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The objectives were to investigate the mechanisms by which glucocorticoids control proteolysis in muscle cells and the relationship between the calpain:calpastatin system and proteolysis in muscle. Female rabbits were treated with 1 mg dexamethasone (Dex)/kg body weight per day for 0, 1, 2 or 4 days after which animals were killed and muscle samples taken for analyses. Dex reduced urinary N-methylhistidine (NMH) 48% (day 4 versus day 1 of Dex treatment) and muscle NMH concentrations by 49% (day 1) to 40% (day 2) respectively, suggesting that protein degradation was reduced. To investigate whether the changes in apparent proteolysis were related to calpains, we examined the effects of Dex on muscle calpain and calpastatin activities. These were unaffected by Dex. This implies that Dex-dependent changes in degradation are not mediated by changes in muscle calpain or calpastatin activities. We studied the effects of Dex on calpain and calpastatin gene expression as a means of clarifying the relationships between proteinase gene expression and proteinase activities. µ-Calpain mRNA concentration was unaffected by Dex but m-calpain mRNA and calpastatin mRNA concentrations were reduced by 42–55% and 40% respectively. Dex had a similar effect on β-actin mRNA. Although calpain and calpastatin genes behaved as housekeeping genes, changes in their expression mimicked apparent changes in proteolysis. The observation that calpain and calpastatin activities were unchanged indicates that additional regulation of the calpain:calpastatin system exists at other sites in muscle cells. To determine whether Dex-dependent changes in proteolysis were mediated indirectly, we assayed the effects of Dex on plasma thyroid hormone concentrations. Dex reduced both plasma triiodothyronine (T₃) and thyroxine (T₄) concentrations by as much as 68% and 67% respectively, and reduced the T₃/T₄ ratio by 74% following 4 days of Dex treatment. Thyroid hormones regulate muscle proteolysis and their lower concentrations in Dex-treated rabbits may account for the reductions in muscle NMH concentration and urinary NMH excretion caused by Dex.

Journal of Endocrinology (1994) 141, 209–217

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