation rates appeared to occur in the rats treated with 10mg of corticosterone/100g body wt. per day, the increases were less than 20%. The free intracellular 3-methylhistidine concentrations were doubled in all groups treated with 5 mg of corticosterone/100g body wt. per day and increased 5-fold in the adrenalectomized rats treated with 10mg of corticosterone/100g body wt. per day, with no change in plasma concentration in any of the groups. It is therefore concluded that: (a) the suppression of protein synthesis is the main effect of glucocorticoids in muscle; (b) marked increases in insulin afford only minor protection against this effect; (c) stimulation of protein degradation may occur, but to a much lesser extent.

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Thyroid hormones and muscle protein turnover. The effect of thyroid-hormone deficiency and replacement in thryoidectomized and hypophysectomized rats.

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